Welcome relief from the heat today with high temperatures below normal in the 60s as cool, onshore flow sets up. The warmth returns by mid-week with highs again touching 90ºF by Wednesday. A strong thunderstorm is possible with a cold front passing through that day. Afterwards, temperatures cool but still reach above average highs in the low-80s on Thursday.
Rest of today – partly sunny with gradually increasing sun, high temperatures in the mid-60s with easterly and northeasterly winds shifting towards the southeast. Overnight lows in the mid-50s.
Weather Prediction Center surface analysis for valid 5AM Monday. Overnight, a cold front pushed southwest of the area, allowing for a much cooler air mass to take hold, with easterly onshore flow off the still as yet cold waters of the Atlantic.
Tuesday – warmer with high temperatures in the low-70s and partly sunny skies. Overnight lows in the mid-60s.
Wednesday – a warm front will pass through and put us in the warm sector of a strong low centered well to our north over Canada. Temperatures are expected to reach around the 90ºF in the city. Later in the day, as a cold front approaches, scattered but possibly strong thunderstorms could roll through. Overnight lows in the upper-60s.
GFS 850 mb temperature anomalies for Wednesday at 5PM. We are forecast to see +7ºC anomalies at this level of the atmosphere, corresponding to much above normal temperatures at the surface.
Thursday– skies clear out with high temperatures in the low-80s. Overnight lows around 60ºF.
Subtropical Storm Ana
Over the weekend, the first named storm of the 2021 Atlantic hurricane season, Subtropical Storm Ana, formed over the open waters of the Atlantic northeast of Bermuda. While the storm itself was never a threat to land, it does mark the seventh year in a row that a named storm formed ahead of the formal start of the hurricane season.
GOES satellite imagery showing the circulation of Subtropical Storm Ana
We can’t draw any conclusions from a single storm, but it is worth noting that nearly all major forecast sources including NOAA are calling for a season of above average hurricane activity.
NASA satellite Moderate Resolution Imaging Spectroradiometer (MODIS) image of the Getty Fire on October 28, 2019, shortly after the fire began. Smoke is visible billowing offshore to the south.
“Extremely critical” fire weather conditions are forecast to impact a large part of Southern California, with the strongest Santa Ana wind event of the year possibly occurring Wednesday before some improvement Friday. Affected areas include the entire Los Angeles metro area, where the Getty Fire continues to burn. Wind gusts at or above 70 mph in higher elevations coupled with very dry air (relative humidity less than 10%) will make it incredibly difficult to make any progress towards containing this fire, and presents a high risk for new fires to start.
Weather Prediction Center surface forecast for 5AM PDT Wednesday
Weather Prediction Center surface forecast for 5PM PDT Thursday
Synoptic Set Up Believe it or not, the strength of the Santa Ana winds will be tied directly to the very cold air mass moving into the Great Basin. A low along the leading edge of this cold air has caused a massive ongoing snowstorm across much of Colorado. Behind this, cold, stable air will move in along with a strong area of high pressure with surface pressures forecast above 1040 mb. At the same time, a coastal low will sweep south along the northern and central CA coast along the leading edge of the colder air accompanying this high pressure. This will set up a tight pressure gradient at the coast favoring strong offshore east-northeast winds, with local forecast offices citing model output of as much as 10-12 mb gradient from Barstow to LAX, over a distance of only 200 miles.
Impacts With the tight pressure gradient forecast above, forecasters are calling for the possibility of gusts in excess of 70 mph in the mountain ranges near the coast with, with lower wind gusts of 35-40 mph at lower elevations. The influx of cold air already less capable of holding moisture that will downslope off coastal ranges and towards the ocean will yield relative humidities of 3-8% (as the air dries further during downsloping due to compressional warming). Santa Ana winds of this magnitude along with such dry conditions easily warrant the Storm Prediction Center designation of extremely critical fire weather.
GFS model output forecast sounding for KLAX at 11AM PDT Wednesday. Note the huge gap between the green dew point line and red environmental temperature line at the surface (5°F dew point, 64°F temperature). This will result in extremely low relative humidity values.
Storm Prediction Center fire weather outlook for Wednesday
Timing Peak potential for winds appears to be during the overnight hours through morning and early afternoon Wednesday. This is the time when the pressure gradient will be maximized. The high pressure referenced above is forecast to weaken overnight into Thursday, while the coastal low should also dissipate. The high pressure is forecast to continue weakening into the weekend, thus ending the most dangerous period of fire weather.
I’m at my college for reunion weekend, and offered to do a detailed forecast for one of the days I’m here. Saturday is shaping up to be a bit of a mixed bag. Temperatures should be summer-like in the mid to upper-80s, but the downside is a risk for possibly strong to severe thunderstorms especially in the early evening hours. These thunderstorm chances are far from being certain though, and there’s a chance we may miss out entirely on any precipitation. In other words, it’s another good day for a precipitation forecast bust!
My Forecast High: 87°F | Low: 62°F | Max sustained winds: 21 mph | Total precipitation: 0.28″ – verification will come from METAR data for the period between 2AM Saturday and 2AM Sunday (06Z Saturday to 06Z Sunday) at LPR (Lorain County Regional Airport).
