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.
Last Thursday, we saw a round of severe thunderstorms develop and roll through the NYC area in the afternoon hours. My instructor (Steve Corfidi) and TA (Phil Lutzak) from my Penn State World Campus Weather Forecasting Certificate program noticed an noteworthy feature in satellite images of the event.
GOES archived visible imagery satellite loop from 1:16 PM through 5:11 PM Thursday, Aug 21, 2019.
You can see that there’s an arcing, convex, wave-like feature oriented southwest-northeast that sweeps east across the Northeast in the visible satellite loop above. If you’re having trouble spotting it check out the series of annotated images below that marks the leading edge of this feature in different points along its progression.
Professor Corfidi noted that this feature seemed to line up well with an area of drier air at the mid-upper levels of the atmosphere, which he picked up in the infrared Channel 8 satellite images. For reference, I’ve superimposed the IR channel on the visible satellite channel from the same time, which is also the 3rd frame of the gallery above.
GOES East IR Channel 8 image at 3:36PM Thursday, Aug 21, 2019
GOES visible satellite image from the same time, annotated to indicate the position of the leading edge of the wave-like feature
GOES IR Channel 8 superimposed on the visible satellite image taken at the same time (3:36PM Thursday, Aug 21, 2019)
What’s more, referring back to the visible satellite loop above, it’s evident that this feature was also partially responsible for firing up strong to severe thunderstorms along the NJ/PA border that eventually tracked east over the NYC area. Storm reports from the day indicate that several of these storms produced damaging wind gusts.
Archived radar image from 3:35PM, Thursday, Aug 21, 2019. Note the positioning of the strong radar echoes and the leading edge of the wave-like feature at this same time.
It’s evident there’s some causative relationship between this wave-like feature and the eruption of afternoon thunderstorms along its leading edge, and this all raises the question: what was this phenomenon? I did some investigation of various upper air analyses from the Storm Prediction Center and found that this phenomenon correlated well with two features at the upper levels of the atmosphere.
Storm Prediction Center analysis of 300 mb divergence valid for 4PM Thursday, Aug 21, 2019
Storm Prediction Center analysis of 400-250 mb potential vorticity valid for 2PM Thursday, Aug 21, 2019
First, we can see that there’s a swath of increased divergence noted at 300 mb (areas outlined in pink) that correlates somewhat with this area of drier mid-upper level air. The second image is perhaps even more convincingly linked to this phenomenon – showing an area of increased potential vorticity. But what does potential vorticity indicate about the atmosphere? In this case, potential vorticity indicates a lowering of the local tropopause – the boundary between the troposphere, where all our weather takes place, and the stratosphere above it. The stratosphere, relative to the troposphere is much drier, and this explains the source of the clear drier region picked up in the GOES Channel 8 infrared images.
Colorado State University depiction of the relationship between the stratosphere and troposphere when there’s an increase in potential vorticity
In fact, there’s a known relationship between potential vorticity and water vapor satellite imagery:
There is a clear relation between PV (potential vorticity) and water vapour imagery. A low tropopause can be identified in the WV imagery as a dark zone. As a first approximation, the tropopause can be regarded as a layer with high relative humidity, whereas the stratosphere is very dry, with low values of relative humidity. The measured radiation temperature will increase if the tropopause lowers. This is because of the fact that the radiation, which is measured by the satellite, comes as a first approximation from the top of the moist troposphere. High radiation temperatures will result in dark areas in the WV imagery.
Potential vorticity in this case was an indicator of increased divergence at upper levels, and this helps explain why severe thunderstorms initiated on the afternoon of Aug 21, 2019, despite the lack of a strong surface boundary providing convergence. This is because divergence and vorticity aloft helps induce convergence at the surface (and may have helped generate a prefrontal trough that day). Divergence aloft is essentially removing air from the top of the column, and since the atmospheric system always attempts to maintain a balance in terms of conservation of mass, momentum, etc, this air leaving the top of the column gets replaced by air flowing in at the surface. This is inflow of air results in convergence, and enhanced lift, as this air rises to replace the air that continues to be evacuated aloft. A source of lift is always a critical ingredient to any severe thunderstorm!
