Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* dangerous, record-breaking heatwave to bake mid-atlantic states *

1:00 p.m. Tuesday, June 30, 2026:

After a relatively cool, cloudy, and damp last weekend of June, partly sunny and warmer conditions returned on Monday with high temperatures in the upper 80s to low 90s across northern MD and southeastern PA. The high pressure system that had been centered over the Northeast on Monday has since shifted into the western Atlantic. On its backside, south-southwesterly winds of 10-20 mph have pumped even hotter air into the Lower Susquehanna Valley this afternoon. Under mostly sunny skies, high temperatures will reach the low-to-mid 90s, but humidity will be relatively low with dewpoints in the upper 50s to mid 60s. Unfortunately, today's heat is just a harbinger of things to come over the next several days. A massive Jet Stream ridge, or "heat dome," will build/amplify over the Eastern United States during the rest of the week and be responsible for an extremely dangerous and record-breaking heatwave from Wednesday through the 4th of July weekend (see below). Large-scale subsidence, or sinking motion, beneath the ridge will thwart clouds and precipitation and also enhance compressional warming. In addition, winds will turn southwesterly on Wednesday and then westerly late this week, further compressing the air via "downsloping" off the Appalachians. Temperatures of 25-26°C at 850 millibars, or approximately 5,000 feet above the ground, support afternoons highs of 102-106°F both Thursday and Friday. They will only be slightly lower both Wednesday and Saturday and generally between 95-100°F. The combination of drought, dry ground, and June's significant rainfall deficit will also aid in higher afternoon temperatures (see below). Evaporation is a cooling process and often leads to slightly lower daytime temperatures during and after periods of excess rainfall due to its continual occurrence. However, dry ground drastically slows or even eliminates the evaporative process. Thus, surface temperatures are often higher and summers, in general, hotter during periods of severe-to-extreme drought, and this case will be no exception.  

The traditionally hottest month of the year is certainly going to kick off with a "bang" in 2026. An Extreme Heat Warning is in effect from 12 PM Wednesday through 8 PM Friday. After a warm but relatively dry Tuesday night with lows around 70°F, humidity will increase dramatically Wednesday morning with dewpoints in the low-to-mid 70s by the afternoon hours. Dewpoints won't be quite as high on Thursday and Friday (mid-to-upper 60s) due to the "downsloping" effect off the mountains (air dries as it compressional warms), but they'll ramp back up into the upper 60s to low 70s on the Fourth of July itself. Thus, heat indices will peak between 105-110°F each afternoon from Wednesday-Saturday and dramatically increase the risk of heat-related illnesses. Blistering sunshine will accompany the blazing heat, and there won't be much relief at night with lows in the mid 70s (outlying areas) to low 80s (urban centers). Since 1920, there have been 45 days with high temperatures of 100°F+ at Millersville University. Thus, the 'Ville reaches 100°F or better about once every 2.5 years on average. However, this feat has not been achieved in nearly 14 years (July 7, 2012), so we are LONG overdue (see below). In fact, the record high temperatures of 101°F and 104°F from July 2-3, 1966, at Millersville University may both be tied or broken on Thursday and Friday. The July 1st record high temperature of 100°F from 1931 is also in jeopardy, but the mercury will likely come up 1-2°F short of the century mark on Wednesday. In the off chance there are indeed 4-straight 100°F+ days (Wednesday-Saturday), that would tie the longest streak for most consecutive 100°F+ days at the 'Ville (July 19-22, 1930). The all-time record high in July at Millersville is 104°F, and that could certainly be tied or perhaps even broken on Thursday and/or Friday. Time will tell.. 

Meteorologists often become hyper-focused on exact high temperatures and whether or not they'll tie, break, or come up just short of record territory. However, none of that really matters in terms of heat-related impacts to health and safety. Regardless of the exact outcome, it's going to be dangerously hot and humid through the Fourth of July holiday weekend. For young children, the elderly, anyone without air conditioning, or those with underlying health conditions, this will be a potentially life-threatening situation. The internal temperature in a house without air conditioning could reach the mid-to-upper 90s by Thursday or Friday afternoon, and it won't drop much at night with lows in the mid 70s to low 80s. In such a situation, the body can no longer cool itself down efficiently, and maintaining proper hydration becomes difficult if not virtually impossible. In order to minimize the risk of contracting heat exhaustion or heatstroke, drink plenty of non-alcoholic beverages and stay out of or limit time in the sun. If you must be outside, take frequent breaks from the heat in the shade or 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°F day. Periodically check up on relatives, friends, and neighbors, especially in the event of a power outage or loss of air conditioning (see below). Wear loose-fitting/light-colored clothing throughout the long-duration heatwave, and apply a cool compress around your neck if outdoors for an extended period of time.

The heat dome will finally get "beaten down," or suppressed, from Sunday into early next week by a series of disturbances and weak cold frontal passages. There won't be one particularly strong cold front to pinpoint, but a gradual cooling trend is expected from Sunday through Tuesday. The first disturbance may sag southeastward through the Commonwealth on the Fourth of July and trigger scattered showers and thunderstorms during the late-afternoon and evening hours. Fireworks displays could certainly be impacted by the storms, but not until Thursday will I have a better idea on the thunderstorm potential and its coverage, timing, and severity. Sunday and Monday may be a "rinse and repeat" of Saturday except with progressively lower temperatures. Highs on Sunday will still likely be in the low-to-mid 90s with heat indices around 100°F, but the heatwave should officially break on Monday due to enhanced cloud cover and more widespread storms. Thereafter, the "heat dome" is expected to fully collapse during the middle of next week and shift back into the Western United States and Four Corners region. A subtle trough, or dip in the Jet Stream, will take its place over the Eastern States, and temperatures should peel back to seasonal norms during the middle of next week with highs in the 80s and lows in the 60s. Talk about some most-welcome relief! Now let's just hope we can "cash in" on the storm chances and get some much-needed rain later Saturday through Monday.. 🤞 -- Elliott