Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* First 90-Degree Heat of the Year; Worsening Drought Concerns; Summer Outlook *

9:00 p.m. Thursday, June 1, 2023:

May 2023 ended as the driest May on record at both Millersville and Harrisburg. A mere 0.29" of rain fell at Millersville with 0.19" at Harrisburg. By comparison, both locations typically receive about 3.75" of rain in May. The previous driest May on record (since 1914) at Millersville occurred in 1939 with 0.50" of rain, and the previous record at Harrisburg (since 1889) was 0.29" in 1902. Since May 14th.. a span of 19 days.. only 0.02" of rain has fallen at Millersville. Rarely do we experience dry spells of such magnitude and persistence, and the latest U.S. Drought Monitor (issued June 1st) reflected the impact of the dry spell with a large expansion of both "abnormally dry" and "moderate drought" conditions. Nearly all of PA and northern MD is now in the "abnormally dry" category, and a narrow tongue of "moderate drought" exists east of the Susquehanna River. The Monthly Drought Outlook for June forecasts a developing and worsening drought across a large portion of the mid-Atlantic States, including most of MD and PA (see below). Despite the record dryness in May, aggregate temperatures were about 1-2 degrees below average across the Lower Susquehanna Valley (see below). This was primarily due to persistently cool nights and a chilly start to the month.

Recently, there has been a large diurnal variation.. or difference between the overnight low and afternoon high.. in temperatures. The parched ground is at least partially to blame for this. Dry ground gives up its heat to the atmosphere quickly at night but is also a great conductor of heat during the day. Evaporation from the ground is a cooling process, and that process has essentially been eliminated on account of very low soil moisture. In fact, the mercury reached 90°F for the first time this year today in both Harrisburg and Lancaster and only fell short of the number by 2 degrees at Millersville (see below). Had the ground been saturated, highs would have been about 2-4°F lower. We'll crank up the heat even more on Friday. Under mostly sunny skies, highs should easily reach the low-to-mid 90s. Talk about an impressive start to "meteorological summer".. phew! With little rain ahead in the foreseeable future, expect large diurnal temperature variations to continue and impacts from the drought to become more detrimental and far-reaching. By mid-June, farmers may need to begin irrigation efforts to prevent crops from dying. Water shortages will also become a concern, and water restrictions could certainly become a reality by the end of June or July. The duration and severity of a drought are very difficult to predict. One soaking rainfall of just 2-3 inches can change everything, but it doesn't look good in the coming weeks..

Not only does June 1st mark the beginning of meteorological summer, but it's also the start of the 2023 Atlantic Hurricane Season. Due to a developing and perhaps strengthening El Niño, wind shear.. or the change in wind direction and speed with height above the ground.. should be higher than normal in both the Atlantic and Pacific Basins this season. In El Niño patterns, the southern branch of the Jet Stream typically becomes dominant over the northern branch. As a result, easterly trade winds in the tropics weaken and can even become westerly in the mid- and upper-levels of the atmosphere. Unlike severe thunderstorms, wind shear is detrimental to tropical storm and hurricane formation. Thus, I expect a near- to slightly-below average Atlantic Hurricane Season with only 10-14 named storms and 4-6 hurricanes. Of those, 1-3 could become major hurricanes. I expect most of these systems to "recurve" back out into the open waters of the Atlantic with a lower-than-normal chance of a significant U.S. landfall.. and that's good news! Fingers crossed.. 🤞