Verification High: 90°F | Low: 61°F | Max sustained winds: 25 mph | Total precipitation: 0.00″ – on high temperatures, hedging up towards the higher end of guidance turned out to be a good idea. It’s yet more evidence that MOS guidance tends to be too cool for breezy warm sectors. If anything, I could have been a touch more aggressive on the high here. Did well on the low temperature, and from what I could see, did decently on max winds too. Like I’d mentioned in the forecast, the bust potential for precipitation on this day was significant. That ended up being the case – KLPR and Oberlin missed out on the heaviest rain and more serious convective activity, leading to barely a trace of measurable precipitation. In retrospect, I probably could have gone even drier with my forecast, but I would still have not felt comfortable going with zeros.
Weather Prediction Center surface forecast for 2PM Saturday
Synoptic Set Up A stationary front eventually lifts through Ohio overnight as a warm front. We’ll spend the day in the warm sector of a surface low centered over central Ontario. At the 850 mb level, winds will not be strong enough to qualify as a low-level jet and relative humidity values likewise do not appear particularly high. At 500 mb, a robust shortwave trough is forecast to push through later in the day, bringing some decent vorticity and upper-level lift. Further up at the 300 mb level, there’s not much evidence of enhanced divergence and lift since we’ll be south of a 300 mb jet streak.
GFS model output for 500 mb vorticity and winds valid at 8PM Saturday. Note the strong cyclonic vorticity values over Northeastern Ohio at this time (bright oranges). This will provide some synoptic scale lift for initiating storms via upper-level divergence.
High Temperatures GFS and NAM MOS and NBM all show Saturday to be a warm day. GFS is the warmest with 87ºF while NBM is coolest with 83ºF. EKDMOS (ensemble MOS) 50th percentile matches comes close to GFS MOS at 86ºF. Because forecast soundings show we’ll be in a well-mixed warm sector with decent though not ideal conditions for warm advection, I am tending towards thinking that a high of 87ºF is in fact possible. We could be even warmer if enough sun breaks out between possible earlier showers and rain later in the day.
GFS 2-meter temperature contours and winds at 2PM Saturday showing decent, though not ideal conditions for warm advection (southwesterly winds of 15 knots at a somewhat sharp angle across closely packed temperature contours). If surface winds were forecast to be stronger, we’d be looking at better chances for robust warm advection and hence warmer temperatures.
Low Temperatures NAM and GFS MOS are again in close agreement with 61ºF and 63ºF lows respectively. NBM is a touch cooler at 59ºF. Although both NAM and GFS show evidence of a nocturnal inversion forming, and calm winds at the surface, both models also show extensive cloudiness or even precipitation ongoing overnight. Because the boundary layer starts out with plenty of moisture, there shouldn’t be much in the way of evaporational cooling. Thus, clouds will actually serve to stave off any radiational cooling, which leads me to believe overnight lows will be on the warmer side of guidance. I’ll go with 62ºF here.
NAM MOS guidance for KLPR
GFS MOS guidance for KLPR
Max Sustained Winds NAM, GFS MOS have maximum synoptic winds averaging about 16 knots. I see no clear reasons to go much higher than 18 knots for a max sustained wind tomorrow, unless we happen have a thunderstorm roll through with sustained winds that are much higher. EKDMOS’ 50th percentile lies right around this figure. Even with well-mixed boundary layer forecast to form tomorrow afternoon, winds above the surface up to 850 mb do not appear to be that strong, only around 20-25 knots. Layer mean wind analysis of the NAM sounding only showed winds of about 16 knots.
GFS forecast sounding for 5PM Saturday
NAM forecast sounding for 5PM Saturday
Total Precipitation There’s a large spread between NAM and GFS on forecast totals tomorrow. This will be the trickiest part of the forecast because there’s clear signs that convective precipitation, with a couple rounds of possibly strong to severe thunderstorms could materialize. Any strong thunderstorm passing overhead could quickly dump a few tenths of an inch of rain. However, if we miss out on any significant thunderstorm action, we could see just a trace of rain instead. It’s still too early to tell the finer details of where storms will initiate at this point. Based on forecast soundings above, the best chance for thunderstorms to roll through would be during the late afternoon and early evening, maybe 5-8PM when moisture is best and instability is maximized from daytime heating. Strong, mostly unidirectional shear profiles with westerly winds increasing from 10 knots at the surface to 50 knots at 500 mb suggest the potential for damaging winds as the biggest severe weather threat.
NAM forecast for 850 mb winds and relative humidity at 2PM Saturday
GFS forecast for 850 mb winds and relative humidity at 2PM Saturday
Going against the potential for significant precipitation, neither GFS nor NAM really show strong signals of low-level jet support. Winds at the 850 mb level don’t look to be particularly strong, and moisture support isn’t looking great either. What’s more, while there’s mention in the local forecast office discussion of a right entrance region of an upper-level jet that would provide some dynamic lift, I’m not seeing that myself. They did also mention that a trough would pass through and provide a focus for some lift. This would be a necessary trigger since there’s no clear frontal boundaries that would provide the convergence and lift necessary to generate showers and thunderstorms. Given the hit or miss nature of thunderstorms, less than ideal conditions for heavy precipitation, and disagreement between GFS and NAM MOS at this time, I find it prudent to hedge down with a forecast for 0.28″ of total precipitation despite MOS guidance averages at 0.48″.
Last Thursday afternoon, April 26, 2019, a line of severe thunderstorms produced potent, damaging winds, some in excess of hurricane force that caused disruptions to regional transportation networks in the DC, Baltimore, and Philadelphia areas. These storms provide an instructive example of what ingredients are required for severe thunderstorms, and how quickly everything can come together on a given day.