Labor Day weekend is shaping up to be a good one overall in terms of sensible weather. A dry cold front passage today sets the stage for high pressure to take control over the remainder of the weekend. The only wrinkle is a warm front forecast to pass Monday that could bring some rain. Looking ahead into next week, a warm up should precede another cold front mid-week. The big weather story this weekend will be much further south of us, as Hurricane Dorian takes aim at Florida as a dangerous Category 4 storm.
Rest of today – sunny with high temperatures in the mid-upper 80s. Overnight lows in the upper-60s.
Saturday – high pressure starts to build from the west, which will result in a cooler weather with northerly winds. High temperatures topping out in the low-80s with lots of sun and overnight lows in the mid-60s.
Sunday – high pressure will progress east, giving us another pleasant day with lots of sun and comfortable temperatures in the low-80s. Overnight lows warm up in the upper-60s with a warm front approaching from the southwest.
Monday (Labor Day) – chance for rain during the day as a warm front, then a weakening cold front approach. High temperatures in the low-80s with mostly cloudy skies. Overnight lows around 70ºF.
Hurricane Dorian Heading Towards Florida
Over the course of the last few days, Hurricane Dorian made a pass over the Northern Lesser Antilles, hitting the Virgin Islands, and Martinique, but sparing Puerto Rico. This was a blessing for an island still recovering from the devastation of Hurricane Maria. However, because Dorian avoided the disruptive influence of high terrain in Puerto Rico, it entered the open waters of the Western Atlantic north of the island ready for continued intensification. Unfortunately, various forecast models have been consistently depicting Dorian making landfall somewhere in Florida early next week as a dangerous Category 4 hurricane packing max sustained winds of 140 mph.
Latest GOES visible satellite imagery of Hurricane Dorian
Overnight going into today, Dorian’s been steadily strengthening and is now showing a markedly more symmetrical structure than before, along with an eye developing in visible satellite imagery. This suggests that the southerly shear and dry air that was afflicting the storm yesterday has abated somewhat. At this point, there is nothing really stopping Dorian from continuing to strengthen to a major hurricane.
National Hurricane Center official forecast for Dorian as of 11AM Friday
Forecasters continue to have high confidence that Dorian will make a turn towards a more westerly direction as high pressure builds over Bermuda and keeps building to the west. This will block Dorian from taking a northerly route away from Florida. Worryingly, forecast models have trended towards a slower and more southerly track overnight, then a turn north after landfall. On this current track, a large part of Florida would be affected by the storm for a considerable amount of time, increasing the risk of damaging storm surge and inland flooding from prolonged heavy rain.
A long duration Nor’easter slowly moves its way eastwards. Conditions improve during the second half of the weekend. A brief warmup to above average temperatures takes place. This doesn’t last, with another cold front passing through to start next week. Tomorrow, keep an eye out on the skies for a possible glimpse at the aurora borealis, typically not visible at these latitudes.
Rest of today – chance for rain shower diminishing as the center of a slow moving Nor’easter continues to progress northeast. High temperatures reaching around 50ºF. Winds will increase on the backside of this departing low overnight. Winds will shift towards the northwest as well, bringing in colder air and overnight lows in the mid-30s.
GOES East satellite loop of the storm system affecting the NortheastOcean Prediction Center surface analysis for the Atlantic Basin. A strong blocking high is inhibiting the forward progress of the Nor’easter impacting our area. This is leading to a long duration coastal flooding and wind event.
Saturday – decreasing clouds as the low above continues progressing northeast slowly. Winds will increase as this low continues to deepen and the pressure gradient builds between it and and area of high pressure further west. Steady winds in the 20-25 mph range with gusts as high as 40 mph are forecast. High temperatures will be a bit below normal as a result in the upper-40s. Overnight lows will be cold, in the mid-30s with clear skies.
Weather Prediction Center surface forecast for 8AM Sunday, still showing a tight pressure gradient between a high over the Ohio Valley and the departing Nor’easter
Sunday – temperatures rebound into the mid-50s as an area of high pressure builds and slides off to our southeast, and winds turn towards the southwest bringing in warmer air. Sunny skies – this will be the best day of the weekend.
