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.
For reference, here’s the post that triggered the following forecast post-mortem analysis. To start, here’s my forecast and the verified totals.
My Forecast High: 70°F | Low: 58°F | Max sustained winds: 30 mph | Total precipitation: 0.10″
Verification High: 76°F | Low: 56°F | Max sustained winds: 24 mph | Total precipitation: 0.16″
Since I did decently at forecasting the low temperature and total precipitation, this analysis will focus primarily on why I missed the mark on the high temperature and maximum sustained winds.
How I Verify Forecasts For an explanation of how I verify my own forecasts, see the methodology outlined in this prior post-mortem post
Post-Mortem Analysis On this forecast, I ended up handling total precipitation and low temperatures but missed the mark on the high temperature badly and to some extent the max sustained winds. Let’s look at what happened here.
High Temperature When I was putting together my forecast on the high temperature, the MOS output for both primary models (GFS and NAM) suggested the day would be largely overcast. Forecast soundings also suggested a deck of low clouds would be present for nearly the entirety of the day before the onset of an initial round of precipitation in the late evening/early overnight hours. Indeed, morning soundings at 8AM seemed to bear out what forecast models were calling for.
Sounding at 8AM from Brookhaven National Laboratory in Long Island, the site of the local National Weather Service forecast office for NYC. Note that the lower atmosphere, below 850 mb, is largely saturated, indicating low clouds and fog
Above the low level clouds, though, there was a noticeable dry layer up to about 500 mb, where high clouds were present. For temperatures, the biggest impact that’s hardest to pick up in advance is a low, thin, but optically thick layer of overcast clouds. Such a layer would suppress temperatures, whereas the lack of such a layer would allow for higher temperatures.
GOES East 1km resolution visible satellite image at 2:56 PM Friday, April 19. Note the relatively small pocket of clear skies right over NYC, western Long Island and Southwestern Connecticut.
In this event, where we were already in the warm sector of a low, any breaks in the clouds allowed for a dramatically warmer high temperature. As it turns out, there was a period of a couple hours in the afternoon during peak heating hours for this time of year that the skies were basically clear over NYC. In the spring, as the angle of the sun’s rays becomes ever more direct, even a small span of time like this with clear skies can result in significantly warmer temperatures. The small geographic scale of this pocket of clear skies, which was largely surrounded by cloudy conditions, shows you how difficult it is to predict something on a small scale like this, even just a day before the event itself!
Max Sustained Winds On max sustained winds, I was off by 6 mph. That’s not too bad in the grand scheme of things. However, looking back at it, I should have realized that my own forecast calling for a small potential for precipitation would likewise mean a lower possibility for strong winds mixing down from 850 mb to the surface.
The slow-moving storm system that brought us heavy rain overnight last Friday into Saturday continues to linger off the coast of the Northeast, finally exiting by tomorrow. Rain chances will exist today, but give way to better conditions tomorrow. More rain is possible overnight into Wednesday, before drier weather works its way in on Thursday.
Rest of today – cloudy, with an increasing chance for showers in the afternoon and evening hours. High temperatures in the mid-60s. Overnight lows in the mid-50s.
High resolution rapid refresh model simulated radar for 4:45 PM today
Tuesday – with this low finally exiting, we should see a sunny day with temperatures warming into the low-70s. The sunny weather doesn’t last, as clouds increase overnight and rain is possible. Lows in the mid-50s.
Wednesday – conditions should improve after the morning hours, when the cold front that produces the rain chances overnight into Wednesday moves offshore. High temperatures should rebound nicely into the upper-60s. Overnight lows will be cooler in the low-50s behind this cold front.
Weather Prediction Center surface forecast for 8AM Wednesday
Thursday – high pressure should take over and result in a sunny day with high temperatures in the low-60s. Overnight lows going into Friday in the low-50s.
A slow-moving storm system will impact the area with potentially heavy rain that could trigger flash flooding overnight into the first part of Saturday. Precipitation chances die down significantly with the passage of this round of rain as a dry slot works into the area. However, lingering chances for showers still exist as this storm makes a slow exit to the east. Temperatures during this period are forecast to be at or above normal.
Rest of today – cloudy, but still warm with temperatures rising to about 70°F. We will be in the warm sector with the influence of southerly winds during the day. Rain chances increasing during the evening hours. Steady rain is forecast late into the overnight hours with temperatures. Some of the rain could be heavy. Thunderstorms are also possible, and these would increase the risk of flash flooding. Overnight lows around 60°F.
Weather Prediction Center surface forecast for 8AM Saturday
Saturday – the north-south orientation of the cold front that will provide lift for these storms overnight will align well with the prevailing southerly flow through most of the atmosphere. The slow eastward progression of this front will allow storm cells to train leading to more flooding potential as rain continues Saturday morning. The cold front associated with this slow-moving low should push through by the afternoon. A dry slot should cut off any precipitation for a while during this time frame. High temperatures should be in the low-60s. Overnight lows should fall into the low-50s behind the cold front.
GFS model output of 850 mb winds and relative humidity for 2PM Saturday, notice the pronounced area of browns, indicating very dry air. This is what is referred to as a “dry slot“, which can often follow on the heels of cold fronts associated with mature mid-latitude cyclones.
Sunday – the upper low associated with this storm system is expected to cutoff (become detached from primary westerly steering currents), resulting in a system that will meander and take a long while to exit the region. As such, Sunday is expected to remain cloudy, with a chance for some showers. High temperatures should be similar to Saturday in the low-60s. Overnight lows will again be around the low-50s.
Monday – temperatures warming up into the upper-60s, but yes, that cutoff low still lingers! Chance for showers continues in the morning. Clouds finally decreasing later and overnight lows in the mid-50s.