Synoptic Set Up (The Big Picture)
On Thursday morning, a low centered over the Great Lakes was progressing north and east. A warm front extended south and east from this low and was moving north, with a noticeable “kink” where there was colder air at higher altitudes along the Appalachians and related foothills. South of this warm front, southerly winds were helping temperatures rise well into the upper-60s and low-70s. A cold front was located a further back and was advancing across Pennsylvania, and the Virginias. This cold front would provide the focus for lift and thunderstorms later in the day, although some more isolated thunderstorms also accompanied the warm front.
Weather Prediction Center surface analysis of this storm at 11AM on April 26, 2019
Above the surface at 850 mb, evidence suggested an axis of relatively saturated air along with a low-level jet of 35-40 knots would develop, providing the moisture necessary for precipitation. Further up in the atmosphere, a negatively tilted 500 mb trough was evident upstream of the area with the Southeast PA region also appearing to be in the exit region of a 300 mb jet streak. Both of these would help enhance lift by providing divergence aloft in the atmosphere as air was removed from the column while decelerating out of the base of the 500 mb trough and 300 mb jet streak respectively.
Fig. 1
Fig.2
Fig. 1: GFS forecast model initialized at 7AM Thursday, April 26, 2019 depicting an axis/tongue of moisture (narrow area of blue) along the PA/NJ border around 5PM that day. Fig. 2: 300 mb analysis for 8PM on Thursday, April 26, 2019. Note the densely packed yellow contours close to the Southeast PA area at this time, indicating strong net divergence in the exit region of a curved jet streak at this level (blue shaded areas with wind barbs showing max winds of 80 knots slowing to 65 knots in the exit region).
Furthermore, winds throughout the atmosphere were strong, and increasing from 35 knots at 850 mb to 60 knots at 300 mb. Meanwhile winds at the surface were light, at 5 knots or so at the from the south. Winds aloft were more from the southwest. So, there was an element of both speed and directional wind shear in the atmosphere this day.
A Sunny Afternoon and Instability
From above, we see that we had several ingredients were taking shape last Thursday: a couple frontal boundaries providing focused lift, moisture at 850 mb, vorticity and net divergence at 500 mb and 300 mb enhancing lift, with strong winds at these levels enhancing wind shear. We still needed one more key component to truly set off some strong to severe thunderstorms: instability. How does instability build up in the atmosphere? The answer has to do with the daytime heating and the sun. That’s why thunderstorms often pop up later in the afternoon when daytime heating is maximized.
Fig. 3
Fig. 4
Fig. 6
Fig. 3: Storm Prediction Center mesoanalysis highlighting areas favorable for severe weather on the afternoon of April 26, 2019. Fig. 4: Storm Prediction Center analysis of 3-hour mixed layer CAPE (convective available potential energy, a measure of instability) change. Note that the pocket of a large increase in instability corresponds to the location of the pocket of clear skies below. Fig. 6: A marked up visible satellite image at 3:16 PM on Thursday, April 26, 2019 showing the approximate position of frontal boundaries extrapolated from the Storm Prediction Center analysis in the preceding image.
Why does daytime heating at the surface lead to destabilization of the atmosphere? This has to do with buoyancy and lapse rates. Lapse rate describes the change in temperature over a given altitude. As the sun heats the surface of the earth up, it shifts the environmental temperature line to the right on a skewT sounding as the one attached below, taken at 2PM on Thursday, April 26, 2019 at Washington Dulles International Airport (KIAD). This tends to increase instability because a warmer airmass above the surface will have greater buoyancy. A large lapse rate combined with enhanced buoyancy allows for air from the surface to rise, and keep rising forming towering cumulus clouds that can eventually build into thunderclouds. As long as a parcel rising from the surface stays warmer than the environmental temperature profile (red line), it will keep rising.
A skewT of a sounding taken at Washington Dulles International Airport (KIAD) at 2PM on Thursday, April 26, 2019. Refer to this post for how to interpret this skewT.
The Storm Prediction Center was well aware that the severe weather potential was maximized for areas that saw clearing skies in advance of the approaching cold front. They also picked up on tornado potential focused on the “kinked” warm front. This is due to the fact that such an orientation of a warm front leads to a situation where surface winds are locally backed, meaning they’re turning counterclockwise over time. This was also paired with a localized pressure fall of 3 mb over the two hours leading up to 3 PM on Thursday.
Storm Prediction Center mesoanalysis of 2 hour pressure tendencies. The area of Southeastern PA, northeast MD, and northern DE had seen a pressure fall of 3 mb leading up to 3PM Thursday.
As was the case with the Lee County Tornado that claimed 23 lives in Alabama on March 3, 2019, these locally backed winds due to the warm front and pressure falls (leading to some isallobaric winds) served to enhance storm relative helicity and create an environment favorable for storm rotation and the possibility for tornadoes. The backing winds also served to increase wind shear and the potential for severe weather. Luckily, in this case, other environmental factors weren’t supportive for a large, strong tornado.
Last Sunday, while I was preparing my post on the snowstorm that was about to hit NYC and the Northeast, the southern side of this same storm system was starting to produce a serious severe weather event in portions of the Deep South. A large, violent, and ultimately deadly EF4 tornado hit parts of Lee County, AL during the afternoon. The tragic toll of 23 confirmed fatalities due to this tornado was more than double the total deaths due to tornadoes in all of 2018. This was also the deadliest single tornado since the EF5 tornado that hit Moore, OK on May 20, 2013. In this post, I’ll share some thoughts and observations about the meteorology behind this event, and about what made this tornado so powerful.