Monday – another cold front approaches from the west and brings a possibility for rain. Temperatures ahead of this front should reach into the low-50s. Behind this, we’ll get another shot of cold air, with temperatures dropping overnight into the low-30s.
Northern Lights Possibly Visible from NYC
On Tuesday, The Space Weather Prediction Center (it exists!) detected a large Coronal Mass Ejection (CME) associated with a potent solar flare. As a result, they have issued a moderate geomagnetic storm watch for Saturday, March 23. Of note, the belt where the charged particles from this CME would impact the Earth’s upper atmosphere and result in an aurora quite a bit further south than where the northern lights are typically visible. In fact, they may even be visible from NYC, although it may be hard to see them due to light pollution.
A winter storm warning is in effect for NYC and the surrounding metropolitan region. This isn’t exactly how we’d all want to start March off! This storm is anticipated to bring some travel impacts to the area, however, for reasons discussed below, this won’t be a blockbuster snowmaker. Watch out for a slog of a morning commute tomorrow. This snow may also stick for a while – a frigid continental polar air mass from Canada will sweep in behind this storm bringing temperatures generally 10-15°F below normal for this time of year. High temperatures in the mid-30s should limit melting.
Headlines
Snowfall totals: I’m forecasting 4-6″ in parts of eastern Queens, southeastern Brooklyn, and lower totals further east into Long Island. Higher totals of 6-8″ are more likely to occur in Manhattan, the Bronx, and points further inland, particularly interior regions of Connecticut. Below are probabilistic forecast maps of various amounts of snow (>= 2″, >= 6″, and >= 8″).
Timing: Precipitation starting in earnest around 8PM. Starting out as a mix of rain/snow near the coast, but transitioning over to all snow later in the evening. The heaviest snow will happen overnight. Because of the fast-moving nature of this storm, precipitation is expected to end rather quickly between 4-6AM Monday morning in the city.
Uncertainties: There is still potential for a wobble in the storm track, further east and south would result in higher snow totals near the coast. Further west and closer to the coast would mean more mixing/rain at the coast and lower snow totals. There will be a rather sharp gradient of increasing snowfall totals spreading across the region (as seen in the previous probabilistic snowfall total forecasts). Mesoscale heavy snow bands will be difficult to pinpoint ahead of time. Some areas could see several inches more than neighboring areas just a few miles south and east.
Synoptic Set Up (The Big Picture)
A storm that’s currently unleashing severe storms with tornadoes across the Deep South now will slide up along the Mid-Atlantic and Northeastern coast of the US. As this storm progresses, it will move close to the 40°N/70°W benchmark, a spot that’s climatologically correlated to heavy snow events along the heavily populated I-95 corridor during the winter. This storm will continue to strengthen as it moves offshore. Snow is expected to develop ahead of the advancing warm front associated with this storm as its precipitation shield advances. Heavier snow is forecast to develop later on as strong isentropic lift associated with the warm front occurs, creating the potential for frontogenesis and some mesoscale bands of very heavy snow. The storm is expected to move quickly along the Northeast coast, such that the duration of precipitation in any one spot is expected to be less than 12 hours.
At the 500 mb level, a shortwave trough will provide positive vorticity and some additional lift/divergence, allowing the storm to continue strengthening. Finally, at the 300 mb level, the surface low will be close to the entrance region of a curved 300 mb jet streak. This will provide yet more divergence and lift, if only for a brief period.
Weather Prediction Center’s surface forecast for 1AM Monday morning
GFS model 500 mb vorticity and height for 1AM Monday
GFS 300 mb height, wind forecast for 1AM Monday, note the brighter red, orange, and white streak over the Northeast, indicating a jet streak.
Evolution of the Storm Track
Over the course of the last three days, forecast models have come into better agreement with this storm tracking close to the 40°N/70°W benchmark (circled in red in the images below). Note the increasingly tight clustering of storm center locations around the benchmark in progressive storm track forecasts from the Weather Prediction Center.
