Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Poor Air Quality Today followed by severe storms on saturday *

3:00 p.m. Friday, July 17, 2026, Update:

The third heatwave of the year scorched the mid-Atlantic States this week, but its intensity and duration both paled in comparison to the early-July scorcher. However, high temperatures still reached the mid 90s in most spots on Wednesday and Thursday, and maximum heat indices topped out between 100-105°F both afternoons. Wednesday's high of 96°F at Millersville University ultimately came up 4°F short of tying the July 15th record high of 100°F from 1997 (see below). A weak cold front moved through the Lower Susquehanna Valley Thursday night, and light, northwesterly winds have since ushered cooler and much drier air into the Commonwealth. Temperatures are in the 80s across the region this afternoon, and dewpoints have fallen from the mid 70s on Thursday into the upper 50s to low 60s at the time of writing. 

Unfortunately, the big story today is the "very healthy" to "hazardous" air quality. Northwesterly flow both at the surface and in the upper-levels of the atmosphere has directed a thick smoke plume from massive wildfires in south-central Canada and northeastern Minnesota into the mid-Atlantic States. The smoke is so thick that it is nearly blotting out the sun across northern MD and southern PA this afternoon. Concentration of fine suspended particles less than 2.5 microns in diameter (PM2.5) is a common measure of air pollution. PM2.5 concentrations exceeded 300 μg/m³ at times from last night through this morning.. some of the highest I've ever seen in the Lower Susquehanna Valley. All of PA is under a Code Purple (Very Unhealthy) Air Quality Alert through this evening for PM2.5 concentrations between 201-300 μg/m³ (see below). Avoid strenuous, outdoor exercise and activity to minimize the effects of air pollution on heart and lung health. Winds will turn southwesterly on Saturday, usher more humid air back into the region, and gradually push the thickest/worst smoke to our north/east throughout the day. Thus, PM2.5 concentrations should gradually lower from late tonight into Saturday evening, and rain from showers and thunderstorms (discussed in the next paragraph) will also help improve air quality. However, sensitive groups such as young children, the elderly, and those with underlying lung/heart conditions may still be impacted, especially during the morning hours. In terms of temperatures, the thick smoke has held them down by some 3-6°F this afternoon, but it will have little impact on them over the weekend. In the wake of Saturday night's cold frontal passage, winds will turn northwesterly again on Sunday, but it will likely take until Sunday night into Monday for another deterioration in air quality. Until the wildfires are extinguished, any wind direction with both a northerly and westerly component may lead to air quality concerns across portions of the mid-Atlantic States. Yuck!

Severe thunderstorms will replace poor air quality as the "big weather story" on Saturday. The Storm Prediction Center (SPC) has placed the entire mid-Atlantic Region in a "slight risk" for severe weather on Saturday and upgraded a large portion of that area to an "enhanced risk" early this afternoon (see below). A vigorous, upper-level disturbance and its trailing cold front will sweep through the Northeast from Saturday into Saturday night and serve as the "triggering mechanisms" for the storms. All modes of severe weather will be on the table in northern MD, southeastern PA and the Lower Susquehanna Valley between 1-10 PM. There may be an initial round of storms in the morning, but that's much more uncertain. Regardless, with high temperatures between 85-90°F and dewpoints in the low-to-mid 70s, the atmosphere will be sufficiently moist and unstable for rapid thunderstorm development and growth. Wind shear.. or a change in wind speed and/or direction with height above the ground.. will increase during the late-afternoon and early-evening hours, as well (see below). Straight-line wind damage from gusts up to 70 mph will be the main threat, but isolated incidents of large hail (>1" in diameter) and a few tornadoes may also occur. The threat for tornadoes appears greatest north of I-76 and also east of the Susquehanna River. Have multiple ways to receive severe weather alerts and warnings, and seek shelter in an interior room or basement if a warning is issued for your location! Downed trees and powerlines, as well as power outages and roof damage, are all expected in the hardest-hit areas. The storms will also bring frequent lightning and torrential downpours with the threat for localized flash flooding of low-lying, urban- and poor-drainage areas. I don't anticipate any stream or river flooding, but standing water on some roadways could cause some vehicles to stall. Areas missed by heavier storms may receive less than 0.25" of rain, while those hit repeatedly by heavy "gullywashers" could pick up 2-3". Be prepared to move indoors at short notice and postpone outdoor plans until Sunday. Storms should push south/east of the I-95 corridor by midnight, and skies will gradually clear late Saturday night with no more than a lingering shower or two through 2 AM Sunday. 