Storm Prediction Center’s Forecasts
One aspect of the event that impressed me was the prescient, geographically accurate, and timely Mesoscale Discussions and convective outlooks that the Storm Prediction Center issued during the course of the day. The SPC already had a handle on the risk for severe weather in parts of the Deep South as evidenced by the convective outlooks they issued Sunday morning.
Storm Prediction Center’s Day 1 Convective Outlook issued at 7:52AM Sunday Mar 3, 2019. The black dot within the orange “ENH” enhanced risk area shows the approximate location of Lee County, AL
Storm Prediction Center similarly recognized the risk for tornadoes on this day. The black dot indicates the location of Lee County, AL. within the hatched yellow area representing a 10% or greater probability of EF2 – EF5 tornadoes within 25 miles of a point risk area
Regarding the enhanced risk area that the SPC identified as possibly being affected by tornadoes:
The most favorable … space for tornadic potential … still appears to be within the enhanced-risk area, where strong deep shear, large low-level hodographs, and at least low-end surface-based buoyancy will juxtapose. Forecast soundings show rapid prefrontal destabilization …. [a]s that occurs, severe potential will steadily ramp up…. a few tornadoes also are possible. Tornado-event density, and risk of significant tornadoes, still is somewhat unclear — being strongly dependent on existence/number of preceding supercells that can develop…
SPC foresaw that the energy (instability) and spin (shear, imparted by strong winds at different levels of the atmosphere) required for strong tornadoes would have a chance to come together in the enhanced risk area. They also identified that the greatest risk would be with any supercells that could form ahead of the main line of thunderstorms that would accompany the cold front later on.
Weather Prediction Center’s surface analysis valid for 1PM EDT Mar 3, 2019, an hour or so before the EF4 tornado struck Lee County, AL
As it turned out, supercells did form ahead of the cold front – one in particular drew the attention of astute SPC forecasters, and this would end up being the supercell responsible for the tornado that hit Lee County. In follow up Mesoscale Discussions regarding the tornado watches over the enhanced risk area, SPC forecasters were remarkably accurate and timely in identifying the risks associated with this supercell and the favorable conditions it would encounter.
Lee County is circled in green in SPC’s Mesoscale Discussion #0145 surface map.
MCD #0145 was issued at 1PM CDT (local time), and contained the following text. The forecasters cited favorable conditions for a strong tornado to form within 30-60 minutes. Just around 2PM, about 60 minutes after this MCD was issued, the EF4 tornado hit Lee County.
A mature supercell located near Montgomery is favorably located within a region of maximized surface pressure falls (3-4mb per 2 hours) immediately east/southeast of the surface low. KMXX VAD shows 500 m2/s2 0-1km SRH when accounting for the observed Montgomery County supercell’s storm motion. Given the ample buoyancy and intense shear profile in place, it appears tornadogenesis will likely occur within the next 30-60 minutes with the possibility of a strong tornado occurring.
Why Conditions Were So Favorable for a Strong Tornado
The following analysis about the mesoscale conditions that favored strong tornadoes on this day came about from a discussion I had with Steve Corfidi, my instructor for the class I took on mesoscale forecasting (severe weather forecasting) as part of Penn State’s Undergraduate Certificate in Weather Forecasting. Steve Corfidi also used to be the Lead Forecaster at the SPC. Suffice to say, I am quite privileged to have been able to glean some insights about this storm from him. These observations are related to another MCD from SPC that day, MCD #0147.
Storm Prediction Center’s MCD #0147, issued at 2:06PM CDT, right around the time the EF4 Lee County Tornado was on the ground. Lee County is circled in green.
In this MCD, the SPC highlights an area of localized surface pressure falls in dashed blue. Steve Corfidi commented this effect is related to “rise and fall pressure “waves” that move across the earth twice-daily in response to solar heating”. As the earth heats up, air warms and rises, and this generates a thermal low since there’s less air over a warmed up spot of the earth than surrounding areas. In this case, this resulted in a localized area of surface pressure falls over the area circled in dashed blue as the day progressed. In response, surface winds will have a tendency to deflect towards the center of the lowering pressure. You can see this by looking at the wind barbs in the chart above: those that are closer to the cold front are more southwesterly, but the ones closer to the blue dashed area are actually more southerly, since they are deflecting towards the north and the localized pressure falls. This is known as the isallobaric effect. This had direct impacts on the favorability of the environment for tornadoes, as Steve Corfidi helped me understand.
My illustration of the situation over Lee County during this tornado.
As winds near the localized pressure falls became more southerly in response to isallobaric effect, this actually increased the vertical wind shear values in the area of the pressure falls (green here, blue dashed area in the SPC analysis, the red 300 mb wind profile barbs are approximated from this sounding). Since vertical wind shear is measured by looking at both the difference in direction and speed of winds at different levels, a change in wind direction at the surface, all else being equal, will result in higher wind shear. Relative to other areas in the warm sector of this storm, this produced an even higher value of storm relative helicity (SRH, as alluded to in MCD #0145) as well as the aforementioned vertical wind shear. I don’t have space to elaborate on why SRH and vertical wind shear are important for tornadoes, I will say that it has to do with enhancing storm rotation, and tornadoes are intense, vertical circulations of rotating air.
One other observation worth mentioning is that the “geometry” of the warm sector maximized the amount of time the supercell could spend in an extremely favorable environment. If you look at the large blue arrow in my illustrated diagram, check out how the approximate mean storm motion was largely parallel to the orientation of the warm front and axis of the maximized surface pressure falls. That meant that as the tornado formed, it was able to keep moving through a favorable environment for much longer than if the storm motion had been more northeasterly, or say southeasterly.