WPC storm track forecast issued 7AM Saturday, the brown spots indicate forecast storm center locations around the benchmark
WPC storm track forecast issued 7AM Sunday, the green spots indicate forecast storm center locations around the benchmark
The tightening clustering of these forecast storm center locations lends greater confidence to the idea that the storm will track very close to the benchmark.
Ensemble Snowfall Totals
The two primary model ensembles (GEFS and SREF) have been edging ever so slightly upwards in their forecast mean snowfall totals, while the model spread has decreased over the weekend
Latest GEFS ensemble plumes for snowfall totals at KLGA
Latest SREF ensemble plumes for snowfall totals at KLGA
These means/spreads were part of what informed my own forecast snowfall totals at the top of this post.
Factors Supporting Heavy Snow
Storm track over or very near the benchmark
Strong isentropic lift and possible frontogenesis (see images below). Strong lift is a critical ingredient for generating heavy precipitation
Possibility of mesoscale bands as a result of this lift, generating heavy snowfall rates
Temperatures probably supporting frozen precipitation through the atmosphere
GFS forecast for 850 mb frontogenesis
NAM 12km forecast for 850 mb frontogenesis and temperature advection (red shades indicate warm advection)
NAM forecast sounding for 1AM Monday
GFS sounding for 1AM Monday
Factors Suggesting Lower Snow Totals
Possibility still remains for storm track to shift further inland, introducing more warm air off the ocean, more rain than snow at the coast
Warm advection associated with the storm’s warm front possibly also affecting snow development. Note how close the overlapping temperature and dew point profiles in the forecast soundings above are to the freezing mark, the dashed blue line the middle of the image that is angled to the right at 45°. Evaporational cooling should help somewhat in staving off warming but if temperatures warm more than forecast, we could see more mixing
Mesoscale bands of heavy snow may not push far enough onshore
Surface temperatures ahead of the storm in the upper-30s near the coast, urban heat island effect could retard snow accumulation
Fast moving nature of the storm, total precipitation window only 12 hours
Small window for the best moisture support at the 850 mb level. No real evidence to suggest a low-level jet carrying a ton of moisture into the region.
NAM model forecast of 850 mb relative humidity and winds. There’s not too big of an area of completely saturated air at this level, and winds are not strong at this level either.
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.
Last Wednesday, a strong Arctic front swept across much of the Northeastern US, impacting many areas with a line of heavy snow showers, then ushering in record-breaking cold. The line of heavy snow immediately preceding the frontal boundary set off Snow Squall Warnings, which many readers would have seen on their mobile devices. The Snow Squall Warning is a new type of warning that went live nationwide on November 1, 2018. I believe that last Wednesday’s event was the first time National Weather Service forecast offices issued this new warning type for a widespread frontal snow squall. In this post, I’ll share my thoughts on the new warning type, and some observations about the event itself.
The New Snow Squall Warning
The relatively new snow squall warning product is, like other existing warnings, an effort by the National Weather Service to inform the public about imminent hazardous weather impacts. The main motivation behind this new warning type is to try and reduce the number of potentially fatal multi-vehicle accidents that can occur in snow squalls. Snow squalls can cause these kinds of accidents because the intense snow and wind in them can rapidly reduce visibility to near whiteout conditions with little advance warning. The heavy snow can also result in quick accumulations that make driving even more dangerous. Snow squalls can occur along frontal boundaries, like what we saw last week, but they can also be isolated or form in conjunction with lake effect snow. Although I haven’t as yet seen a clear-cut definition of what triggers this new warning, the criteria I have seen tie in directly with the hazards mentioned above: visibility less than 1/4 mile (whiteout conditions), strong wind gusts (above 35 mph, it appears), heavy snow, and surface temperatures below freezing.
Archived text of the snow squall warning issued for NYC by the local National Weather Service. Credit: Iowa State University‘s archive tool for NWS warnings.