It’s been four months (!!) since I’ve done one of these detailed forecasts. I’m a bit out of practice but trying to get back into the groove. Lucky for me, I picked a day to forecast that has a decent bust potential for precipitation. I’ll discuss that below.
Friday is going to start off decently enough. It should end up being the warmest day of the week. However, it’s also going to be the start of a protracted period of unsettled weather that will affect the weekend. A flash flood watch is in effect late overnight Friday going into Saturday for this reason.
My Forecast High: 70°F | Low: 58°F | Max sustained winds: 30 mph | Total precipitation: 0.10″ – verification for temperatures and precipitation will come from METAR data for the period between 2AM Friday and 2AM Saturday (06Z Friday to 06Z Saturday) at LGA. Wind speed verification will draw on the daily climate summary from the National Weather Service.
Verification High: 76°F | Low: 56°F | Max sustained winds: 24 mph | Total precipitation: 0.16″ – I did decently forecasting the low, and the total precipitation. Hedging way down on precipitation totals towards the NAM solution on a slower moving storm was a good idea. On the low temperature, I think a 2°F margin is acceptable here. Being off by 6°F on the high temperature here constitutes a forecast bust that I’ll put a post-mortem together for – as it turns out the bust happened here and not with precipitation. Winds were also not as strong as anticipated. I should have hedged a bit down too with less intense precipitation anticipated.
Weather Prediction Center surface forecast for 8PM Friday
Synoptic Set Up We begin the period in the warm sector of a low with a center forecast to sit over southwestern Quebec. A secondary surface low is forecast to form over the OH valley and strengthens as it briefly moves northeast. This low is then forecast to stall out and retrograde northwest as it matures and occludes. This will lead to a protracted period of precipitation, with the primary focus being along a cold front moving through Sat morning. This may result in the best chance for heavy rain being outside the forecast period.
At the 850 mb level, a strong low-level jet will provide efficient moisture transport. At 500 mb we’ll be downstream of a longwave trough, with an embedded closed low that becomes cutoff (this mid-level cutoff low will be tied to why the secondary surface low retrogrades). The best vorticity and divergence (lift) associated with this upper low seems to move through after forecast period according to NAM forecast model. GFS has a bit earlier timing, and hence brings precipitation in earlier Friday. Finally, at the 300 mb, there’s not much evidence of enhanced divergence and lift at this level since, we’ll be east of any favorable entrance/exit regions of a 300 mb jet streak.
NAM forecast model’s depiction of the 500 mb layer at 11PM Friday. The cut off low referenced above is circled in green. The shades of yellow-purple indicate increasingly strong vorticity, which is linked to divergence and enhanced lift.
High Temperatures GFS and NAM MOS are in pretty close agreement here. It should be warmer than average, since we’ll be in the breezy warm sector of a low as mentioned above. Limitations on warmth will be evaporational cooling due to precipitation, but this doesn’t appear to be a factor until late in the forecast period, past normal times for peak temperatures. The other factor is thick cloud cover. However, temperatures will largely be driven by warm advection tomorrow and not necessarily solar heating. EKDMOS suggests a temperature over 70ºF. In deference to this ensemble model, I’ve sided with the warmer GFS and EKDMOS (as well as National Blend of Models) and call for a high temperature of 70ºF.
GFS MOS output
NAM MOS output
Low Temperatures NAM and GFS MOS are again in close agreement. With warm advection happening during the course of the overnight hours (strong winds at large angles to isotherms, southerly flow), have no reason to disagree that temperatures could actually go up early then back down a touch before rising again with daybreak, and I’m going with an overnight “low” of 58ºF that is well above average for this time of year.
NAM showing temperature contours and wind direction/speed for 2AM Friday. Note the direction of the wind barbs (pointing from south) is crossing at large angles of relatively closely packed isotherms. This is a set up favorable for warm advection (transport of warm air).
Max Sustained Winds NAM is lower on winds than GFS, but the ensemble EKDMOS skews towards faster GFS solution. This makes sense with heavy rain possible and downward momentum transfer. Winds are forecast to back from south-southwest to south-southeast during the period. These directions don’t typically produce the strongest winds climatologically. However, a strong 45-60 knot low-level jet aloft mixing down with precipitation could easily top 20 knots, especially if elevated convection and thunderstorms materialize. I’m going with the EKDMOS 50th percentile range and going with 25 knots.
The NAM forecast sounding at 5PM tomorrow shows considerably drier low-mid levels.
In contrast, the GFS forecast sounding shows a deep saturated layer indicative of ongoing precipitation by 5PM tomorrow.
Total Precipitation There’s a large spread between NAM and GFS on forecast totals (more than 0.50″) because the GFS is coming in with precipitation starting much earlier in the forecast period than the NAM. The local forecast office seems to buy more into the NAM solution due to its consistency with slower onset of heaviest precipitation. There is some indication as well of MUCAPE present in SREF plumes, could result in some elevated convection that would boost precipitation totals. Any faster timing and/or burst of elevated convection could cause heavier rain to fall earlier, resulting in a forecast bust.
Counting against that, I don’t see any strong signal for frontogenesis at the 850 mb level enhancing lift. SREF doesn’t show too much in the way of strong lift either. Still, the fact that SREF ensemble shows much heavier precipitation totals are not far off to the west during this forecast period makes me a bit nervous.
Lastly, moisture isn’t a total lock, and there’s some evidence of drier air offshore and pockets of dry air moving through from the west at times. NAM is notably drier than GFS at both outset of the forecast and throughout the forecast period, as seen in the comparison forecast soundings above. Training of storm cells possible with steering currents set up parallel to the forecast orientation of the cold front.
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.