In lieu of an early week forecast this week, I’m opting to share some observations about weather I experienced in Mexico last week while on vacation. I stayed in Isla Mujeres, a small island located about 13 miles off the coast of Cancun. Temperatures were of course quite warm. It was also unusually windy for this time of year down there, though nothing quite like the windy weather NYC experienced today with some peak winds recorded at over 50 mph nearing 60 mph.
METAR READINGS FROM JFK and LGA showing peak winds of 56 mph and 58 mph respectively recorded at 12:04AM and 8:31AM respectively todaY
I’m only just getting adjusted back to cold temperatures, and am not looking forward to snow possibly falling Wednesday night and another storm bringing wintry precipitation Friday. On the bright side, we are now only about 3 weeks off from the vernal equinox and the start of spring!
On the Servicio Meteorológico Nacional (SMN) – National Meterological Service of Mexico
Since I was going to be in Mexico, I started checking out the Mexican government’s weather service page. Check out the surface analysis below that’s overlaid on what looks like a GOES East image from last Thursday (02/21/2019).
Click to enlarge this surface analysis from the Mexican SMN
Unlike our own National Weather Service, the SMN numbers frontal systems that move through Mexico. Notice the stationary front in the center of the image is labeled as “Frente No. 38” (Front #38) and you can see “Frente Frio No. 40” (Cold Front #40) crossing from southern California into northern Baja California in the upper left corner of the image. They also number their winter storms. The “B” (representing a low pressure center) over Nevada is labeled as “Octava Tormenta Invernal” (Eighth Winter Storm). “Corriente en Chorro Polar” (polar jet stream), “Corriente en Chorro Subtropical” (subtropical jet stream) are familiar features to us, which we seen streaking across the northwest and central portions of Mexico respectively. A “Corriente de Bajo Nivel” (low-level jet) is seen flowing from the east towards the Yucatan. Here’s a translation of the text in the lower left panel:
Systems affecting Mexico The Eighth Winter Storm over the southwestern US combined with cold front #40 in northwestern Mexico will favor showers with some strong storms, very cold temperatures, and wind gusts over 60 km/h in the northwest and northern Republic, and also the potential for snow or sleet in mountainous areas of Baja California, Sonora, and Chihuahua, extending gradually towards Durango. Front #38 with stationary characteristics extends over the western Gulf of Mexico and will generate clouds with isolated rain in the eastern and northeastern parts of the country.
Servicio Meteorológico Nacional of mexico
Synotpic Conditions – the Tropical Big Picture
The consistent breezy south-southeasterly winds I felt on Isla Mujeres were tied to that low-level jet (LLJ) pictured above. This LLJ enhanced the general easterly trade winds in the area. This was a result of the influence of a high anchored over the Western Atlantic, and a low over northern Colombia pictured in the OPC surface analysis below (issued Friday 2/22 02:35Z) “funneling” the winds.
In this analysis of the Western Atlantic from NOAA’s Ocean Prediction Center, you can see that a broad high pressure center was anchored near Bermuda. Meanwhile, a low sat over northern Colombia. The clockwise flow around the high and counterclockwise flow around the low in proximity to each other act to enhance the easterly trade winds found in the tropics.
A
sounding from Philip Goldson International Airport near Belize City
(the closest sounding station I could find to Cancun) showed clear
evidence of a well-mixed layer from the surface to just about 900 mb. It
felt like in Cancun, this mixed layer extended a bit further up into
the 850 mb level where the LLJ sat because the winds were stronger.
Sounding taken above Philip Goldson International Airport near Belize City at 7PM Feb. 20, 2019, showing a well-mixed boundary layer representative of the area around the Yucatan Peninsula during these few days
By
way of brief explanation, well-mixed layers like the one shown above
provide favorable conditions for faster moving winds aloft to transfer
their momemtum downwards, in this case all the way to the surface. It
shouldn’t be a surprise that a deep well-mixed layer also existed today
over NYC – enabling the strong winds aloft to mix down, leading to some
very strong winds and gusts.
Sounding from Upton, NY for at 7AM Feb. 25, 2019, showing a deep well-mixed layer down to the surface from around 800 mb with strong winds 35-50 knots through most of this layer
Aside
from the winds, the weather followed a pretty standard tropical pattern
with clouds building in the afternoon and isolated showers. Despite how
flat the Yucatan Peninusla is, it nevertheless provides at least some
small potential for lift and convergence for air flowing off the
Caribbean Sea. This is because there’s actually a significant difference
in frictional properties of land and water, which makes sense since the
surface of the ocean is considerably “smoother” than the corresponding
forested Yucatan. One other notable trait was that the base of rain
clouds in the area took on a distinctively blue hue, which I imagine was
a reflection of the characteristically blue waters of the Caribbean
Sea.
The latest seasonal forecast from the Climate Prediction Center suggests a 90% chance of an El Niño forming during this winter. Because El Niño (and its opposite, La Niña) occurs when there are sea surface temperature anomalies over large portions of the equatorial Pacific, it can affect sensible weather across the world. However, even if an El Niño does form, and is potentially strong, it doesn’t mean it’s the only determining factor for climate outlooks in our region.
Climate Predicition Center’s latest ENSO Outlook as of December 13, 2018. The CPC’s forecast probability that El Niño will form and persist through April 2019 exceeds 80%.