Snow squall warnings are functionally similar to severe thunderstorm warnings, which makes sense because snow squalls and severe thunderstorms share some sensible weather impacts and meteorological properties. In this case, with a frontal snow squall, my professor and seasoned forecaster Steve Corfidi observed that “Essentially, a winter cold-frontal snow squall band is simply a summer cold frontal squall line with its bottom two-thirds or so chopped off. For all practical purposes today you simply experienced the passage of a narrow, fast-moving band of convective cirrus!”
Snow Squall Event in NYC
The snow squall that hit NYC last week was associated with a strong Arctic front. As I’ll discuss below, this frontal boundary provided the necessary lift to generate a narrow band (along the east-west dimension) of heavy snow along much of its length. Light snow began falling around 3:30PM by my estimate. The intensity of the snow picked up moderately over the next 15 minutes. However, it wasn’t until close to the end of the event that snowfall rates truly kicked into high gear, along with the winds. During a span of about 5-10 minutes, as the snow and wind rapidly picked up, visibility dramatically decreased, with scenes like the one below typical.
Storm Prediction Center’s Mesoscale Discussion graphic showing the general set up of the snow squall event
Image taken looking north along Sixth Avenue from the 12th floor of my office building at approximately 3:50PM. The building that is barely visible as a dark grey blob in the middle is about 400 feet (2 blocks north), showing you just how bad visibility was at the peak of this snow squall.
Once the worst of the snow squall cleared, conditions rapidly improved, with visibility recovering quickly and precipitation ending rather abruptly. Following the passage of the Arctic front, forced subsidence with the much colder and denser air behind the front sinking to the surface helped mix down some very strong wind gusts, and helped usher in some of the coldest air of the season.
Why the Squall Seemed to Peak at the End
The snow squall started off as a few flurries, and for most of the duration of the event, it seemed like that was all we’d get. Then, within a very brief span, the squall peaked in intensity, and as quickly as it had peaked, it was over. So, why did this event appear to unfold this way to us as observers on the ground? It all has to do with the profile of the winds above surface relative to the Arctic front and the squall line.
A sounding collected at KPIT (Pittsburgh) earlier during the day, close to the time that the same snow squall line was progressing through Pennsylvania. This sounding is thus a reasonable approximation for what the atmosphere looked like over NYC later in the afternoon.
In the sounding above, we can observe that the wind barbs on the right side of the sounding are generally increasing in speed up to 600 mb – triangles represent 50 knots, each full tick represents 10 knots, and half a tick is 5 knots. At the surface, winds were west-northwest at 10 knots, but at 600 mb, winds were at 90 knots! Quite a difference. The second thing to note is above 850 mb, the wind barbs are oriented roughly at the same angle, indicating winds from the same direction at these levels (west-southwest) . This is what forecasters refer to as a “unidrectional” wind profile. The result here is that we had a set up where there was significant vertical speed shear. This has tangible effects on the structure of the clouds/convective activity within the snow squall, as shown below.
Simplified illustration showing the scenario during the snow squall
In the diagram above, as the leading edge of the Arctic front progresses, the air ahead of it is mechanically lifted above the dome of cold air behind the frontal boundary. Once the air reaches the LCL (lifting condensation level), it’s saturated and clouds begin to form. In this case, there’s enough lift and available moisture that precipitation begins to fall. Temperatures at the time supported all snow. Lift provided by the front would continue allowing the clouds to grow until they hit a stable layer – I won’t go into specifics about this but suffice to say that at this point, the cloud can’t keep growing vertically. This results in the cloud spreading out horizontally, creating an “anvil”. Because the wind speed is so much faster at this level, the anvil is sheared away from the direction of oncoming wind producing a “leaning” effect.
This radar image from the Newark TDWR (Terminal Doppler Weather Radar) at 2:58PM last Wednesday has a good depiction of the light snow falling ahead of the main squall line that’s coming from the sheared anvils of the main convective line. Note the scattered lobe of light snow ahead of the solid band of darker blue hues indicating heavy snow.
This is a diagram showing the effect of the speed shear on the evolution of the snow squall line’s cloud structure.
Because the winds were coming from the west-southwest, the anvils leaned in the opposite direction to the east-northeast. For us on the ground, that meant the light snow preceded the heart of the action, “the worst of the storm” that was closer to the leading edge of the Arctic front itself. Had winds aloft been weaker, or from a different direction, suppose more parallel to the frontal boundary itself, the contrast between the light and intense snow wouldn’t have been as dramatic.
