Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Warmer and more typical, summertime pattern into mid-June *

2:00 p.m. Tuesday, June 4, 2025:

Due to an onslaught of soaking rainfall events and closed-off, upper-level lows, May 2025 did indeed conclude as the 4th-wettest on record at Millersville University since 1914. Many parts of northern MD, southeastern PA and the Lower Susquehanna Valley received 2-3 months' worth of rain in just 4 weeks (May 3-31). However, the excessive rainfall in May was largely beneficial due to the long-term drought. In fact, it essentially wiped out both the drought and annual rainfall deficits. All areas north of the Mason-Dixon line should be pulled out of the "moderate drought" category and downgraded to "abnormally dry" or "none" on this week's U.S. Drought Monitor update. A wild temperature rollercoaster in May.. featuring abnormally warm conditions during the first 18 days of the month and unseasonably chilly ones thereafter.. ended up "near normal," in aggregate (see below).

Sunday, June 1, marked the start of meteorological summer, but it felt nothing at all like the season. Gusty, northwesterly winds of 15-30 mph accompany partly-to-mostly cloudy skies and highs in the mid-to-upper 60s, and lows bottomed out in the mid-to-upper 40s Sunday night. The weather was more typical of mid-to-late April than early June, and a jacket or sweatshirt were essential for outdoor activities. Fortunately, a big warmup commenced on Monday and will reach maturity late this week. The weekend chill has been wiped away, and there are no signs of the damp, raw and unseasonably chilly pattern of late May returning this month. Instead, a pattern more typical of summertime should emerge with plenty of 80-degree days and bouts of isolated-to-scattered showers and thunderstorms. The high-latitude blocking that developed over Greenland during the middle of May has vanished and won't be making a comeback anytime soon. In turn, the persistent trough, or dip in the Jet Stream, over the eastern United States and has been replaced by faster, zonal (west-to-east) flow and even a subtle northward bulge in the Jet Stream. As a result, high temperatures returned to normal (upper 70s to low 80s) yesterday afternoon.. the first 80-degree day at the 'Ville since May 17th.. and will soar into the mid-to-upper 80s this afternoon.

South-to-southwesterly flow on the backside of a sprawling high pressure system over the western Atlantic will pump increasingly warm and humid air into the mid-Atlantic States through the end of the week (see below). High temperatures will be 5-10°F above average, or in the mid-to-upper 80s, through Friday. In addition, dewpoints have climbed into the low 60s this afternoon, and they'll be in the uncomfortable mid-to-upper 60s both tomorrow and Friday (see below). Thus, heat index values could creep past the 90-degree mark late this week and peak between 91-93°F. While not "dangerously hot," these will be the hottest conditions since last August or September, so it will be imperative to drink plenty of non-alcoholic beverages and take breaks from the heat. The mercury itself may even reach 90°F in a few spots Thursday afternoon, but the first widespread 90-degree day of the year is still likely a couple weeks away. Low temperatures will also be much higher than in recent weeks and in the upper 50s to low 60s tonight, low-to-mid 60s Thursday night and mid-to-upper 60s Friday night. Time to turn on and crank up the air conditioning unit!

The large-scale subsidence, or sinking motion, beneath and to the north/west of the aforementioned high pressure system will also thwart shower/thunderstorm development over the next 48 hours. By Friday morning, no measurable precipitation will have fallen across northern MD or the Lower Susquehanna Valley in 6 days.. the longest stretch of dry weather since late April. However, smoke from Canadian wildfires several thousand miles away will dim the sun but lead to spectacular sunsets the next two evenings. Fortunately, the smoke will mostly be confined to the mid- and upper-levels of the atmosphere and not have too much of an impact on air quality like in 2023 (see below). However, a Code Orange Air Quality Alert is in effect for southeastern PA and the eastern half of the Lower Susquehanna Valley through this evening, and another may be issued tomorrow. Sensitive groups such as children, the elderly and those with lung or heart diseases should avoid strenuous activity and outdoor exercise (see below). Smoke and air pollution concentrations should be lower on Friday.

Despite the dry conditions through Friday morning, it can only be so warm and humid for so long during the summer months before shower/thunderstorm chances return. That will happen later Friday afternoon through Saturday along and ahead of a slow-moving cold front sagging southeastward through the Commonwealth. Isolated storms may reach areas north/west of Route 15/I-81 after 3-4 PM Friday, but the bulk of the activity should hold off until Friday night and Saturday. At this point, any severe weather later Friday through Saturday appears isolated and in the form of locally damaging wind gusts. The atmosphere will be sufficiently moist and unstable, but winds both at the surface and aloft appear light. In addition, there won't be much upper-level energy along the front or change in temperatures/dewpoints from one side to the other. Thus, any violent storms will probably be short lived and of the "pulse" variety. Areas between I-95 and Route 15/I-81 may actually be in the least-favorable zone for severe weather due to the frontal timing. In these areas, most of the activity may occur later Friday night through midday Saturday. Storms may then re-intensify Saturday afternoon, mainly south/east of I-95, while drier air seeps back into south-central PA and northern MD. Timing of frontal passages is always challenging to finesse more than 24-48 hours in advance, so much can still change. A difference of just 3-6 hours can significantly alter the corridor most at risk for severe weather. Regardless, the tornado threat will essentially be "nil" in this case, and frequent lightning and locally heavy downpours will be the primary hazards from the storms. Be prepared to move outdoor activities inside on Saturday, and have multiple ways to receive severe weather alerts/warnings.

Behind the front, an area of high pressure will settle over the Northeast from Saturday night into Sunday and promote slightly cooler and less humid conditions. However, high temperatures and dewpoints should still be in the low 80s and mid-to-upper 50s, respectively, under mostly sunny skies on Sunday. Low temperatures both Saturday and Sunday nights will likely peel back into the upper 50s to low 60s, but that's perfectly normal in early June. At least we may get a chance to turn off the A.C. and open up the windows for 24-36 hours. By early next week, winds will have already turned southerly again, so temperatures and dewpoints/overnight lows may be back in the low-to-mid 80s and low-to-mid 60s, respectively, by Monday and/or Tuesday. In addition, the next cold front may be sweeping southeastward from the Great Lakes region and triggering the next round or two of scattered showers and thunderstorms. I'll be on vacation and away from E-mail and Twitter(X) through Sunday but will have more on that early next week. To be honest, the pattern of a few dry days followed by a few days with shower/thunderstorm chances may "rinse and repeat" multiple times this month. High temperatures on most days will be between 75-90°F, and monthly rainfall should end up "slightly above normal." This June looks like the typical, summertime month to me.. -- Elliott