Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* An Active/Stormy Pattern Through Mother's Day with a Weekend Cooldown *

12:30 p.m. Tuesday, May 7, 2024:

Following a "surprise" record-breaking 90-degree day on May 2nd at Millersville (see below), a "backdoor" cold front.. or one that moves from northeast-to-southwest out of New England instead of the traditional west-to-east fashion.. slid through eastern PA Friday night. Behind the front, strengthening, east-to-southeasterly flow off the Atlantic caused low-level moisture to increase and low clouds to develop east of the Appalachians by Saturday morning. The easterly flow regime remained in place over the weekend, and a pair of disturbances caused periods of light rain and drizzle to plague the Lower Susquehanna Valley. Due to the clouds and rain, high temperatures were confined to the low-to-mid 50s on Saturday and upper 50s to low 60s on Sunday. Needless to say, the weather was absolutely lousy on graduation day for the second time in just 3 years.. yuck!

Fortunately, the disturbances moved off the Eastern Seaboard on Monday, and the easterly flow was finally disrupted. Clouds hung tough for most of the day across northern MD and southeastern PA, but the air mass across the region was considerably warmer than the one over the weekend. As a result, high temperatures at least returned to normal and reached the upper 60s to mid 70s. The combination of partial clearing and high dewpoints overnight led to the development of locally dense fog across the eastern half of the Lower Susquehanna Valley early this morning. In fact, visibility dropped to near-zero in spots, and a Dense Fog Advisory was in effect through 9 a.m. However, the fog has long since burned off, and the rest of the afternoon will turn out partly sunny, warm and rather humid with highs in the mid 70s to perhaps 80°F and dewpoints in the low-to-mid 60s. After a raw and dreary weekend, the air is back to feeling more like summer than mid-spring! A quasi-stationary frontal boundary draped along the Mason-Dixon Line will be the focal point for isolated showers and thunderstorms this afternoon (see below). However, most of the activity will be brief, and many areas won't receive any precipitation. A vigorous disturbance will then track from the eastern Great Lakes region into southern New England tonight. It will be in close enough proximity to bring scattered showers and thunderstorms to the southern-tier counties of PA after midnight, but severe weather is not anticipated. Due to the warm and "soupy" air mass in place, overnight lows will only fall into the low-to-mid 60s and be around 15-20°F above average.

Behind the disturbance, skies should gradually clear on Wednesday, and westerly winds of 12-25 mph will direct even warmer air into the Lower Susquehanna Valley. High temperatures are expected to be around 15°F above normal Wednesday afternoon, or generally in the mid-to-upper 80s (see below). Any lingering showers will exit southeastern PA and the Philadelphia metro. area by 7-8 a.m. Thus, I don't anticipate any weather-related concerns for the Wednesday morning commute, and shorts and t-shirts will be the attire of choice for most tomorrow afternoon!

On the heels of tonight's disturbance, a stronger and more potent storm system will move from the Ohio Valley into the mid-Atlantic States spanning Wednesday night to Thursday. Clouds will increase ahead of the system Wednesday night, and widespread showers and thunderstorms are expected on Thursday. However, the center of low pressure is now expected to track along or just south of the Mason-Dixon Line and limit the severe weather threat across southern PA. Given this storm track, east-to-southeasterly flow off the Atlantic would promote much lower temperatures (60s/70s) Thursday afternoon and a lot of low-level cloud cover. This would essentially prevent the atmosphere from becoming unstable and cause strong-to-severe thunderstorms to be limited to areas south of Interstate 70. A northward shift in the storm system's track would mean a greater risk for severe weather across southern PA, but I don't currently expect this scenario to play out. Due to the widespread showers and storms, a widespread 0.50-1.00" of rain is likely from Thursday into Thursday night, but this won't be nearly enough to cause flooding issues. Still, be alert for standing water in low-lying and poor-drainage areas during the Thursday afternoon commute, and reduce speed when encountering heavier downpours on highways and interstates. Mostly cloudy skies and scattered showers will linger on Thursday night, but thunderstorms should weaken and push farther south/east Thursday evening. Behind the storm system, winds will shift to the northwest later Thursday night into Friday and usher cooler and less humid air into the Commonwealth. An upper-level low trailing behind the system will move through the state on Friday and keep the threat for scattered showers around. However, they will be far lighter and more isolated than on Thursday, and much of the day may even turn out dry. Temperatures should still be in the 50s around daybreak but only rise into the mid 60s Friday afternoon. When combined with a 12-25 mph breeze and mostly cloudy skies, a jacket or sweatshirt may be necessary for outdoor activities. It will undoubtedly be a noticeably cooler end to the week.

A major pattern reversal.. featuring a northward bulge in the Jet Stream over western North America and a deep, southward dip over the eastern United States.. will commence late this week and reach maturity over Mother's Day Weekend. Embedded, smaller disturbances rounding the base of the larger, Jet Stream trough will trigger rounds of showers throughout the weekend, but specific timing of these showers remains nebulous at this distance. Additionally, partly-to-mostly cloudy skies and a 10-20 mph west-to-northwesterly breeze should accompany much cooler conditions this weekend. Highs on both Saturday and Sunday will probably be stuck in the 60s (about 5-10°F below normal), and overnight lows will be in the seasonably cool mid-to-upper 40s. Given the cooler and showery weather, you'll want to keep a jacket/sweater and umbrella handy at all times. Outdoor activities for Mother's Day won't be washed out but may have to be shifted indoors for a period of time. Showers will be of the hit-and-miss variety, so rainfall amounts should not exceed 0.25-0.33" in any given location this weekend. Currently, Sunday appears to be the more active day with greater shower coverage, but that will depend on the exact timing of each disturbance. By early next week, an area of high pressure should build into the mid-Atlantic States and promote drier and brighter conditions. Additionally, the large trough over the Eastern States should begin to flatten out, and a gradual warming trend should take place from Monday-Wednesday. High temperatures should rebound into the upper 60s or low 70s on Monday, and they may be back in the upper 70s to low 80s by the middle of next week. As is the expected theme this month, cool shots will be brief with a far greater number of "warm" days than "chilly" ones. Check back on Friday for more precise details about the weather for Mother's Day! -- Elliott