Thoughts on Improving the Snow Squall Warning
This was the first widespread use of this new warning product, and it’s not surprising that this led to some confusion. I had several people ask me when the warning was issued and the snow started “How long is this going to last?”. Some people even did the exact opposite of what the warning is intended to prevent: they rushed out to “beat the snow” since it started off light and they didn’t realize it would be over in a short span of time.
I think that these warnings could be improved if a specific duration of the event were mentioned in the warning text, something along the lines of “Expect snow squall conditions to last between 30-45 minutes”. Some other weather forecast offices issue warnings with such text. As discussed above, frontal snow squalls are similar in nature to their warm season relatives. While people are used warm season convective activity ending pretty quickly, intense snow squalls here are often caused by the mesoscale bands accompanying Nor’easters. These can last several hours. In general, many winter weather warnings are long duration, which I believe contributed to some of the confusion that people had about this new type of warning.
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).
The last weekend of the year gets off to a mild, rainy start as a storm system moves through. Things cool off and dry out the remainder of the weekend but it looks like another rain event and warm spell hits for New Year’s Eve.
Rest of today – rain through most of the day. Gusty winds around 30 mph possible. High temperatures well above normal in the mid-50s due to a surge of warm advection accompanying the warm front initially responsible for widespread rain. At the 850 mb level, a low-level jet develops, helping enhance moisture convergence and allow for heavier rains. A flood watch is actually in effect for areas of New Jersey in the metro NYC region. Lows going into Saturday will be quite warm in the mid-40s.
Storm Prediction Center mesoanalysis of 850 mb moisture transport. The maroon vectors indicate the magnitude and direction of moisture transport. The long vectors over the NYC region show efficient moisture transport due fast moving winds of a low-level jet at the 850 mb level. The shaded contours show values of theta-e (equivalent potential temperature), while the details of this are quite complex, the easiest way to understand theta-e is that high values indicate very moist airmasses that can be prime for development of convective activity, as an example. In this case, we can tell that the potential for heavy rain exists within areas of high theta-e (dark-green to red shaded areas).
Saturday – a strong cold front will move through during the afternoon which should result in high temperatures peaking early in the day around 50°F, even though it will remain mostly sunny all day. Behind this cold front, Canadian high pressure builds yielding much colder overnight lows into Sunday in the low-30s.
Sunday – much colder day on tap with Canadian high pressure in control. High temperatures around normal for this time of year near 40ºF even with plenty of sun. Overnight lows into Monday are forecast in the mid-30s.
Monday (New Year’s Eve) – clockwise return flow from the southwest on the western side of the exiting high pressure warms temperatures up into the upper-40s ahead of the next storm. The timing of this storm looks like it will result in a rainy New Year’s Eve in the city. This may sound less than ideal, but recall that we rang 2018 in with near record cold. It’ll be downright balmy in comparison this time with overnight lows forecast to barely budge in the mid-40s.
On Friday, I forecast that Sunday and Monday would be mostly sunny and that we wouldn’t see any precipitation on either day. Updated forecast data shows that there is potential for some light mixed precipitation overnight Sunday into Monday. Below, I’ll provide a detailed forecast update for Sunday
My Forecast High: 42ºF | Low: 33ºF | Max sustained winds: 25 mph | Total precipitation: 0.02″ – verification for temperatures and precipitation will come from METAR data for the period between 1AM Saturday and 1AM Sunday (06Z Sunday to 06Z Monday). Wind speed verification will draw on the daily climate summary from the National Weather Service.
Verification
High: 44ºF | Low: 36ºF | Max sustained winds: 24 mph | Total precipitation: 0.00″ – this was a decent forecast in terms of wind speed, and total precipitation. Precipitation didn’t start falling until late in the forecast period, so it ended up being a good call to hedge down below some of the more aggressive model output that suggested earlier/heavier precipitation. Layer mean wind analysis aided my forecast, though again, noting a possible small surface inversion in the NAM forecast sounding led me to make a wise decision to hedge down on max sustained winds. On temperatures, the overnight lows going into Sunday ended up being on the warmer side of the forecast envelope. Here, I mixing and downsloping played a bigger role than I anticipated.
