Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Bouts of Showers and Thunderstorms to Impact Memorial Day Weekend *

12:30 p.m. Tuesday, May 21, 2024:

Following yet another cool, damp, and showery Saturday, the second half of the weekend turned out to be much warmer, brighter, and quite pleasant. After a cloudy start, partial sunshine made a welcome comeback Sunday afternoon, and high temperatures topped out in the upper 70s for most. Monday's weather was essentially a repeat of Sunday's with the exception of slightly warmer conditions (highs in the low-to-mid 80s). An upper-level ridge, or northward bulge in the Jet Stream, developed over the eastern U.S. early this week in response to a deep trough over the Intermountain West. The Jet Stream ridge will remain in place through Wednesday before slowing breaking down through Memorial Day Weekend. A series of subtle disturbances and relatively weak cold fronts will be responsible for the ridge's gradual breakdown. Given the track and timing challenges associated with each system, the forecast over the holiday weekend is anything but straightforward. Quite frankly, your best bet will be to keep an umbrella handy at all times and be prepared to move outdoor activities inside for a few hours. I don't foresee any "washouts" between Friday-Monday, but there will certainly be multiple bouts of showers and storms throughout the weekend. I'll do my best to parse which time periods may be most active, but understand that exact details can and will change over the next 2-3 days. I'll be on vacation and away from Twitter/email beginning late Thursday afternoon through Monday morning but will post a video update of the forecast on Thursday morning. In the meantime, here's my "best shot" at the forecast through Memorial Day..

On the backside of a large high pressure system over the western Atlantic, south-to-southwesterly winds will usher increasingly hot and humid air into the Lower Susquehanna Valley over the next 36 hours. Under mostly sunny skies, high temperatures will reach the mid-to-upper 80s in most spots this afternoon with dewpoints in the low 60s (see below). It won't be oppressively hot, but you'll definitely notice the humidity. After a mainly clear and mild Tuesday night with lows around 60°F, high temperatures will be around 10 degrees above normal tomorrow (upper 80s to perhaps 90°F in spots) with dewpoints in the mid-to-upper 60s (see below). As a result, heat indices may reach the low-to-mid 90s for a couple hours Wednesday afternoon. If you have prolonged, outdoor plans, drink plenty of fluids, wear light-colored clothing, and take breaks from the heat in an air-conditioned room or building.

A disturbance traversing the Commonwealth from west-to-east may spark isolated showers and thunderstorms late tomorrow afternoon (after 4-5 PM) through tomorrow night, but more numerous storms should erupt on Thursday in association with a cold frontal passage. Some of the storms could produce locally heavy downpours, frequent lightning, and isolated damaging wind gusts up to 60 mph during the afternoon hours. However, widespread severe weather is not anticipated, and the greatest threat appears to be along and south/east of the I-95 corridor Thursday afternoon and evening. Leftover clouds from Wednesday night's convection should limit instability on Thursday, and the front may already be draped along the I-95 corridor by the early-afternoon hours. Despite the faster arrival of the front, high temperatures will still reach the low-to-mid 80s Thursday afternoon. Humidity will still be high with dewpoints in the mid-to-upper 60s, so the "summer stickies" are going to remain with us through the end of the week.

The air mass behind the front won't be much different from the one ahead of it. Thus, dewpoints and overnight lows will remain in the 60s through at least Saturday night and perhaps even through Memorial Day. Any lingering showers and thunderstorms should exit northern MD and southeastern PA Thursday evening, and I'm optimistic that Friday actually turns out dry and pleasantly warm. A weak bubble of high pressure will briefly settle into southern New England later Thursday night and Friday, and it should be in close enough proximity to suppress shower/thunderstorm development during this time. Thus, high temperatures should be back in the low-to-mid 80s on the first day of the holiday weekend under partly-to-mostly sunny skies. Don't forget a water bottle and sunscreen before heading outside! 🧴 Unfortunately, the next disturbance will reach the mid-Atlantic States later Friday night into Saturday and trigger another round of scattered showers and thunderstorms. The air will still be sufficiently warm, moist, and unstable for the most intense storms to contain locally heavy downpours, lightning, and strong winds on Saturday, so keep an eye on the sky and move indoors if lightning is seen or thunder heard. The system won't have enough energy to produce much, if any, severe weather, but wind gusts could reach 45-50 mph in a few spots. As is often the case during the late spring and summer months, shower and thunderstorm coverage will likely be most widespread Saturday afternoon. Despite the storms and enhanced cloud cover, highs will still be in the upper 70s to low 80s with the "juicy" air mass still firmly in place.

Behind the disturbance and its associated weak cold front, another bubble of high pressure should settle into the mid-Atlantic States on Sunday. Thus, I also foresee generally dry and pleasantly warm conditions on Sunday with highs in the low 80s. In fact, Sunday's weather may be virtually identical to Friday's, so those should be the best two days for outdoor barbecues, picnics, and family gatherings. Humidity may also be slightly lower on Sunday with dewpoints in the upper 50s to low 60s. Of course, the nice weather probably won't last for Memorial Day itself. A stronger disturbance and more organized area of low pressure over the Central States Sunday afternoon is expected to track east-northeastward into the Ohio Valley Sunday night and reach southern New England by Monday (see below). Along and to its south, scattered showers and thunderstorms should return Sunday night and persist into Monday morning. If the system moves fast enough, then Monday afternoon could potentially turn out dry. However, a slower system would mean continued shower and thunderstorm chances into Monday evening and the possibility of severe weather. I'll have a better idea of the potential on Thursday and discuss it in my video update.

Regardless of the exact outcome, rainfall amounts through Memorial Day will likely average between 0.75-1.50", but locations hit repeatedly by heavier downpours may receive up to 3" of rain. Given 6-hour flash flood guidance values currently between 2-5" throughout the mid-Atlantic States, I don't expect any stream or river flooding through early next week. However, localized flooding of low-lying, poor-drainage, and urban areas is always a possibility during summertime thunderstorms. For those vacationing to the Jersey, Delaware, or Maryland beaches, the weather should be relatively similar to that discussed above. Ocean temperatures are only in the upper 50s to around 60°F, so temperatures will be around 5 to as much as 10 degrees lower each afternoon along the immediate coastline. With water temperatures so low, I advise wearing a wetsuit or limiting exposure to avoid cold water shock. Remember, the exact details of this forecast are tricky and subject to change, so tune in on Thursday for my final Memorial Day Weekend forecast! -- Elliott