After a rainy start to the week, high pressure takes over during the mid-week period. Another storm system moves in Thursday. Behind this next bout of rain, a dry and mostly sunny weekend should await us.
Rest of today – skies clearing as a cold front pushing. Winds picking up with some subsidence (sinking air, since air mass behind the cold front is cooler and drier). High temperatures reaching into the upper-70s with this subsidence and some downsloping from the northwest. Overnight lows in the mid-60s.
Weather Prediction Center surface forecast for 8AM Wednesday
Wednesday – high pressure remains firmly rooted over the area. High temperatures in the mid-70s with mostly sunny skies. Overnight lows in the low-60s.
Thursday – a developing low closer to the coast and another one further inland over the Great Lakes will bring showers and thunderstorms to the area. First, like Monday, a warm front will lift through earlier in the day, then a cold front is expected to follow later on. High temperatures in the low-70s due to rain and cloud cover. Overnight lows in the upper-50s.
NAM model for 1000-500 mb thickness and surface reflectivity at 8AM Thursday, showing a distinct area of rain associated to a low emerging off the coast of the Delmarva Peninsula
Friday – with a cool start, temperatures top out in the mid-70s. Mostly sunny in the wake of the cold front Thursday. Overnight lows in the low-60s.
Temperatures cool off after a warm holiday weekend. A frontal boundary will linger in the area this week allowing multiple disturbances to move along it. This will result in multiple chances for rain but with generally mild temperatures except today. Looking into the weekend, it does appear that high pressure takes over and gives us a nice Saturday at the least.
Rest of today – mostly cloudy, temperatures cool, in the upper-60s. Showers early should give way to a period of dry weather, however, stronger showers and thunderstorms could hit later this evening around 8PM ahead of an approaching low sliding along a warm front just to our south. We are just outside Storm Prediction Center’s slight risk area for severe thunderstorms, and these storms should weaken as they near the more stable marine layer, in particular if clouds hold and no sun peaks through. Overnight lows staying steady in the upper-60s.
Weather Prediction Center surface forecast for 8PM Tuesday
Wednesday – a cold front is forecast to push through and allow for some drying out on Wednesday with partly sunny skies. High temperatures should rise into the upper-70s. Dry weather doesn’t last because yet another warm front pushes through later in the evening with more showers and thunderstorms possible. Overnight lows in the low-60s.
Thursday – another impulse of low pressure and warm front moves through the area bringing continued chances for rain. High temperatures in the upper-70s under mostly cloudy skies. Overnight lows again in the low-60s and with more chances for rain.
GFS model output for surface pressure, precipitation, 500 mb thickness at 6AM Friday
Friday – cold front sweeps through and allows for sunny skies to return, this should be a really nice day with high temperatures in the upper-70s. Overnight lows about 60°F.
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.
A wet start to the weekend this Friday with multiple rounds of rain moving through. Saturday shapes up to be a better day at least in terms of being dry. A second disturbance/low moves through Sunday bringing yet more chances for rain before skies clear up to start next week. Temperatures behind this second storm/cold front will be below normal for this time of year.
Rest of today – periods of rain during the afternoon as a warm front works its way across the area. Some thunderstorms possible. Second round of more focused/heavier rain later in the evening around 7-8PM as a trailing cold front pushes through. This front will have decent upper level support with some indications of a vorticity maxima at 500 mb and us being in the exit region of a 300 mb jet streak. High temperatures today topping out around the upper-50s even with the rain as we’re expected some time in the warm sector of the low bringing us this stormy weather. If the warm front below fails to push north through here though, we may only see temps in the mid-50s. Overnight lows should bottom out around 50°F.
Weather Prediction Center’s most recent surface analysis, showing a marked warm front south of NYC. Some thunderstorms and showers have popped up ahead of this frontal boundary due to isentropic lift (overrunning) as warm air from the south slides over colder air at the surface further nortHigh resolution rapid refresh simulated radar for 8PM tonight showing a line of showers with some thunderstorms possible.
Saturday – with the trailing cold front clearing the area tonight, precipitation should end and Saturday should end up being a decent day though with more clouds than sun. High temperatures should be able to get into the low-60s, edging into the mid-60s if more sun breaks out. Overnight lows noticeably cooler with a colder, drier airmass behind today’s cold front, in the mid-40s.
Sunday – a second low approaches from the Ohio Valley, again bringing some potential for rain. This rain is expected to be on the lighter side. Temperatures should reach into the upper-50s. Overnight lows in the low-40s.
Weather Prediction Center’s surface forecast for 8AM Sunday
Monnday – start of next week should be sunny, with Sunday’s storm system moving offshore. High temperatures will be below normal though, with the cold front associated with Sunday’s low bringing a reinforcing shot of cooler air. High temperatures probably topping out in the upper-50s. Overnight lows in the mid-40s.