Synoptic Set Up (The Big Picture)
An occluded low pressure center tracking over the Great Lakes will bring a weak cold front through the area Sunday night into Monday. Above the surface, a limiting factor will be the lack of deep moisture or strong winds at the 850 mb level. At the 500 mb level, a negatively tilted shortwave trough will supply good positive vorticity and divergence. However, there isn’t a clear signal for favorable entrance/exit regions of a jet streak at the 300 mb level to provide additional lift.
High Temperature
MOS forecasts show high temperatures Sunday in the mid-40s, with NAM (North American Model) coming in at 45ºF and GFS (Global Forecast System) pointing to 46ºF. NBM (National Blend of Models) – a consensus based forecast model, showed cooler highs at 42ºF. I tend to side with high temperatures on the cooler side due to persistent low overcast clouds during much of the day. Low overcast clouds are a known bias for statistical forecast models like NAM and GFS because they can effectively block out solar radiation, keeping things cooler than otherwise expected.
NAM forecast sounding valid 1PM Sunday, showing multiple layers in the atmosphere where the environmental temperature profile (red) and dew point (green) approach each other, which would suggest multiple layers of clouds hindering solar heating.
Low Temperature
The same MOS data above has lows on Saturday of 33ºF for NAM, 34ºF for GFS and 32ºF for NBM. I’ve gone with the middle of the road, in line with NAM at 33ºF because despite relatively clear skies overnight, strong winds are forecast to be blowing from the west to west-northwest, resulting in some downsloping and staving off any radiational cooling.
Max Sustained Winds
Overnight, forecast soundings indicate the presence of a well-mixed near surface layer. A well-mixed layer allows for faster winds aloft to transfer down to the surface relatively efficiently, and as such, the layer mean wind method can be applied to forecast maximum sustained winds in these cases. We can calculate layer mean by averaging wind speeds at every pressure level within the well-mixed layer shown in forecast soundings that are available in forecast model output. Performing a layer mean wind analysis for NAM and GFS produces an average of about 28 mph. However, noticing that there is a possibility for a small surface based inversion in the NAM forecast sounding, I’ve cut this down to 25 mph. In the chart below, the layer mean wind ends up being 26.71 knots (31 mph).
Pressure (mb)
Speed in m/s
Speed in knots
997
8.20
15.94
975
12.90
25.08
950
14.60
28.38
925
15.90
30.91
900
17.10
33.24
GFS forecast sounding valid for 1AM Sunday, the black circled section shows a well mixed layer where the environmental temperature profile (red line) largely parallels the nearest dry adiabatic lapse rate line (light blue). Note, at the top of this circled area, the environmental temperature profile almost touches the dew point temperature, indicating a layer of thin clouds.
Total Precipitation
An approaching cold front will provide convergence and lift at the surface. However, at the 850 mb level, conditions look pretty dry through much of the day Sunday. 500 mb level divergence and lift looks decent, but the 300 mb level support doesn’t look great. Precipitation, if it occurs, won’t happen until the overnight hours going into Monday, and if it does happen, signs point to no more than a light event. I’m forecasting 0.02″, or just a trace of precipitation by 1AM Monday. Temperature profiles aloft support snow, though it appears that near the surface, temperatures will be above freezing, so it’s more likely that we’ll see a mix of rain and snow.
GFS forecast for 500 mb vorticity and height valid 10PM Sunday. The light blue line shows the axis of a negatively tilted shortwave trough west of the NYC area. This supports divergence downstream (east) of the axis which induces surface pressure drops and growing storms.GFS 850 mb forecast for relative humidity and wind. The NYC area at this time is far from being saturated at this level.By 1AM Sunday, the atmosphere starts to become saturated to basically the surface, indicating ongoing precipitation. Most of the precipitation looks to fall in a zone of the atmosphere that’s below freezing, but temperatures at the surface are above freezing.
