Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* some much-needed rain tonight; first frost likely thursday night *

12:30 p.m. Tuesday, October 7, 2025:

In retrospect, the first week of October could certainly be classified as a "blowtorch" and virtual extension of summer. High temperatures have been in the low 80s across most of the Lower Susquehanna Valley since Saturday, and there has not been a day with a high temperature below 70°F since June 27th. In addition, no measurable rain has fallen at Millersville University since September 26th. However, both are about to change, and one in a big way. South-to-southwesterly winds of 10-20 mph on the backside of a high pressure system over the western Atlantic are still pumping unseasonably warm and humid air into the region this afternoon, but a much cooler, drier and more fall-like air mass is "waiting in the wings." High temperatures will still be around 80°F this afternoon.. about 10°F above average for early October.. with dewpoints around 60°F, but a cold front will sweep through the Commonwealth from tonight through Wednesday morning and put an end to our "extended summer." First, we'll receive some much-needed, beneficial rain along and just ahead of the front. Showers and some localized heavier downpours will impact northern MD and southeastern PA between ~6 PM this evening and 10 AM Wednesday. The showers will overspread the region from southwest-to-northeast between ~6-11 PM this evening, continue overnight and then taper off between ~6-10 AM Wednesday morning. Due to the abnormally dry conditions and parched ground, the rain will be largely beneficial (see below). The only real impacts to travel will be wet roadways and standing water in some low-lying, urban- and poor-drainage areas during the Wednesday morning commute. In terms of rainfall amounts, numerical computer models are all over the place. The ECMWF and ICON models suggest less than 0.50" of rain falls across most of northern MD and south-central PA, but the NAM and HRRR show as much as 1-2" of rain. Given the lack of instability and timing of the rain, I favor the lower amounts.. but only time will tell (see below)..

Behind the front, northerly winds of 12-25 mph will usher much cooler and drier air into the Lower Susquehanna Valley on Wednesday. After a mostly cloudy start, skies will gradually clear during the late-morning and early-afternoon hours (see below). In stark contrast to the last few days, high temperatures will be "near normal" on Wednesday (near 70°F) and then end up 5-10°F below average on Thursday (low 60s). The change in humidity will be even more extreme. Dewpoints should peak in the low-to-mid 60s tonight, tumble into the 40s by Wednesday evening and then bottom out in the upper 20s to low 30s Thursday night. Temperatures will only drop into the low-to-mid 60s tonight on account of the high humidity, clouds and showers, but lows will end up in the low-to-mid 40s Wednesday night under clear skies.

As mentioned above, Thursday will be the coolest day since late May despite abundant sunshine. It won't be as windy as tomorrow, but a north-northeasterly breeze of 8-16 mph will make the outdoor air feel even chillier. I then expect the first widespread frost of the season across northern MD and the Lower Susquehanna Valley Thursday night. A Canadian high pressure system will settle into southern New England and northern PA late this week and promote efficient, radiational cooling after sunset. Winds will be very light or calm Thursday night, and dewpoints should be in the upper 20s to low 30s under clear skies. Temperatures often bottom out near the dewpoint on clear, crisp fall nights, and this case should be no exception. In fact, temperatures may bottom out between 30-32°F in the deepest valleys and most outlying areas early Friday morning. They'll be significantly higher (38-40°F) in urban centers such as York, Lancaster and Harrisburg. In the majority of locations, lows will range from 32-36°F (see below). Regardless, the frost will likely put an end to the growing season in most spots, so bring tender plants/vegetation inside or apply a protective covering over them Thursday evening.

In terms of the weather, Friday will be virtually identical to Thursday with high temperatures in the low-to-mid 60s under sunny skies. Winds will be even lighter (5-10 mph or less), so it should feel slightly milder and more comfortable outside Friday afternoon. However, you'll definitely need to grab a jacket, sweatshirt or coat before heading out the door to work or school late this week and put shorts, t-shirts and flip-flops in the closet for awhile. Friday night won't be as cold with lows in the upper 30s to perhaps mid 40s, but it will take until the weekend for temperatures to be back around average. The aforementioned high pressure system will shift into the western Atlantic this weekend, and southeasterly flow on its backside should at least boost highs back into the mid-to-upper 60s from Saturday through early next week. There is some potential for a low pressure system to develop along or just off the southeastern U.S. or Carolina coastline over the weekend and then back into the mid-Atlantic States early next week, but the more likely outcome is for a track out-to-sea and continuation of partly-to-mostly sunny and pleasant conditions across the Lower Susquehanna Valley. Check back on Friday for an update on the potential system, and enjoy the "change of seasons" and "sweater weather" late this week. The "warm-weather party" is over.. ☹️ -- Elliott