Definition: What is El Niño?
The term El Niño refers to the large-scale ocean-atmosphere climate phenomenon linked to a periodic warming in sea-surface temperatures across the central and east-central equatorial Pacific (between approximately the date line and 120oW)… [CPC] declares the onset of an El Niño episode when the 3-month average sea-surface temperature departure exceeds 0.5oC in the east-central equatorial Pacific [between 5oN-5oS and 170oW-120oW].
There are links between a pattern of weakening trade winds and the onset of El Niño, though there’s no conclusive understanding of the mechanics that lead to the formation of this effect. Either way, this post will focus more on possible effects of El Niño. The key lies in the geographic extent of El Niño, impacting much of the central and east-central Pacific. Since the oceans play a pivotal role in governing global atmospheric patterns, it’s no surprise that El Niño can have global weather impacts.
As you can see, impacts from a classic El Niño bring warmer than normal weather to the northern part of the western US, and cooler and wetter conditions to the Gulf Coast/Deep South. Though not official yet, it does appear an El Niño was already in progress September-November, and possibly into December. This has already brought copious rains to the Southeastern US.
Green and blue hues indicate areas that received above normal precipitation.
One of the primary ways that El Niño affects global weather is by altering the intensity, orientation, and physical extent of the subtropical jet at the 200 mb level. Over the southeastern US, El Niño promotes a stronger subtropical jet streak – this can lead to the formation of stronger than usual storms over this portion of the country, bringing above normal precipitation patterns we see above.
CPC’s analysis of atmospheric anomalies, in this sequence of images, you can see the elongation of the subtropical jet (area of yellows, oranges, reds) flowing east from Asia. Similarly, a stronger subtropical jet streak is seen over parts of the US.
El Niño doesn’t have particularly strong impacts on our area, and this is borne out by the CPC’s seasonal 3-month outlook for this winter. It appears we may see slight chances for above normal precipitation here, but about equal chances of temperature anomalies.
CPC 3-month temperature outlook
CPC 3-month precipitation outlook
Notice, however, that some of the areas forecast to experience above normal temperatures do map well with a classic El Niño’s impacts (Alaska, parts of the Pacific Northwest, extreme Northern Plains), as do parts of the southern tier (Texas, Gulf Coast).
A major news story unfolded over the weekend as the Southeastern US got slammed with a snowstorm that dropped uncommon snow totals over the area, causing widespread travel disruptions. This region of the country is not accustomed to snowstorms of this scale and many municipalities were not prepared for it. Making matters worse, there was a major forecast bust in this storm, which shared key characteristics with a similar forecast bust that led to a high impact snowstorm hitting NYC a few weeks ago on November 15th (and may have prompted the ouster of the director of NYC Office of Emergency Management). For example, Richmond, VA had a forecast going into Sunday for only 1″ of accumulating snow, but in fact received 11.5″ when all was said and done – a near record-breaking storm.
Below, I’ll provide a “post-mortem” analysis of why forecasters missed the mark so badly in this case. The overall lesson here underscores the difficulty of forecasting snow when temperatures are expected to be hovering close to freezing, especially in coastal storms where the precipitation gradient can be quite sharp.
Dry air at the outset of the storm
Soundings from KWAL (Wallops Island NASA Launch Facility, which we can use as a reasonable proxy for areas in Virginia heavily impacted by snow) at the outset of this storm showed very dry air at the low levels of the atmosphere. This is indicated by the large gap between dew points (green line) and the environmental temperature (red line) on the Skew-T diagram below.
Since I think most people reading this are probably not familiar with Skew-Ts, let me provide a brief exposition. These charts are densely packed with data and can be difficult to read. To orient yourself, know that the y axis on these represents pressure levels from the surface (~1000 mb) all the way up to almost the very limit of the atmosphere at 100 mb. Pressure levels are also related to altitude, though this relationship is not linear because it depends on temperature. The x axis on these charts shows temperature in degrees Celsius. However, note that the lines of temperature are actually slanted at a 45 degree angle and not straight up. The dotted blue line to the right marks the 0 degree mark, critical for determining whether precipitation is frozen or not.
So back to the Skew-T at hand – notice that above the 700 mb layer, the dew point (green) and environmental temperature (red) lines were essentially overlapping. This indicates a layer of air that’s reached saturation since by definition, dew point is the temperature to which the air would need to be cooled to be saturated. When you see a thick layer of dew points and temperatures meeting, it generally indicates ongoing precipitation (thinner layers like this can indicate clouds). In this case, what’s happening is that precipitation is falling from about 400 mb down, but from 700 mb and below, the air is very dry.
With this set up in place, we have excellent conditions for evaporational cooling. As precipitation from above starts to saturate the layers below (some of the precipitation evaporates into the dry layer), the temperature actually cools because evaporation is a phase change of water that requires an input of energy (heat). This is exactly the same mechanism that occurs when you exercise and sweat, or when you step out of a shower (even a cold one) and feel cooler. The net effect of the evaporational cooling in this case, like in the storm that hit NYC in November, was to keep environmental temperatures below freezing for longer than expected (shifting the red environmental temperature line to the left on a Skew-T), allowing snow to fall and accumulate for a longer period as well.
The issue for forecasters here, and for NYC on November 15, was that the models were not all in agreement about how dry the low levels of the atmosphere would be at the outset of the storm. Forecasters are trained not to rely solely on just one model’s depiction of upcoming events, even though in this case, some models had what turned out to be a much more accurate take on dry air. As we’ve seen, the difference of a degree or two when temperatures in the atmosphere are close to the freezing line can have serious consequences for tangible weather impacts.
