Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* A Cooler But Seasonable and More Tranquil Pattern Into Late April *

12:15 p.m. Tuesday, April 16, 2024:

After a cool and blustery start to the week, a warm front moved through the Lower Susquehanna Valley Sunday morning, and temperatures were "off to the races" behind it. In fact, highs reached the upper 70s to mid 80s from east-to-west across the region, and skies were mostly sunny until late in the day. A slow-moving cold front sagged southward through the Commonwealth from Sunday afternoon through Monday afternoon and triggered two rounds of severe weather. The first impacted the northern half of Pennsylvania Sunday afternoon and evening, but those storms weakened dramatically prior to reaching south-central PA. However, showers and generic, run-of-the-mill storms still survived the journey off the Appalachians, and most areas picked up 0.10-0.50" of rain Sunday night (see below). On Monday morning, the frontal boundary was still draped across central PA, and temperatures skyrocketed back into the upper 70s to mid 80s south of the Turnpike shortly after midday. As a result, the atmosphere become highly unstable, and isolated severe thunderstorms erupted in northern MD and the southern-tier counties of PA. Most areas were "missed" by the storms, but there were several reports of large hail between 1.0-1.5" in diameter in parts of Adams and York Counties between 3:00-4:30 p.m.

The cold front finally pushed south of Interstate 70 Monday evening, and north-to-northwesterly winds have since ushered a drier and more refreshing air mass back into the region. This afternoon's weather will be absolutely spectacular and virtually "unbeatable" by mid-April standards. After a seasonably cool morning with lows in the low-to-mid 40s, temperatures were already in the mid-to-upper 60s as of midday and will peak in the comfortable low-to-mid 70s by 3-5 p.m. An area of high pressure is centered directly overhead, so winds will be light throughout the day with only a few high clouds in the sky (see below). If possible, get outside and take advantage of the beautiful weather after work or school. It will be a perfect day for outdoor activities such as running, hiking, biking, fishing, golfing, or sunbathing.. "paradise" for warm-weather lovers like myself! 😎

Contrary to last Friday's expectations, temperatures actually did manage to reach or exceed the 80-degree mark in most areas south of the Turnpike on Sunday and/or Monday. For the first time since October 28, 2023, the mercury touched 80°F at Millersville on Sunday, and the feat was achieved again on Monday with a high of 81°F (see below). However, temperatures will be much lower and closer to average over the next 7-10 days, and I don't expect any more days in the 80s until the last few days of April or perhaps early May.

As mentioned on Friday, a storm system currently over the Canadian province of Alberta will skirt eastward along the U.S./Canadian border through week's end and ultimately reach southeastern Canada by Saturday. It will not plunge southward into the western United States but instead "camp out" over eastern Canada from Saturday through most, if not all, of next week. As a result, the high pressure system near Bermuda and upper-level ridge over the eastern U.S. won't have a chance to amplify, or strengthen, late this week. Instead, it will be squashed and suppressed to the south. However, the aforementioned storm system and its associated dip in the Jet Stream won't drop south.. or amplify.. over the eastern U.S. this weekend or next week. Instead, the Jet Stream should remain fairly "zonal," or oriented primarily in a west-to-east fashion, during the foreseeable future. The Madden-Julian Oscillation (MJO).. or an eastward-moving wave of convection between the Indian and western Pacific Oceans.. is also inactive and projected to remain so through early May. MJO inactivity during the spring and summer months favors a zonal/progressive weather pattern and less amplified Jet Stream, lending further credence to a cooler.. but rather seasonable and more tranquil.. weather pattern through next week. Generally speaking, this type of Jet Stream configuration usually promotes weaker storm systems, lighter precipitation events, and few late-season cool shots, and I expect this case to be no exception.

In the short term, clouds will increase tonight in advance of an approaching disturbance, and showers are then expected to overspread northern MD and the Lower Susquehanna Valley from west-to-east between 8 a.m. and Noon on Wednesday. Due to the clouds and showers, high temperatures should occur around or shortly before midday and be around 4-8 degrees lower than today, or generally in the 60s. After showers arrive, temperatures may fall back into the mid-to-upper 50s for awhile tomorrow afternoon. A second disturbance will move eastward through the Commonwealth from Wednesday night into Thursday and bring additional showers to southeastern PA. There may be a 2-4 hour period of steadier, light rain Wednesday night, but only some areas of drizzle should linger for the Thursday morning commute and throughout the rest of the day. Additionally, a backdoor cold front will move through eastern PA Wednesday night and slash temperatures by another 5-10 degrees on Thursday. It is uncertainty how far south/west the backdoor front will progress, but the possibility of a large temperature gradient, or difference in temperature, across eastern/central PA exists Thursday afternoon. Temperatures could easily peak in the 50s around Philadelphia but in the 70s near Chambersburg. Exact details of this nature are nearly impossible to predict until the morning of, so follow @MUweather on Twitter(X) for updates! Regardless of the exact outcome on Thursday, rainfall totals through Thursday afternoon should not exceed one half of an inch across northern MD or south-central PA. You'll want to keep an umbrella handy from Wednesday morning through Thursday afternoon, but flooding of streams, creeks, or low-lying and poor-drainage areas won't be a concern. Thank goodness!

Moist, east-to-southeasterly flow off the Atlantic will continue on Thursday night and Friday, so low clouds and patchy drizzle will be the main themes during this time, as well. Due to the persistent cloud cover, overnight lows shouldn't fall below the mid 40s to low 50s through Thursday. A weak cold front will finally move through the mid-Atlantic States from Friday afternoon into Friday night and scour out the low clouds and easterly flow. High temperatures on Friday will likely be "near normal," or generally in the low-to-mid 60s, ahead of the front. A few showers may accompany its passage later Friday afternoon or evening, but the atmosphere will be far too stable and the front too weak for any severe weather.. or even thunderstorm.. concerns. Behind the front, a 3- to 4-day stretch of cooler, drier, brighter, and rather pleasant conditions are a "good bet." North-to-northwesterly winds will usher progressively cooler air into the mid-Atlantic States from Saturday into Sunday, but high temperatures still won't be very far below normal. In fact, the weekend, as a whole, may be the nicest of the month so far. Partly sunny skies should "rule the roost" both days, and highs should be in the mid 60s and upper 50s, respectively. Sunday morning will be chilly with lows in the upper 30s to low 40s, but temperatures rebound quickly on account of the strong April sun. You may need a jacket or sweater for Sunday morning church services, but long pants and t-shirts should be the "attire of choice" for most during the afternoon hours. Next week should start on a similar note with chilly nights (30s/40s) giving way to pleasantly cool afternoons (60s). Average highs in late April are in the mid 60s to around 70°F with normal lows in the low-to-mid 40s. Thus, aggregate temperatures from Saturday-Tuesday should be a few degrees below normal, but that's still quite nice this time of the year. No complaints! -- Elliott