The same storm system that impacted the Deep South this weekend with severe weather, including several tornadoes that caused fatalities, swept through the area overnight. Some severe wind gusts were reported, and area transit networks experienced some delays due to storm-related damage. As this storm departs east, it will continue deepening, resulting in strong winds the remainder of today. Another storm system is forecast to move in to the region Thursday and going into the weekend. Temperatures during the week are expected to be mostly around normal for this time of year.
Rest of today – windy, with decreasing cloud cover. Steady west winds between 20-30 mph later in the day. This will help bring colder air into the area, with highs already peaking this morning in the low-60s then dropping back to the mid-50s by this afternoon. Overnight lows in the low-40s with partly cloudy skies.
Tuesday – sunny day, with winds calming and seasonable high temperatures in the low-60s. Overnight lows in the mid-40s and mostly cloudy.
Weather Prediction Center surface forecast for 8AM Wednesday
Wednesday – increasing clouds as a backdoor cold front works its way through. High temperatures in the low-60s. Overnight, mostly cloudy with lows in the mid-40s.
Thursday – chance of rain as a low tracks slowly from the Plains to the Great Lakes. East of this low, a slow moving warm front could touch off showers during the day and overnight Thursday. High temperatures around 60°F, overnight lows quite mild due to the influence of the approaching warm front and warm advection, in the low-50s.
A cool start to the weekend should give way to a nice couple of days before another storm moves in to start next week. During this period, high temperatures will start well below normal, then rise above average before finally settling around average. Looking ahead, another storm system appears to impact the area late next week, bringing a colder weekend next week in its wake.
Rest of today – cool, cloudy with high temperatures only in the mid-40s. Increasing rain chances towards the evening and overnight hours. Rain should taper off overnight then we should see overnight lows stabilize in the low-40s as warm advection takes hold.
Saturday – clouds diminishing as the low responsible for the rain moves over the Atlantic to our south. High temperatures in the mid-60s, maybe a touch warmer if we see clouds break earlier than forecast. Overnight lows milder in the upper-40s.
Weather Prediction surface forecast for 8AM Sunday
Sunday – high pressure stays in control and gives us a pleasant, mild day with high temperatures in the low-60s. The high pressure and nice weather doesn’t stick around long though, with another storm system approaching during the overnight hours going into Monday. Overnight lows should hold around 50ºF as rain starts to fall.
Monday – despite clouds and rain sticking around, temperatures will surge to about 70°F with a warm front forecast to pass through.
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.
As we spring forward this Sunday, our weather pattern finally takes a step forwards towards spring as well. Rain will accompany this warmup, then we start next week with temperatures close to average for this time of year.
Rest of today – Increasing clouds as an area of low pressure moves offshore well to our south. High temperatures in the upper-30s. A 500 mb (upper level) shortwave trough passing through overnight could provide enough lift for some scattered flurries. Lows dipping to around freezing.
Saturday – High temperatures in the mid-40s with lots of sun as high pressure temporarily exerts its influence. Overnight lows in the mid-30s. Rain starts overnight going into Sunday as a storm system draws near.
Weather Prediction surface forecast for 8AM Sunday
Sunday – rain during the morning with improving conditions later. Overcast to start with cloud cover decreasing as the storm departs to the east. High temperatures around 50°F. Overnight lows in around 40°F
Monday – sunny and seasonable with highs in the upper-40s. Overnight lows cooler than Sunday in the mid-30s.
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.
What a difference a week makes. Last week, we saw some of the coldest air to hit NYC in quite a while. This week, we’ll have the chance to hit 60°F. Two low pressure systems will impact the area, the first is not forecast to produce precipitation, while the second will likely bring rain. Both of these lows will bring a surge of warm air in their warm sectors prior to the passage of the trailing cold front accompanying them. Enjoy the balmy February weather while you can, because the warmth won’t last.
Rest of today – mostly sunny with high temperatures reaching into the low-50s. Patchy fog developing overnight with low temperatures near 40°F.
Tuesday (Lunar New Year)– a mild start to the day gives us a shot to reach into the upper-50s. We’ll be in the warm sector of the first low. If a deck of low clouds builds in and persists, we might be a bit cooler in the mid-50s. On the other hand, if we get a relatively sunny day instead we could see high temperatures around 60°F. Overnight lows considerably cooler around 30°F with a dry cold frontal passage from this first low pressure system.
Weather Prediction Center’s surface forecast for 7PM Tuesday, showing the “dry” passage of a cold front
Wednesday – a cooler start to the day should result in temperatures only reaching the low-40s. Clouds increasing, and rain expected to develop overnight into Thursday with low temperatures in the mid-30s.
Thursday – a few degrees warmer with high temperatures in the mid-40s and rain. Rain expected to taper off a bit after the warm front associated with the second low to hit us moves north of the area. This will set us up for a warm day on Friday as well. Overnight lows going into Friday not much cooler than day time highs, in the low-40s.