Frontogenesis and mesoscale (localized) banding
When coastal storms form off the East Coast during the winter, the temperature differential between the warmer air south of the storm’s core and the colder air to the north can lead to frontogenesis, which is the process of the formation of a frontal boundary. In these storms, the result is a coastal front. During this process, a mesoscale circulation forms as atmospheric dynamics attempt to restore equilibrium between cold and warm airmasses. This circulation can greatly enhance lift, a critical ingredient for heavy precipitation, as well as helping cool the air columns. For coastal storms during the winter, the result of strong frontogenesis is the development of narrow, but intense localized bands of heavy precipitation. The difference between an area impacted by a band like this can easily be more than 0.50″ of liquid equivalent, which if you convert to snow using a standard 10:1 snow-to-liquid ratio is 5″! The trouble with these mesoscale features, as is the case with thunderstorms, is that even the most advanced forecast models do not have sufficient resolution to accurately capture features on these scales. That means it’s often difficult to know for certain if/where/when one of these bands sets up and for how long – a critical, high impact detail that can make or break any forecast.
Analysis of frontogensis at 1PM on Sunday – the blue-green hues over Virginia indicate areas of strong frontogenesis
As it happened, with this storm, stronger frontogenesis than forecast took shape. The North American Model (NAM) actually had a pretty good handle on this, but as with the NYC storm, forecasters didn’t put all their eggs in one basket and side with this solution.
NAM’s forecast for frontogenesis valid at 3PM Sunday – the very tightly packed purple lines are an indication of intense frontogenesis
Cold air damming
Along the Eastern Seaboard, certain orientations of high pressure systems can lead to an effect known as cold air damming. This occurs when high pressure centers of Canadian origin set up northeast of the mid-Atlantic and Southeast. Anti-cyclonic clockwise flow around these highs brings cold air around the core of this high into the East Coast with easterly winds. At some point, these winds start to hit the eastern flank of the Appalachian mountains. Because cold air has higher density, the mountains provide an effective barrier to the westward (and upward) progress of this cold air. This then leads the air to gradually turn to the left (south) and progress further south than would otherwise be possible without the cold air damming effect. This is visible from the following surface analysis where you can see surface isobars linked to the high pressure center “sagging” south along the eastern edge of the Appalachians. This phenomenon can provide a critical shot of cold air in advance of a storm that can tip the balance from a rain event to a snow/mixed/frozen event. Forecasters probably did have a decent handle on this, but I mention it because it would have helped in keeping cold air in place prior to and during the beginning of the event.
Given that this scenario has unfolded twice this season, a key takeaway for forecasters should be to have heightened awareness of snowfall totals exceeding model consensus when one or more of those models is indicating the possibility for both strong frontogenesis with a coastal storm like this and very dry air preceding such a storm. Ideally, forecasters and emergency managers should be in close communication about probabilities of exceeding forecast totals as soon as evidence and observations show a colder scenario unfolding. If possible, these details should be passed on to the general public by highlighting the uncertainty that exists and probabilities, even if they’re not high, of exceeding forecast totals dramatically. Municipalities should have a fallback plan for fast mobilization of personnel and equipment for snow removal in the event that a forecast bust of this magnitude starts to look more likely during the early onset of a storm when we can verify things like dew points, and observe trends of mesoscale bands on radar.
What follows is a discussion lab that I wrote for this past week’s WxChallenge forecast competition as part of Penn State University World Campus’ METEO 410 capstone class in weather forecasting. I’m sharing this to give folks a glimpse into the forecasting process we’ve been learning, and because this discussion lab garnered some plaudits from my instructor for providing a really good analysis with attention to detail.
Model Guidance
12Z NAM MOS and 18Z GFS MOS (model output statistics) today agreed on a high temperature of 46°F for Day 7 (06Z Thursday to 06Z Friday). 00Z Wednesday’s NBM run was forecasting 44°F. 18Z EKDMOS shows ~46°F in the 50th percentile, with 50°F in the 90th percentile, and 41°F in the 10th percentile.
EKDMOS (Ensemble Kernel Density MOS), producing a probabilistic forecast of severeal different variables. This shows maximum temperatures. The green bar shows the 10th percentile, red bar the 50th percentile, and blue bar the 90th percentile
Synoptic Set Up
By Thursday, the cold front of the occluded low that will bring precipitation Wednesday is forecast to have pushed through KCAR. During this frontal passage, winds will veer from the SSE towards the west. Winds are not forecast to be particularly strong, however, westerly winds would downslope a bit, enhancing wind speeds as well as warming temperatures a touch.
Weather Prediction Center (WPC) surface forecast for 00Z Friday November 2 (8PM Thursday, November 1, 2018)
Typically, we’d expect temperatures to be cooler behind a cold front due to cold air advection (CAA). Checking dynamical model forecast 2-meter temperatures, there’s not really evidence of large temperature gradients around KCAR. Even though winds will be blowing from areas of cooler temperatures towards warmer temperatures, the lack of a large gradient and low wind speeds do not suggest strong CAA.
