This weekend is shaping up to be similar to last weekend in some respects, with Saturday being a nearly ideal day with plenty of sun, while Sunday is cloudier with some chance for rain. The big difference is unlike last weekend, we’re not anticipating a coastal storm or record-breaking cold! In fact, we should see our first 80°F+ day in some time by Monday before a chance for showers and thunderstorms cools things off.
Rest of today – current visible satellite observations show clouds moving in from the west, so the sunny start to the day will give way to mostly cloudy conditions later. Temperatures will be mild with the influence of warm southwesterly winds, topping out in the mid-70s. Later in the evening, a cold front will push through, bringing a chance for scattered showers and possibly a thunderstorm or two. Overnight lows expected in the mid-50s.
Weather Prediction Center surface forecast for 2PM today
Saturday – even though we’ll have a cold front push through, sinking air behind the cold front and downsloping winds from higher terrain in the north should keep things around average for this time of year in the low-70s with mostly sunny skies. Overnight lows continue to be in the mid-50s.
NAM output for 500 mb height and vorticity, valid for 8AM Saturday. A noticeable reversal of the upper air pattern from earlier this week is visible with a deep longwave trough over the Western US, and ridging over the Eastern US. This will result in great weather for us tomorrow, while areas of the Plains downstream of the trough picture above see the potential for severe weather.
Sunday – warm front approaching from the south during the day Sunday will result in more clouds than sun, and perhaps a scattered rain shower. High temperatures still around average for this time of year in the low-70s. Overnight lows warm up into the low-60s as we enter the warm sector of the storm system approaching us from the west that’s expected to bring severe weather to parts of the Central US this weekend.
Monday – with the region forecast to be in the warm sector of the parent low shown below, and a prolonged period of southwesterly surface winds bringing warm air into the area, temperatures are expected to warm up into the low-80s. This day will have a summery feel, with some humidity to the air and a chance for showers and thunderstorms later in the day as a cold front approaches from the west. Overnight lows should remain mild around 60°F.
Weather Prediction Center surface forecast for 8AM Monday
After a very cool, wet start to the week, temperatures should moderate and conditions will gradually improve as we approach the weekend. A pattern change at mid and upper levels is apparent in forecast models going into this weekend. We finally break free of persistent troughing that brought the cold, rainy weather this weekend going into the beginning of this week. Instead, we’ll get to look forward to sunnier, warmer weather more in line with the calendar.
Rest of today – if we don’t reach 55°F, today will be the second day in a row we set a new record low and record low maximum at KLGA. High temperatures are forecast to be in the low-50s with lots of clouds. Needless to say, it’s been quite cold and below average for this time of year. The noreaster that brought us miserable conditions yesterday is steadily moving east. However, a surface trough and moisture is forecast to rotate around its back side as it keeps retreating east. This could result in scattered showers later in the afternoon and evening. Overnight lows are expected to be quite cool again, in the mid-40s.
Weather Prediction Center surface forecast for 2PM today
Wednesday – temperatures should rebound nicely out of the gate as we finally get some clearing skies. High temperatures should reach into the mid-60s. Chance for showers overnight going into Thursday as a cold front approaches from the north. Overnight lows much warmer in the mid-50s.
Thursday – the warm up continues despite cloud cover associated with this approaching cold front. High temperatures expected to touch 70°F for the first time in a while. This passing (and slowing) cold front could continue to touch off a couple showers going into Friday. Overnight lows slightly cooler in the low-50s.
Weather Prediction Center surface forecast for 8AM Thursday
Friday – cooler with high temperatures in the upper-60s and partly sunny skies. Overnight lows into Saturday in the mid-50s. Pattern change is coming with large scale troughing forecast for the western US while a ridge builds east, essentially the opposite of the pattern we’ve had the past few days.
Mother’s Day weekend here in NYC unfortunately looks to be quite rainy. We start off with the potential for rain later today. Saturday is shaping up to be the sole dry day with high pressure briefly building before giving way to a prolonged period of wet weather Sunday and Monday. So, if you’re making any outdoor plans for mom, consider bumping them up to Saturday!
Rest of today – cloudy, with temperatures around 70°F. We will sit in the warm sector of an elongated low centered over western Quebec as a warm front has already passed overhead. This will allow plenty of mild, moist air to stream northward with southerly winds ahead of the trailing a cold front associated with this low. This cold front will serve as the trigger for some showers and thunderstorms later this evening as it swings through. Overnight lows behind this cold front in the mid-50s.
Weather Prediction Center surface forecast for 8PM Friday
Saturday – high pressure briefly builds in during the day. This will give us the best weather of the weekend with mostly sunny conditions and seasonable high temperatures in the upper-60s. Ideal weather for any outdoor activities. This nice weather doesn’t last with clouds building into the overnight hours and low temperatures dipping to the low-50s
Sunday – another storm system approaches from the south. A warm front at the leading edge of this storm seems likely to stall out. This will bring a prolonged period of chances for showers, cloudy skies, and cooler temperatures with easterly onshore winds. High temperatures in the mid-50s. Overnight lows quite cool in the upper-40s as rain chances continue.
GFS model output for 2PM Sunday
Monday – a coastal low forms along the frontal boundary mentioned above. At this time, it appears this coastal low tracks close enough to make Monday another rainy day with cool temperatures, highs in the mid-50s, 10-15°F below normal for this time of year. Overnight lows looking even colder than Sunday if prolonged rain and a northeasterly wind flow materializes with this coastal low, current forecasts pointing to the mid-40s.
We have a beautiful, sunny start to the week for a change. This will be short-lived though, as a cold front will arrive late Tuesday and bring the first chance for rain this week. More rain arrives later this week with a storm system moving in. Looking ahead to the weekend, we should see a decent Mother’s Day weekend, but chances for rain do show up on Sunday.
Rest of today – mostly sunny, high temperatures in the upper-60s. High pressure in control today will give us a pleasant, spring day, right about average for this time of year. Overnight lows in the low-50s.
Weather Prediction Center surface forecast for 2PM today
Tuesday – most of the day should be dry with high temperatures in the low-70s. Partly cloudy but increasing clouds late as a cold front moves into the area. This frontal boundary will be the focus for any rain that develops in the evening and overnight hours. Overnight lows in the low-50s.
Weather Prediction Center surface forecast for 8PM Tuesday
Wednesday – should have another decent day after the rain moves through overnight. High temperatures in the mid-60s with partly sunny skies. Overnight lows going into Thursday in the low-50s with increasing clouds.
Thursday – mostly cloudy, chance for showers as a warm/stationary front approaches from the south. High temperatures cooler, around 60°F with overnight lows in the mid-50s.
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.
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.
