Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Stagnant and oppressively humid pattern with daily t-storm chances *

1:30 p.m. Wednesday, July 9, 2025:

Following a beautiful 4th of July weekend with low humidity, pleasant warm afternoons and comfortable nights, hotter and more humid conditions returned early this week. High temperatures and dewpoints crept back into the upper 80s to low 90s and low-to-mid 70s, respectively, on Monday and Tuesday, and maximum heat indices reached or exceeded the century mark in many spots. A vigorous disturbance and subtle cold front sparked numerous strong-to-severe thunderstorms across southeastern PA and the Delmarva Region Tuesday afternoon. The storms fed off the moist and unstable air mass in place and dumped as much as 2-3" of rain in parts of Lebanon, Lancaster and Harford (MD) Counties (see below). In addition, there were nearly 100 reports of severe weather.. primarily in the form of wind damage.. across the mid-Atlantic States on Tuesday. Southeastern PA and the I-95 corridor were hit particularly hard with widespread tree- and powerline damage (see below). At one point, dozens of flash flood and severe thunderstorm warnings were in effect simultaneously across the region, and 2 tornado warnings were issued for the northwestern suburbs of Philadelphia.

Unfortunately, flash flooding issues may recur and be exacerbated later this afternoon into tonight. Another vigorous, upper-level disturbance will swing through the mid-Atlantic States and trigger more heavy, drenching thunderstorms across northern MD and southeastern PA between 2 PM and Midnight (see below). The disturbance is taking a similar track to yesterday's disturbance, albeit somewhat farther south. Thus, today's Flood Watch only covers Adams, York and Lancaster Counties (from 3-11 PM), as well as all of southeastern PA (from 5 PM today to 2 AM Thursday) and northern MD (from 2 PM to Midnight). Fewer storms and much lighter rainfall amounts are expected north of the Turnpike.

With dewpoints in the low-to-mid 70s and temperatures in the upper 80s to low 90s, the atmosphere is sufficiently moist and unstable for rapid thunderstorm development and growth. Precipitable water (PWAT) values, or a variable meteorologists utilize to measure the atmosphere's moisture content, are near record-high levels for July 9th and over 2" across much of southeastern PA, MD, DE and VA. PWATs are used to help forecast rainfall rates and pinpoint areas of potential flooding, and values over 2" indicate a significant risk for flash flooding. Numerous thunderstorms have already erupted over WV and southwestern VA early this afternoon, and they will only intensify east of the Blue Ridge/Appalachians later this afternoon and evening. The primary threats from the storms will be blinding downpours with rainfall rates of 2" per hour or more, locally heavy rainfall amounts of 2-4", frequent lightning, small hail and damaging wind gusts of 60-70 mph. 6-hour Flash Flood Guidance (FFG) values remain low and on the order of 1-2" across much of Lancaster, Chester and Berks Counties (see below). A measly 0.50-1.00" of rain in just 1 hour will cause flash flooding in parts of these counties. As much as 1-2" of rain may fall in that amount of time this afternoon and evening, especially in locations impact by multiple, slow-moving storms. The highest rainfall amounts (3-4") should be farther south than on Tuesday, so the flooding threat will be even greater across MD, northern VA and in the highly-populated/urbanized I-95 corridor (see below). Downed trees and powerlines, as well as power outages, from the most intense storms are most likely south of the Mason-Dixon Line in a region of maximum instability and forced ascent/rising motion ahead of the upper-level disturbance. However, a few incidents of wind damage may occur in the southern-tier counties of PA, primarily south of the Turnpike. To be honest, today's "Storm Reports" map from the Storm Prediction Center (SPC) could look similar to Tuesday's, except shifted a few hundred miles farther south/west.

Due to the heightened risk of flash flooding over the next 12-18 hours, avoid roadways next to smaller streams and creeks. In addition, never drive through floodwaters! Instead, always turn around and seek an alternate route. It's impossible to judge the depth of water covering a roadway using only the naked eye, and just 12-18" of standing water will float most vehicles. 2 feet of rushing water can sweep most vehicles away. Do not try to cross flooded roadways or smaller streams on foot, either. It takes even less water to sweep a person off their feet, and sharp objects or dangerous debris are usually embedded within floodwaters. Reduce speed, increase following distance and turn on your vehicle's hazard lights in any blinding downpours, as well. Today's storms will impact the PM commute in some areas, so be alert for blowing objects and debris on roadways too. Have multiple ways to receive severe weather alerts this afternoon, and seek shelter in an interior room or basement if a warning is issued for your location. To sum it all up, make wise decisions this afternoon and, if necessary, move to higher ground. Keep an eye on the sky and head indoors if lightning is seen or thunder heard.

Regardless of the eventual rainfall distribution through this evening, any lingering showers should push east of the I-95 corridor by 1-3 AM Thursday morning. Looking ahead through the end of the week and upcoming weekend, we are going to be stuck with oppressive humidity and a daily chance of showers and thunderstorms. The overall weather pattern could be described as a "stagnant" one with very little flow (light winds) throughout the entire atmosphere. The Jet Stream has long since lifted well north over Canada, and southerly winds on the backside of a semi-permanent high pressure system over the west-central Atlantic.. often deemed the "Bermuda High" by meteorologists.. will continue to pump warm, moist air into the eastern United States. Embedded, smaller-scale disturbances within the slow, zonal (west-to-east) flow across the nation will combine with the heat and humidity to spark isolated showers and thunderstorms over the next several days. As is often the case during the summer months, shower and thunderstorm coverage from tomorrow through Sunday will be greatest during the afternoon and evening hours (2-10 PM). Fortunately, a weak bubble of high pressure will settle over the Great Lakes region late this week and limit shower and thunderstorm coverage to the "isolated" variety on Thursday and Friday. The high should shift into northern New England and the Canadian Maritimes over the weekend and at least partially exert its influence down the eastern slopes of the Appalachians. Thus, shower and thunderstorm coverage on Saturday and Sunday may be similar to Thursday and Friday, and far less than today. However, you'll need to keep an umbrella handy through the weekend and be prepared to move outdoor activities inside for a couple hours each afternoon. I certainly wouldn't cancel any outdoor plans, but they may be interrupted on one or more days.

Any severe weather from Thursday-Sunday should be highly localized and solely in the form of sporadic wind damage. That being said, locally heavy rainfall and flash flooding of low-lying, urban- and poor-drainage areas will still be problematic due to the slow-moving nature of the daily storms. Due to somewhat enhanced cloud cover over the next several days, high temperatures will generally be in the mid-to-upper 80s with lows in the upper 60s to mid 70s through Sunday night. Anyone hoping for the even the slightest break in the stifling humidity will be sorely disappointed through at least early next week. 70-75°F dewpoints aren't going anywhere in the foreseeable future.. yuck! A subtle cold front may try to sag southeastward into the mid-Atlantic States on Monday and trigger more widespread showers and thunderstorms. If so, then another round of severe weather might occur early next week before a brief stint of dry weather next Tuesday-Wednesday. I'll post a brief update on Friday and write another in-depth discussion during the middle of next week, so check back then for more details. The "Dog Days" of summer have arrived and are living up to their name.. 🐕 -- Elliott