Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* more Showers and storms This Weekend, but a Reprieve Next Week *

Brief 1:00 p.m. Friday, July 18, 2025, Update:

For the first time since July 8th, the mercury exceeded 90°F at Millersville University on Thursday. When combined with dewpoints in the low-to-mid 70s, heat indices peaked in the low 100s Thursday afternoon. Fortunately, a cold front swept through the Commonwealth last night and put an end to the dangerous heat. In its wake, northerly winds of 5-10 mph have ushered a drier, less humid and more refreshing air mass into the Commonwealth this afternoon. Dewpoints have fallen into the mid 60s across the Lower Susquehanna Valley, and high temperatures will only be in the low 80s under mostly cloudy skies. In fact, temperatures should not reach 90°F again in most spots until late next week. A few light showers may skirt the southernmost 20-25 miles of PA (generally south of the Turnpike) between 3-7 PM, but the bulk of the activity should remain south of the Mason-Dixon Line (see below). 

The aforementioned cold front will lift back north as a warm front later tonight into Saturday and be the focal point for scattered showers and thunderstorms during that time. Dewpoints will climb back into the upper 60s to low 70s tomorrow, and the higher humidity and instability will fuel locally heavy downpours. Due to the saturated ground and recent onslaught of drenching storms, isolated incidents of flash flooding may occur. However, the storms should not be as widespread nor as intense as in recent weeks. High temperatures should be in the low-to-mid 80s again tomorrow afternoon on account of extensive cloud cover. A quick-moving disturbance may spark additional showers and storms Saturday night, and a cold front sweeping through the mid-Atlantic States from northwest-to-southeast on Sunday will trigger a final round of isolated showers and thunderstorms during the afternoon hours. Fortunately, severe weather is not anticipated through the weekend, and rainfall amounts through Sunday evening should not exceed 1-2", even in the hardest-hit areas (see below). Ahead of Sunday's cold front, it will turn oppressively hot and humid with highs in the mid-to-upper 80s, dewpoints in the low-to-mid 70s and heat indices between 95-100°F. If you have prolonged, outdoor plans, drink plenty of fluids to stay hydrated and take frequent breaks in an air-conditioned room or building. Keep an umbrella handy each day, but I certainly wouldn't cancel any outdoor activities. Just be prepared to move them inside for a bit.

Looking ahead into next week, the forecast has actually taken a surprisingly nice turn. Behind Sunday's cold front, northerly winds will usher a cooler and noticeably less humid air mass into the mid-Atlantic States. Skies will clear Sunday night, and there should be very few clouds in the sky from Monday-Wednesday. A Canadian high pressure system will settle over the eastern Great Lakes region, southeastern Canada and New England during the middle of the week and promote an extended stretch of dry weather for the first time since July 4th weekend. In a truly refreshing turn of events, dewpoints should tumble through the 60s Sunday night and end up in the mid-to-upper 50s Monday afternoon. Better yet, they'll remain in the 50s through Tuesday and only climb back into the low-to-mid 60s on Wednesday. Plenty of sunshine will accompany the lower humidity, so high temperatures should be in the comfortable low 80s both Monday and Tuesday afternoons. They'll probably top out in the mid-to-upper 80s on Wednesday, but it will still feel pretty good outside due to the relatively dry air mass in place. In addition, you should be able to sleep with the windows wide open Monday and perhaps even Tuesday nights. Low temperatures will bottom out in the upper 50s to low 60s Monday night and low-to-mid 60s Tuesday night.. a few degrees below average for late July. Overall, the weather will be picture-perfect for outdoor activities early next week, and flood- and storm-weary residents across northern MD, southeastern PA and the Lower Susquehanna Valley will finally be able to "catch their breath" and enjoy a break from the onslaught. By the end of the next week, the aforementioned high pressure system will shift into the western Atlantic. On its backside, southwesterly flow will return and direct hotter and more humid air back into the region. In fact, the heat may once again be of the "dangerous" variety with highs in the 90s and heat indices over 100°F. Details regarding the intensity and length of the potential heatwave are uncertain at this distance, so check back on Tuesday for an update! Scattered showers and storms will likely return late next week or the following weekend, as well, but the large-scale pattern looks more progressive through the end of July with a lower risk for flash flooding and extreme rainfall amounts. Thank goodness! -- Elliott

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