Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* warm and oppressively humid pattern with more rounds of storms *

1:00 p.m. Tuesday, July 15, 2025:

As anticipated, flooding woes continued to plague parts of southeastern PA and the Lower Susquehanna Valley over the last few days. Slow-moving thunderstorms dumped 3-4" of rain across eastern Adams County on Saturday, 3-4" of rain in western York County on Sunday and 6-8" of rain in northwestern Lancaster County yesterday. Millersville University received 2.17" of rain from yesterday's torrential downpours, but 6" of rain fell just 5 miles away in Mountville. Mount Joy got hit the hardest by the storms and picked up about 7.5" of rain in just 2-3 hours. Catastrophic flash flooding ensued, and there were numerous reports of water rescues, stranded vehicles, road closures and flooded homes. In fact, all lanes of Route 30 were shut down for several hours Monday evening between the Columbia/Marietta and Mountville exits. This morning's map of 24-hour rainfall totals from the Middle Atlantic River Forecast Center (MARFC) looked eerily similar to the June 30th flash flooding event in Lancaster and Lebanon Counties (see below). After facing moderate-to-severe drought for over a year, we've now had far too much of a good thing in the past 2 months. Enough is enough already!

Unfortunately, another vigorous disturbance will swing through the mid-Atlantic States from Wednesday afternoon into Wednesday night and trigger numerous showers and heavy, drenching thunderstorms during that time. An area of high pressure over the central Appalachians will provide storm-weary residents of southeastern PA and the Lower Susquehanna Valley with a much-needed respite from the wet weather through tonight, but the break won't last for long. As has been the case in recent weeks, the main threats from tomorrow's storms will be locally heavy rainfall amounts of 2-3", torrential downpours with rainfall rates up to 2" per hour or more, and localized but potentially catastrophic flash flooding (see below). It will only take 0.50-1.00" of rain in one hour across much of the region to cause flash flooding, and tomorrow's storms can easily dump that much rain (or more) in just 30-60 minutes. In addition, the most intense storms may produce wind gusts up to 50 mph and cause isolated power outages. Storms should be most numerous between ~2-11 PM before weakening and dissipating overnight. Remember to never drive through floodwaters! Instead, turn around and seek an alternate route. Avoid roadways near smaller streams/creeks and in low-lying and poor-drainage areas, as well. It only takes about 2 feet of rushing water to sweep most vehicles away. You'll need to keep an umbrella handy and be prepared to move outdoor activities inside tomorrow afternoon and evening. Expect areas of slow travel and drastically reduced visibility during Wednesday's PM commute, and there may be additional road closures in a few spots. The storms should not be as intense or slow-moving as in recent days, but the atmosphere will still be very moist and unstable with dewpoints in the low-to-mid 70s and high temperatures in the mid-to-upper 80s. 

From Thursday into Friday, a weak cold front will drop from the Great Lakes region into the mid-Atlantic States. Additional showers and thunderstorms may erupt Thursday afternoon well in advance of the front, but most of the activity should remain northwest of the Lower Susquehanna Valley. Any showers and storms south/east of I-81 will likely be of the isolated- to widely-scattered variety. The bigger story on Thursday will be the extreme heat and oppressive humidity. With highs around 90°F and dewpoints in the mid 70s, heat indices should peak between 100-105°F during the afternoon hours under mostly sunny skies. To minimize the risk of contracting heat-related illnesses, drink plenty of non-alcoholic beverages to stay hydrated. Take frequent breaks from the heat in an air-conditioned room or building, as well, and wear light-colored, loose-fitting clothing to stay as cool as possible. In terms of any severe weather or additional flooding concerns on Friday, things are trending in the right direction. The front will likely sag through south-central PA later Thursday night or Friday morning during the coolest and most stable part of the day, so the next round of storms may very well erupt south of the Mason-Dixon Line Friday afternoon. If so, then only a stray shower or two might impact the southern-tier counties of PA, and most of the day could just turn out dry under partly sunny skies. Regardless of the exact outcome, it will still be a very warm and muggy afternoon with highs in the mid-to-upper 80s and dewpoints in the low 70s. An area of high pressure should then settle into the Northeast from Friday night into Saturday and promote drier and slightly less humid conditions during that time. However, low temperatures and dewpoints will only bottom out in the mid 60s to around 70°F early Saturday morning with highs back in the seasonably warm mid 80s Saturday afternoon under partly-to-mostly sunny skies. 

Looking ahead into the second half of the weekend and early next week, we are going to remain stuck in an oppressively humid regime with frequent chances for showers and thunderstorms. The overall weather pattern could be described as "stagnant" with very little flow (light winds) throughout the entire atmosphere. Frequent disturbances will ridge along the northern periphery of an elongated, Jet Stream ridge across the southern tier of United States. Meteorologists often refer to this zone as the "ring of fire" due to the onslaught of showers and thunderstorms within it, and this case will be no exception. One such disturbance will move through southern New England and the mid-Atlantic States from Saturday night into Sunday and bring more showers and storms to the region. Behind the disturbance, another high pressure system may briefly settle into the Northeast on Monday and promote partly sunny and dry weather. However, the next round of showers and storms might arrive as early as Monday night and/or Tuesday. Storms on Saturday night/Sunday and Monday night/Tuesday are expected to be of the "scattered" variety. Some residents may experience more heavy downpours, flooding woes and/or damaging wind gusts while others see little if any rain. Hence is the norm in the summer months. In an active pattern such as this, timing of each disturbance is subject to change by several or more hours up to 2 days before their arrival, so check back on Friday for an update on the forecast from Sunday through the middle of next week. 70-degree dewpoints will make a comeback Saturday night and probably stick around through most if not all of next week, so expect overnight lows to remain in the uncomfortable upper 60s to mid 70s. As a result, cooling demands will continue to run high for the foreseeable future. Welcome to the "Dog Days" of summer.. 🐕 -- Elliott