This week starts off right around average for this time of year, with pleasant, sunny conditions. High pressure remains in control for a good part of the week and temperatures start creeping up as the high pressure continues moving east and offshore. Clockwise flow around this exiting high pressure from the southwest and west will help warm temperatures up before the next chance for rain comes in Wednesday and Thursday.
Rest of today – clear skies with high temperatures in the low-70s. Overnight lows in the mid-50s.
Weather Prediction Center surface forecast for 8AM Tuesday
Tuesday – high pressure still in control, temperatures again in the low-70s with plenty of sun. Overnight lows warming up into the low-60s.
Wednesday – the high pressure center will be southeast of us and a warm front will push through. This should allow temperatures to reach into the low-80s with partly sunny skies. We could see an afternoon shower or thunderstorm as a result of increased instability. Overnight, a prefrontal trough and cold front is forecast to push through and lead to more chances for showers and thunderstorms with low temperatures in the mid-60s.
Pivotal Weather rendering of GFS model output for 1000-500 mb thickness and precipitation, 2AM Thursday
Thursday – partly sunny skies again with continuing chances for shower and thunderstorms later in the day. High temperatures in the low-80s. Overnight lows in the mid-60s.
We’ll get a much needed respite from deluging rains at least for part of the weekend as high pressure initially takes hold of the weather. This should give us two days to start with slightly above average warmth and mostly sunny skies. By Sunday, the next storm system to affect the region will be closing in and we can expect more rain and thunderstorm chances. Timing of this system now appears to clear the area by Monday and result in cooler but sunny conditions starting next week.
Rest of today – early fog has burned off and we’re in line to have a nice day with sunny skies and high temperatures topping out in the low-80s except at the immediate coast where a sea breeze is expected to develop with onshore flow forecast later in the day. Overnight lows in the low-60s.
Weather Prediction Center surface forecast for 8AM Saturday
Saturday – another great day with temperatures in the low-80s (except again at the coasts with sea breezes) and mostly sunny skies. Clouds on the increase late and overnight with lows again in the low-60s.
NAM (North American Model) output for 500 mb vorticity and heights on Sunday 8PM showing a pronounced upper low over James Bay in Canada with a trough extending into the Ohio Valley. This will provide upper level divergence and instability to spur storms on Sunday.
Sunday – a warm front is forecast to push through overnight into Sunday and set the stage for rain and thunderstorms later in the day as a cold front approaches from the west. Temps probably topping out a touch cooler in the upper-70s with more clouds hanging around. Overnight lows around 60°F.
Monday – at this time, expecting the aforementioned cold front to sweep through the region before the day Monday. That should result in cooler highs in the low-70s with mostly sunny skies. Overnight lows in the upper-50s.
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.
I’m at my college for reunion weekend, and offered to do a detailed forecast for one of the days I’m here. Saturday is shaping up to be a bit of a mixed bag. Temperatures should be summer-like in the mid to upper-80s, but the downside is a risk for possibly strong to severe thunderstorms especially in the early evening hours. These thunderstorm chances are far from being certain though, and there’s a chance we may miss out entirely on any precipitation. In other words, it’s another good day for a precipitation forecast bust!
My Forecast High: 87°F | Low: 62°F | Max sustained winds: 21 mph | Total precipitation: 0.28″ – verification will come from METAR data for the period between 2AM Saturday and 2AM Sunday (06Z Saturday to 06Z Sunday) at LPR (Lorain County Regional Airport).
Verification High: 90°F | Low: 61°F | Max sustained winds: 25 mph | Total precipitation: 0.00″ – on high temperatures, hedging up towards the higher end of guidance turned out to be a good idea. It’s yet more evidence that MOS guidance tends to be too cool for breezy warm sectors. If anything, I could have been a touch more aggressive on the high here. Did well on the low temperature, and from what I could see, did decently on max winds too. Like I’d mentioned in the forecast, the bust potential for precipitation on this day was significant. That ended up being the case – KLPR and Oberlin missed out on the heaviest rain and more serious convective activity, leading to barely a trace of measurable precipitation. In retrospect, I probably could have gone even drier with my forecast, but I would still have not felt comfortable going with zeros.
Weather Prediction Center surface forecast for 2PM Saturday
Synoptic Set Up A stationary front eventually lifts through Ohio overnight as a warm front. We’ll spend the day in the warm sector of a surface low centered over central Ontario. At the 850 mb level, winds will not be strong enough to qualify as a low-level jet and relative humidity values likewise do not appear particularly high. At 500 mb, a robust shortwave trough is forecast to push through later in the day, bringing some decent vorticity and upper-level lift. Further up at the 300 mb level, there’s not much evidence of enhanced divergence and lift since we’ll be south of a 300 mb jet streak.
GFS model output for 500 mb vorticity and winds valid at 8PM Saturday. Note the strong cyclonic vorticity values over Northeastern Ohio at this time (bright oranges). This will provide some synoptic scale lift for initiating storms via upper-level divergence.
