Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Hot and Humid with Daily Thunderstorm Chances Through Wednesday *

2:00 p.m. Friday, August 5, 2022:

Due to the combination of excessive heat and high humidity, Thursday turned out to be the hottest day of the year across the Lower Susquehanna Valley. The mercury soared to 95°F at Millersville University, and heat index values reached or exceeded the century mark in many parts of southeastern PA. The hot and oppressive air mass was also extremely unstable, and that instability was released late Thursday afternoon and evening in the form of scattered showers and thunderstorms. In fact, a severe thunderstorm warning was issued for southeastern York and southern Lancaster Counties around 6:15 p.m. Although there were no reports of wind damage in the two counties, strong-to-severe thunderstorms caused numerous downed trees and power lines across northern MD. As much as 1.5-2 inches of rain fell in areas hit repeatedly by torrential downpours, while other spots barely received measurable rainfall (see below). This large variation in rainfall amounts is actually quite typical of summertime thunderstorms, and we'll see more of the same over the next week.   

South-to-southwesterly flow on the backside of the "Bermuda High" - what meteorologists call the large-scale, Jet Stream ridge that often flares up over the western Atlantic in July and/or August - will continue to pump heat and humidity into the mid-Atlantic States through early next week. As a result, high temperatures will reach the upper 80s to low 90s every afternoon through Tuesday. Dewpoints will remain between 70-75°F through the period, causing peak heat index values to reach the mid-to-upper 90s each afternoon.. true "Dog Days of Summer"-type weather. Don't expect much relief from the heat at night either. Temperatures will only drop into the low-to-mid 70s through Tuesday night, making it imperative to follow heat safety tips over the next 5-6 days. Make sure to drink plenty of non-alcoholic beverages, wear lightweight clothing, take frequent breaks from the heat in an air-conditioned room or building, and eat plenty of fruits and vegetables in order to avoid contracting heat-related illnesses. It's often easy to forget that extreme heat is the number 1 weather-related killer in the U.S., highlighting the importance of following these tips. 

As for precipitation, there will be a near-daily chance of showers and thunderstorms through the middle of next week. A dying cold front currently in the process of stalling out over central Pennsylvania will trigger another round of scattered showers and thunderstorms late this afternoon and evening (see below). Lingering cloud cover over southeastern PA has limited instability through the early-afternoon hours, but clouds have given way to sunshine across the western half of the Lower Susquehanna Valley. This clearing trend will progress eastward into the mid-afternoon hours, allowing the atmosphere to become increasingly unstable. Slow-moving showers and thunderstorms have already erupted across the higher terrain of south-central PA, western MD, and WV, and these will continue to expand in coverage throughout the rest of the day. Due to very weak flow aloft and the moisture-laden air mass in place, thunderstorms will be capable of producing torrential downpours and moving over the same areas repeatedly, raising the risk for localized flash flooding. In addition, thunderstorms will produce frequent, cloud-to-ground lightning and wind gusts up to 50 mph in a few spots. If you hear thunder or see lightning, move indoors immediately to avoid being struck by lightning or hit by falling tree limbs or blowing debris. Always turn around and seek an alternate route if you encounter water covering the roadway ahead. It's impossible to tell how deep the water is, and only two feet of moving water can sweep a vehicle away.

Showers and thunderstorms will diminish during the mid-to-late evening hours with the loss of daytime heating, but yet another round of scattered showers and storms will erupt Saturday afternoon and evening with the frontal boundary still nearby. The storms will pose the same threats as the ones late this afternoon and evening and will also not impact every location. This is why meteorologists often use the term "scattered" to describe thunderstorms in the summertime. Although some areas will receive as much as 3-4 inches of rain over the next 36 hours, others will receive less than one tenth of an inch. Much like today, the showers and storms will diminish Saturday evening a few hours after sunset. By Sunday, the stalled frontal boundary should reverse course and move back into New England. As a result, showers and thunderstorms will diminish in coverage and become more isolated in nature. The only reason there will still be isolated thunderstorms around on Sunday is due to the hot, humid, and unstable air mass in place. Monday may be the one day in the next six where we don't have to worry about showers and thunderstorms. Although it will still be hot and humid with highs in the low 90s and dewpoints in the 70s, winds will turn westerly and cause drying in the mid- and upper-levels of the atmosphere. In addition, a weak, surface high-pressure system will build into the southern mid-Atlantic and promote large-scale subsidence (sinking motion), resulting in mostly sunny skies. Unfortunately, the more tranquil weather won't last long..

By Tuesday afternoon and evening, another cold front will drop southeastward from the eastern Great Lakes into the Commonwealth.. and it's appearing more likely that the front will indeed stall over southern PA and northern MD from Tuesday night through Wednesday. This means that scattered showers and thunderstorms Tuesday afternoon and evening will likely be followed by yet another round of storms on Wednesday. Although much of the rain may still be considered beneficial by farmers and gardeners, flash flooding could again become a concern in any areas hit repeatedly by heavy downpours.. and localized damaging wind gusts can also not be ruled out. High temperatures will likely reach or exceed the 90-degree mark again on Tuesday before clouds and precipitation limit highs to the mid-80s on Wednesday. Although dewpoints may slowly fall into the upper 60s by Wednesday afternoon, true relief from the sticky air won't arrive until Thursday. In the wake of the front, an area of high pressure should settle over the Northeast late next week into the following weekend. If this occurs (and I suspect it will), then northwesterly winds would direct a cooler, less humid, and more refreshing air mass into the Commonwealth. At this point, I think it's a "good bet" that next Thursday-Saturday turn out mostly sunny and comfortable with highs generally in the low-to-mid 80s, lows in the 60s, and dewpoints in the upper 50s to mid 60s. Consider this the light at the end of the steam-filled tunnel! -- Elliott