As for my 2023 summer outlook, things look relatively bleak in the rainfall department. We're still in a transition phase from La Niña to El Niño, and it will take the atmosphere several more months to respond to these changes. Both the northern and southern branches of the Jet Stream are currently quite weak and should remain that way through August. Put simply, there is just not enough energy available in the atmosphere to spawn significant storm systems or potent cold fronts, and I don't foresee many changes until autumn. Thus, I expect drier-than-normal conditions this summer across much of the mid-Atlantic region. In ENSO-neutral (neither La Niña nor El Niño) summers, there tends to be a general lack of rainfall in south-central and southeastern PA. I do not expect record dryness this summer, but overall rainfall should end up below average by a few to several inches. Given that we're already experiencing abnormally dry conditions, continued drier-than-normal weather will only exacerbate the situation. As for heat, the worsening drought should promote an above-average number (about 25-35) of 90-degree days yet again this summer. There is often a correlation between drought and the core of the heat during summer months. This year, I expect the hottest conditions (relative to average) to extend from the Plains through the Midwest, Ohio Valley, and mid-Atlantic States. After a near- to perhaps slightly-above normal June, I expect several heat waves in July and August with above-normal odds for a 100-degree day or two. In aggregate, I expect temperatures from June through August to average 1-3°F above normal. Unlike many summers, our heat this season should be primarily of continental origin and come from "over the top" instead of from the western Atlantic or Gulf of Mexico. Fortunately, this means that a lot of our heat should be accompanied by low humidity. Regardless, any temperatures in the mid 90s to 100°F are dangerously high and make practicing heat safety essential.

Right on cue, heat will be the big story to end the week on Friday. For the first time this year, highs should soar into the low-to-mid 90s, or about 15°F above average for June 2nd. Fortunately, dewpoints will only be in the 50s, so the heat index will be the same as the actual air temperature. Despite this, you'll still want to drink plenty of non-alcoholic beverages, wear light-colored clothing, and take frequent breaks from the heat in the shade or an air-conditioned building if you have prolonged outdoor plans. Never leave small children or pets unattended in cars. On a 95-degree day, a car's interior temperature can rise to approximately 130°F in just 30 minutes. A backdoor cold front.. or one that moves from northeast-to-southwest instead of northwest-to-southeast.. will slide southwestward out of New England on Friday night and Saturday. A few scattered showers should accompany the frontal passage on Saturday, but these will be light and of the hit-or-miss variety. On average, most locations will receive one tenth of an inch of rain or less.. not enough to put even the slightest dent in the drought. After a mild Friday night with lows in the 60s, winds will shift to the northeast behind the front and usher a much cooler air mass into the Lower Susquehanna Valley this weekend. High temperatures on Saturday may occur in the late-morning or around midday before slowly falling during the afternoon. Depending on the timing of the frontal passage, highs may reach the low-to-mid 80s (slower front) or be held in the mid 70s (faster front). Regardless, temps should fall into the 60s to perhaps low 70s by sunset and only be in the low-to-mid 50s by sunrise on Sunday.

An upper-level low will develop over the Canadian Maritimes this weekend and drift southward into the open waters of the western Atlantic early next week. At the same time, another upper-level low will "close off," or become detached, from the northern branch of the Jet Stream over southeastern Canada and drift southward into New England during the middle of next week. Here's the problem: we'll be on the southern and western flanks of each system in a region of subsidence, or sinking motion. That doesn't bode well for much, if any, rainfall next week. Sure, there could be a stray shower in spots on Wednesday and/or Thursday, but most areas will stay dry. Where it rains, amounts won't exceed a few hundredths of an inch. Like I said earlier, there just isn't much drought relief on the horizon. Due to the trough, or dip in the Jet Stream, over the eastern United States this weekend and much of next week, heat won't be making a comeback until at least the middle 10 days of June (see below).

Highs should be in the 70s each afternoon from Sunday through at least Thursday, with the one exception perhaps coming on Monday. Downsloping, northwesterly winds in advance of another weak, backdoor cold front may boost highs into the low 80s Monday afternoon. The two coolest days of the week will most likely be Wednesday and Thursday with highs only in the low-to-mid 70s. A north-to-northwesterly breeze of 10-20 mph should accompany the lower temperatures, making it feel a bit chilly outside at times, especially during the morning hours. The good news in this dry, northwesterly flow pattern? Overnight lows will be in the 50s from Saturday night through at least Thursday night, so you'll be able to turn off the A.C. and sleep comfortably with the windows wide open. Sure, temperatures from Sunday through Thursday will likely average 2-4°F below normal, but can you really complain about that in early June? I think not! -- Elliott