High Temperatures GFS and NAM MOS and NBM all show Saturday to be a warm day. GFS is the warmest with 87ºF while NBM is coolest with 83ºF. EKDMOS (ensemble MOS) 50th percentile matches comes close to GFS MOS at 86ºF. Because forecast soundings show we’ll be in a well-mixed warm sector with decent though not ideal conditions for warm advection, I am tending towards thinking that a high of 87ºF is in fact possible. We could be even warmer if enough sun breaks out between possible earlier showers and rain later in the day.
GFS 2-meter temperature contours and winds at 2PM Saturday showing decent, though not ideal conditions for warm advection (southwesterly winds of 15 knots at a somewhat sharp angle across closely packed temperature contours). If surface winds were forecast to be stronger, we’d be looking at better chances for robust warm advection and hence warmer temperatures.
Low Temperatures NAM and GFS MOS are again in close agreement with 61ºF and 63ºF lows respectively. NBM is a touch cooler at 59ºF. Although both NAM and GFS show evidence of a nocturnal inversion forming, and calm winds at the surface, both models also show extensive cloudiness or even precipitation ongoing overnight. Because the boundary layer starts out with plenty of moisture, there shouldn’t be much in the way of evaporational cooling. Thus, clouds will actually serve to stave off any radiational cooling, which leads me to believe overnight lows will be on the warmer side of guidance. I’ll go with 62ºF here.
NAM MOS guidance for KLPR
GFS MOS guidance for KLPR
Max Sustained Winds NAM, GFS MOS have maximum synoptic winds averaging about 16 knots. I see no clear reasons to go much higher than 18 knots for a max sustained wind tomorrow, unless we happen have a thunderstorm roll through with sustained winds that are much higher. EKDMOS’ 50th percentile lies right around this figure. Even with well-mixed boundary layer forecast to form tomorrow afternoon, winds above the surface up to 850 mb do not appear to be that strong, only around 20-25 knots. Layer mean wind analysis of the NAM sounding only showed winds of about 16 knots.
GFS forecast sounding for 5PM Saturday
NAM forecast sounding for 5PM Saturday
Total Precipitation There’s a large spread between NAM and GFS on forecast totals tomorrow. This will be the trickiest part of the forecast because there’s clear signs that convective precipitation, with a couple rounds of possibly strong to severe thunderstorms could materialize. Any strong thunderstorm passing overhead could quickly dump a few tenths of an inch of rain. However, if we miss out on any significant thunderstorm action, we could see just a trace of rain instead. It’s still too early to tell the finer details of where storms will initiate at this point. Based on forecast soundings above, the best chance for thunderstorms to roll through would be during the late afternoon and early evening, maybe 5-8PM when moisture is best and instability is maximized from daytime heating. Strong, mostly unidirectional shear profiles with westerly winds increasing from 10 knots at the surface to 50 knots at 500 mb suggest the potential for damaging winds as the biggest severe weather threat.
NAM forecast for 850 mb winds and relative humidity at 2PM Saturday
GFS forecast for 850 mb winds and relative humidity at 2PM Saturday
Going against the potential for significant precipitation, neither GFS nor NAM really show strong signals of low-level jet support. Winds at the 850 mb level don’t look to be particularly strong, and moisture support isn’t looking great either. What’s more, while there’s mention in the local forecast office discussion of a right entrance region of an upper-level jet that would provide some dynamic lift, I’m not seeing that myself. They did also mention that a trough would pass through and provide a focus for some lift. This would be a necessary trigger since there’s no clear frontal boundaries that would provide the convergence and lift necessary to generate showers and thunderstorms. Given the hit or miss nature of thunderstorms, less than ideal conditions for heavy precipitation, and disagreement between GFS and NAM MOS at this time, I find it prudent to hedge down with a forecast for 0.28″ of total precipitation despite MOS guidance averages at 0.48″.
The long holiday weekend appears to be mainly dry, and even features a day that could make for a good beach getaway. We do have a chance for rain overnight Saturday into Sunday but this shouldn’t be a damper on anyone’s plans. Sunday’s shaping up to be the warmest day of the weekend although Memorial Day itself may end up being the nicest all around day.
Rest of today – breezy and sunny with high temperatures in the mid-70s. The strong pressure gradient between a departing surface low and a high pressure center moving east from the Great Lakes is what’s causing these breezy conditions. Winds should die down overnight as the high moves overhead and the pressure gradient relaxes. Overnight lows in the upper-50s.
Weather Prediction Center surface forecast for 8PM Friday
Saturday – as the high pressure continues moving east, clouds will increase due to the influence of an advancing warm front. This frontal boundary may serve as the focus for some showers and thunderstorms overnight Saturday into Sunday. High temperatures Saturday expected to be in the low-70s due to increasing cloud cover. Overnight lows in low-60s.
GFS model output for surface pressure, precipitation, 500 mb thickness valid 4PM Sunday
Sunday – we’ll be in the warm sector this day. Clouds and rain are expected to subside early and allow for temperatures to warm into the mid-80s and possibly higher. This will be the best day of the weekend to hit the beach though it’s not a lock for sunny weather all day. Overnight lows in the mid-60s.
Monday (Memorial Day) – anticipating this will be the best day of the weekend with sunny skies and mild temperatures in the upper-70s. Overnight lows in the low-60s.
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
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.
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.