It’s worth noting that the cold front appears to have anafrontal characteristics (precipitation behind the front seen in the WPC surface forecast) – this has implications on cloud cover behind the front. Both sets of MOS guidance show overcast conditions throughout the day. Forecast soundings suggest the main effect of the cold front is a drying out of the layer between approximately 900 mb to 600 mb initially, though by 18Z Thursday this layer dry layer tops out 700 mb (NAM has a smaller dry layer, between 900 mb and 750 mb – not pictured). Outside of this dry layer, clouds appear likely both near the surface and also from the top of the dry layer to as high as 200 mb (300 mb in NAM). In fact, it appears that the column above the dry layer will be saturated, and precipitation will be falling at upper levels during periods of the day, which explains why the dry layer shrinks from the top down as moisture works its way down through the column.
GFS forecast sounding
Closing Thoughts
Because of the likelihood of persistent, seemingly thick overcast during peak heating, I’m hesitant to side with the MOS consensus of 46°F, which I think is too warm. Even if precipitation doesn’t reach the ground, evaporational cooling may still be a factor. I think NBM’s 44°F is reasonable given the current data. I wouldn’t go too low into the low-40s because of warming impacts of downsloping westerly winds and the lack of any strong CAA.
Results
Subsequent MOS runs actually trended up, as high as 49°F. However, because of the factors outlined above, I continued to hedge down from MOS guidance, and submitted a finalized forecast of 46°F on the day. The actual high ended up being 45°F. Because I hedged down, I was able to minimize my error points for the day and ended up climbing to the top of the class leaderboard.
As part of the WxChallenge competition and Penn State University World Campus’ METEO 410 capstone course on weather forecasting, we are required to write up climatologies for cities that we will be forecasting for during the competition. I thought I would share the latest one I put together for Philadelphia, PA, which will be our forecast city for the next 2 weeks in the competition.
Climatology for Philadelphia, PA (KPHL)
City Name / Station ID: Philadelphia, PA (Philadelphia International Airport, KPHL)
Time Period: November 6-November 16
Topography and Geography
Local Time Zone: Eastern Standard Time (UTC -5)
Station Elevation: 10 feet above sea level.
Station Location: Philadelphia International Airport (KPHL) lies on the north bank of the Delaware River, 6.75 miles southwest of City Hall in downtown Philadelphia.
Important Topographical Features: Philadelphia is located in the southeasternmost corner of Pennsylvania, along the border with New Jersey to the east defined by the Delaware River. Philadelphia lies along the Fall Line, and there are rolling hills oriented southwest-northeast immediately west and north of the city. These hills have elevations of 200-500 feet. The Appalachian Mountains are further north and west, though many of these can be characterized more as narrow ridges. The elevations of these ridges range from 1000-1500 feet. East of the city are lowlands of the coastal plain in New Jersey. Although KPHL isn’t directly on the coastline, there are significant bodies of water within 55 miles of the site, including Chesapeake Bay to the southwest, Delaware Bay to the south, and the Atlantic Ocean to the southeast and east. Lastly, although not technically a topographical feature, the city of Philadelphia is a sizable urban agglomeration that can have effects on local microclimates via differential heating (urban heat island effect).
Winds
Wind Roses:
Frequency (percentage) of the single most common wind direction: West-northwest, occurring around 11.5% of the time.
Directions that are most and least common: Most common wind directions: southwest (~10.25%), west (~10%), northwest (~9.5%), west-southwest (~8.75%). Least common wind directions: southeast (2.5%), east-southeast (~2.75%), south-southeast (3%).
Direction(s) most likely to produce the fastest winds: west-northwest, and northwest have the highest likelihood of producing winds in excess of 21.5 knots. Due west is not far behind either.
Direction(s) least likely to produce the fastest winds: The least common wind directions (east-southeast, southeast, and south-southeast) also are least likely to produce winds exceeding 16.5 knots. Among these, southeast winds have the lowest frequency of producing winds in excess of 16.5 knots.
Impacts of wind direction on local weather: Winds from the westerly-northerly directions flowing towards KPHL would all experience some degree of downsloping (not particularly strong), as they flow over and down the higher terrain in these regions as discussed in the section on topography. Southwesterly-easterly winds all have the potential to transport moisture into the KPHL area, as they would flow over Chesapeake Bay (southwest), Delaware Bay (south), and the Atlantic Ocean (southeast-east). Southwest winds are quite common – the southerly-easterly winds are significantly less common, but still occur collectively about 17% of the time. The LCD mentions both the Appalachian Mountains and the Atlantic Ocean as moderating influences, as winds from the former warm via downsloping; and winds from the advect cooler marine air in the warm season, and milder air in the cold season.
While northeasterly are generally uncommon, east-northeast winds are somewhat more frequent, occurring about 6.5% of the time. Winds from these directions are noteworthy for a couple impacts. First, when KPHL lies north of a deepening coastal low, these winds can enhance moisture transport from the Atlantic Ocean while also possibly bringing milder air from the ocean when the sea surface temperatures exceed surface temperatures during winter. Second, when a high pressure center approaches KPHL from the west, these winds can bring result in cold air damming as they would eventually pool cooler air at the base of higher terrain west of KPHL before turning south. This scenario would bring about cooler temperatures than otherwise expected. Though less of a concern during the cold season, there could be scenarios in which a strong enough sea breeze could penetrate far enough inland during the warm season to suppress temperatures at KPHL. On the other hand, the urban heat island effect induced by the city of Philadelphia should have year-round impacts in terms of generating an inbound wind from outlying suburbs towards the city center (which KPHL is very close to), while also resulting in warmer temperatures than surrounding areas.
Maximum observed two-minute wind speed for the month (or months) in knots: 40 knots (converted from 46 mph)