As mentioned above, north-northwesterly winds of 8-16 mph will then usher cooler and MUCH drier air into the Commonwealth on Sunday. Dewpoints will drop into the upper 50s to low 60s by the afternoon hours, and highs will be in the comfortable low 80s under mostly sunny skies (see below)! Needless to say, the weather will do a total 180 from Saturday and be ideal for outdoor activities and/or cleanup and recovery efforts from the storms.

A Canadian high pressure system will then settle into the Commonwealth from Sunday night into Monday and promote mainly clear skies and pleasantly cool conditions. In fact, low temperatures may bottom out in the mid-to-upper 50s early Monday morning with highs in the low-to-mid 80s again during the afternoon hours. Air quality concerns may return due to the northwesterly flow, but to what extent is currently nebulous. The next threat for showers and thunderstorms.. and perhaps another round of severe weather.. returns during the middle of next week, but numerical computer models differ significantly in the timing of the offending cold front. As such, I won't try to finesse those details right now since there's plenty of other adverse weather to get through over the next 36 hours. Check back on Monday for a video update and Tuesday for my next SWD! -- Elliott

1:00 p.m. Tuesday, July 14, 2026:

As anticipated last week, the second weekend of July wrapped up on a partly sunny and seasonably warm note with high temperatures in the mid 80s Sunday afternoon. The week then kicked off on an even nicer note with more sunshine, lower humidity and similar temperatures on Monday. Unfortunately, much hotter and somewhat more humid conditions have returned this afternoon, and another heatwave will bake northern MD and southeastern PA through the end of the week. The upper-level, Jet Stream ridge, or "heat dome," that was responsible for the record-breaking, early-July heatwave will pay us another visit this week. It's currently in the process of shifting farther east and will become oriented from northwest-to-southeast over the next few days, ultimately extending from the Midwest to the southern mid-Atlantic States (see below). Downsloping, westerly winds off the Appalachians will combine with mostly sunny skies to boost high temperatures into the low 90s this afternoon, upper 90s to low 100s on Wednesday, and mid-to-upper 90s Thursday. The July 15th record high temperature of 100°F from 1997 at Millersville University may be tied or broken, and a Heat Advisory is in effect for the Lower Susquehanna Valley from 11 AM to 8 PM Wednesday (see below).

Both humidity and overnight lows will not be quite as high this go around, but dewpoints and low temperatures will still be in the mid-to-upper 60s and low-to-mid 70s, respectively, through Thursday. As such, maximum heat indices will top out in the mid-to-upper 90s this afternoon, between 103-107°F Wednesday afternoon, and between 97-101°F on Thursday. Smoke from Canadian wildfires will dim the sun this afternoon and again on Thursday/Friday, so air quality may also deteriorate and become poor late this week. It won't be as bad as in 2023, but sensitive groups may still be impacted and face adverse health effects (see below). Drink plenty of non-alcoholic beverages to stay hydrated, limit time in the sun, and take frequent breaks from the heat in an air-conditioned room or building. Eating a diet high in fruits and vegetables can replenish electrolytes and help with hydration, as well. Never leave children or pets unattended in a vehicle! The internal temperature of a car can reach lethal levels in a matter of minutes on a 100-degree day. Periodically check up on relatives, friends, and neighbors, especially the elderly and those without air conditioning or with underlying medical conditions.

A weak cold front will sag southward through the Commonwealth on Thursday night and begin to "beat down," or suppress, the Jet Stream ridge, but high temperatures should still reach or exceed 90°F on Friday. Only a stray shower or thunderstorm is expected to accompany or precede the frontal passage Thursday evening, and the air mass behind it should be noticeably drier with dewpoints back down in the upper 50s to perhaps low 60s Friday afternoon. However, higher humidity will make a quick comeback by Saturday in advance of a storm system and its trailing cold front. The system will track from the Great Lakes region into New England spanning Friday night to Saturday night. South-to-southwesterly winds to its south will boost dewpoints back into the upper 60s to perhaps low 70s across the Lower Susquehanna Valley by Saturday afternoon. Due to extensive cloud cover, high temperatures will fail to reach 90°F for the first time in 5 days, but the atmosphere will still be sufficiently moist and unstable for thunderstorm develop and growth. It's too early to talk about severe weather prospects, but be prepared to move indoors at short notice and reschedule outdoor plans to Sunday. Keep an umbrella handy, as well. As is often the case during the summer months, storms will likely be most widespread during the afternoon and evening hours on Saturday. Much like this past weekend, there is uncertainty as to how far south.. and how quickly.. the front progresses. That being said, I once again favor a drier, seasonably warm and less humid outcome from Sunday into Monday with high temperatures in the mid-to-upper 80s each afternoon. However, numerical computer models have a tough time resolving such smaller features more than 3-4 days in advance, so check back on Friday for an update and higher-confidence forecast. Until then, stay cool and hydrated this week! -- Elliott