The unsettled weather pattern this week continues into the weekend. Rain is possible tonight into Saturday, then again overnight Saturday into most of the day Sunday. Entering next week, it appears we get a break from rainy weather, but looking ahead, another period of prolonged chances for rain could occur mid-week next week. Temperatures will alternate between below normal and around average, depending on whether there is rain in the forecast.
Rest of today – cloudy, with temperatures in the upper-50s. A warm front lingers to our south slowly moving north. Eventually, a cold front will push through overnight into Saturday. This could bring a brief period of rain, and maybe a thunderstorm. Overnight lows in the mid-50s.
Weather Prediction Center surface forecast for 8AM Saturday
Saturday – rain chances should end by late morning. High temperatures should be able to reach around 70°F despite cloudy skies. Another storm system will move in overnight going into Sunday. Rain chances will ramp up steadily after midnight. Moderate to heavy rain is possible. Temperatures overnight in the mid-50s.
North American Model (NAM) high resolution simulated radar for 11AM Sunday
Sunday – washout of a day as low pressure moves though. Plenty of moisture available for this system and that could lead to some heavier precipitation. Poor conditions for the Five Borough Bike Tour, with highs around 60°F, as a cool onshore easterly wind accompanies this storm. Overnight lows in the low-50s.
Monday – nice rebound day with high temperatures reaching to around 70°F. Finally should see some sun with high pressure briefly building in the wake of Sunday’s storm and before the next frontal boundary to impact the area. Overnight lows in the mid-50s.
A cool start to the week will lead into a period of extended unsettled weather, with multiple disturbances bringing chances for rain throughout the remainder of the week. During this period, temperatures will remain generally below normal under the influence of these cloudy conditions and rain, though not by much.
Rest of today – increasing clouds with high temperatures rebounding into the upper-50s from a cool start in the mid-40s. The first of several quick-hitting storm systems should move in overnight, bringing generally light precipitation leading into early tomorrow morning. Overnight lows are expected to be warmer in the upper-40s with clouds staving off radiational cooling as occurred overnight into today.
Weather Prediction Center surface forecast for 8AM Tuesday
Tuesday – brief period of dry weather Tuesday as a ridge of high pressure builds in. If more sun than clouds materializes during the afternoon, we could see temperatures nearing 70°F, though temperatures could end up in the low-60s if clouds linger all day. Rain chances increase into the overnight hours with overnight lows near 50°F.
Wednesday – mostly cloudy and cooler with a stationary front setting up near or just south of NYC. This will bring easterly onshore flow and keep temperatures in the mid-50s. As another low develops west of us and slides up along this stationary frontal boundary, rain chances will increase through Wednesday. Overnight lows in the low-50s.
Weather Prediction Center surface forecast for 8AM Thursday
Thursday – temperatures expected to rebound into the mid-60s with the stationary front changing into a warm front and slowly pushing north. Clouds and rain yet again. Mild overnight lows in the low-50s.
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.
Two distinct storms will affect weather in the area this weekend going into next week. The first is the remnant of the once mighty “bomb” cyclone that brought an abrupt return of winter conditions to the northern Plains states and Upper Midwest. The second will be a storm developing over Texas that could be a severe weather threat down South. Neither should result in a washout as the bulk of precipitation associated with these storms is timed to fall later in the day or overnight. Temperatures will be largely above normal during this period.
Rest of today – cloudy with chance of drizzle early. High temperatures around 60°F. Overnight lows in the mid-50s. Winds are forecast to turn to the south ahead of an anticipated cold front and in the wake of a warm front passing. Rain should accompany this frontal passage overnight.
Weather Prediction Center surface forecast for 2AM Saturday
Saturday – warm day in store with highs reaching into the low-70s despite mostly cloudy skies as the cold front passing overnight is forecast to stall and become stationary during the day. Overnight lows in the mid-50s again.
Sunday – shower chances increase as the next storm approaches. Temperatures will be cooler with highs in the low-60s due to clouds and rain. A second, more vigorous cold front is forecast to pass overnight into Monday. Rain, heavy at times, possibly with thunderstorms as overnight lows fall into the mid-50s.
GFS model output for 8PM Sunday
Monday – rain associated with this second storm should clear out around lunchtime. Cloudy day forecast now, but could see more breaks in the clouds and warmer temperatures in the upper-60s.
This weekend will see a warm start but then a cool down into the below normal range going into next week. A rumble of thunder and showers will precede this cool down. Weather looks pleasant for St. Patrick’s Day festivities and the NYC Half Marathon this weekend.
Rest of today – mostly cloudy with some breaks of sun after a warm front passes over shortly and we sit in the warm sector of the same storm that tore through the central US earlier this week. As the trailing cold front pulls through this evening, it could provide enough lift for some additional showers and possibly a couple thunderstorms. High temperatures will be above average in the low-60s, mid-60s if some sun breaks through while we’re in the warm sector. Overnight lows behind the cold front should be in the low-40s.
Weather Prediction Center surface forecast for 8PM Friday
Saturday – bright, sunny day with high pressure building behind the cold front. Temperatures cooler, around 50°F. Overnight lows much cooler in the low-30s.
Sunday – cold start to the NYC Half Marathon in the low-30s but high temperatures will end up in the mid-40s with lots of sun. Overnight lows again around the freezing mark
Monday – increasing clouds with a possibility for some precipitation but chances are too low now to call. Highs in the mid-40s and overnight lows back down in the low-30s again.
After a chilly start to March, things will take a turn towards much more seasonable weather this week. Still, temperatures will be slightly below average on several days. Precipitation chances don’t pop up until Friday when an approaching cold front should bring a warm up and then rain.
Rest of today – sunny, with a high near 50°F. Overnight lows around freezing.
Tuesday – a weak cold front passes through in the morning, but there will not be enough moisture for precipitation. High temperatures will be cooler than today, in the mid-40s with mostly sunny skies. Overnight lows will be cooler around 30°F.
Wednesday – sunny skies and a high temperature again in the mid-40s. Clouds will be on the increase overnight as a warm front approaches from the southwest. Overnight lows will be a touch warmer in the upper-30s.
Weather Prediction Center surface forecast for 8PM EDT Wednesday
Thursday – warmer but mostly cloudy with high temperatures in the mid-50s. Overnight lows expected to be in the mid-40s with the chance of rain increasing.
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.
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.
