Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* warm Pattern persists through start of may *

11:30 a.m. Tuesday, April 29, 2025:

Following a seasonable but windy end to the weekend, a sprawling high pressure system settled over the Commonwealth on Monday and promoted sunny skies, light winds and pleasantly warm conditions with high temperatures in the mid 70s for most. The area of high pressure has since shifted into the western Atlantic, and south-to-southwesterly winds of 12-25 mph on its backside will usher unseasonably warm air into northern MD, southeastern PA and the Lower Susquehanna Valley this afternoon. Under mostly sunny skies, high temperatures will soar into the low-to-mid 80s.. some 15°F above normal by late-April standards (see below). As discussed last Friday, a cold front will traverse the mid-Atlantic States from northwest-to-southeast spanning this afternoon through Wednesday morning and cut the anomalous warmth short, but high temperatures will still be above normal both Wednesday and Thursday. 

The cold front will trigger widespread, strong-to-severe thunderstorms across the Ohio Valley and western NY/PA this afternoon and evening, but the storms should rapidly weaken and collapse south/east of the Appalachian Front later tonight. The front should sweep through the I-81 and I-95 corridors between ~4-10 AM Wednesday morning, so I only expect a few showers to impact the region tonight. In addition, most of the upper-level energy associated with the front will pass through New England and southeastern Canada, and the atmosphere will be quite stable due to the late-night and early-morning timing of the frontal passage (see below). Thus, areas south/east of I-81 should be spared from any severe weather, and a gusty outflow boundary from the dying storms could be the only real indication of the frontal passage. Any isolated showers should clear the I-95 corridor by 7-8 AM, and rainfall amounts will generally be negligible and under 0.10". Due to extensive cloud cover, higher humidity and a persistent breeze overnight, temperatures will still be in the 60s around daybreak. 

Behind the front, Wednesday will turn out mostly sunny and less humid.. but still rather warm.. with highs in the mid-to-upper 70s. After peaking around 60°F tonight, dewpoints will tumble into the mid-to-upper 30s by tomorrow afternoon. Northerly winds of 8-16 mph will usher the drier air mass into the region and make the air feel even more refreshing. Another high pressure system will settle into the Great Lakes region tomorrow and shift into the western Atlantic by Thursday (see below). On its backside, winds will turn south-to-southeasterly again by Thursday afternoon, and the southerly flow will strengthen from Thursday night into Friday ahead of another cold front. As a result, dewpoints will rise back into the upper 40s to low 50s by Thursday afternoon and peak in the upper 50s to low 60s later Thursday night into Friday. After a mainly clear and comfortable Wednesday night with lows in the upper 40s to low 50s, high temperatures should reach the mid 70s to perhaps 80°F again Thursday afternoon under partly sunny skies.

Scattered showers will then return Thursday night, but any heavier/steadier activity should remain well north/west of the Lower Susquehanna Valley. An area of low pressure will develop over the Midwest later Wednesday night into Thursday and track into southeastern Canada and northern New England by Friday afternoon. A weak, trailing cold front may spark additional showers and a few thunderstorms across northern MD and southeastern PA on Friday, but I don't anticipate any severe weather due to a lack of upper-level energy and the storm system's track (nearly 1,000 miles to our north). Friday's showers and storms will only be of the isolated-to-scattered variety. Ahead of the front, partial sunshine and strong, downsloping flow off the Appalachians should boost high temperatures into the low-to-mid 80s early Friday afternoon. Overall, it won't be a bad end to the week whatsoever. Keep an umbrella handy and be alert for a few brief, heavy downpours, but most of the day should be dry.

On the heels of Friday's cold front, another storm system may form right over the mid-Atlantic States on Saturday. A vigorous, upper-level disturbance is expected to swing through the Ohio Valley from Friday night into Saturday. In stark contrast to the setup both tonight and on Friday, the disturbance will trek through the Commonwealth and be in close proximity to the surface low pressure system. Thus, I'm going to keep a close eye on Saturday's severe weather potential. After a brief lull Friday night, showers and potentially violent thunderstorms could erupt in the I-81 and I-95 corridors Saturday afternoon. Temperatures will only fall into the upper 50s to low 60s Friday night, and they should quickly shoot up into the 70s again later Saturday morning. Throw 60-65°F dewpoints, triggering mechanisms (upper-level disturbance and secondary cold front) and some wind shear into the mix, and the stage could be set for scattered severe thunderstorms. Time will tell..

The forecast from Sunday through the first full week of May is rife with uncertainty, but odds favor another prolonged stretch of mostly sunny, pleasantly warm and generally dry conditions. The Jet Stream will continue to exhibit a pronounced northward bulge over the Central, Northern and Eastern States through the first 10 days of May. The final piece of the Tropospheric Polar Vortex (TPV) has settled over northwestern Canada and will meander across northern portions of the country during the next 2 weeks. As a result, the last vestiges of winter's chill have essentially exited the Lower 48. By contrast, unseasonably warm weather should "run rampant" over the eastern United States through early May with high temperatures in the 60s, 70s and 80s most afternoons going forward. I don't expect any more prolonged or noteworthy "chilly shots" until October or November, and we've very likely dealt with the last frost/freeze concerns until autumn. Overall, the Jet Stream should remain relatively weak and be located over central/southern Canada through at least the middle of May. Thus, our abnormally dry and tranquil pattern will continue with no "big rainmakers" in the foreseeable future. The only "fly in the ointment" could come in the form of a rogue, cutoff low next week, but I'd give that scenario a mere 20% chance of occurring. To sum it up, the weather pattern looks about as warm and quiet as possible for this time of year. That's good news for outdoor enthusiasts but bad news in terms of the drought. Instead of improving, it will only worsen in the days and weeks to come. The old saying "drought begets drought" continues to play out in real time. Check back Friday for an update on the forecast from this weekend through next week. I should be able to finesse temperature, sky cover and other details by then.. -- Elliott