Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Keeping an Eye on Friday Night for Potential Brush with Light Snow *

3:00 p.m. Tuesday, December 6, 2022:

After a mostly sunny and seasonable start to the first full week of December, the weather has taken a turn for the worst today. A disorganized, upper-level disturbance is combining with moist, southeasterly flow off the Atlantic to bring periods of light rain, fog and drizzle to the Lower Susquehanna Valley. Due to the low clouds and showers, temperatures are stuck in the low-to-mid 40s across most of the region.. a truly damp, raw, and dreary afternoon. Warmer air several thousand feet above the ground will stream northward into the mid-Atlantic overnight, so temperatures should remain nearly steady in the 40s before rising into the 50s Wednesday afternoon. A cold front will traverse the Commonwealth from west-to-east on Wednesday, and another weak disturbance moving northeastward along it will spawn additional showers. Fortunately, the showers should be more isolated and lighter than today, with the steadier and more organized activity remaining across central and northern PA. In fact, dry weather may return as early as Wednesday afternoon and promote smoother travel for the evening commute. Until then, expect slow travel this evening and again Wednesday morning as a result of the wet roadways and areas of reduced visibility. Exercise caution while driving, making sure to increase following distance and remain within the speed limit to lower the risk of hydroplaning. Rainfall amounts through tomorrow won't be particularly high and generally on the order of one quarter to one half of an inch. Behind the front, an area of high pressure building into southeastern Canada will cause winds to turn northerly Wednesday night and Thursday, resulting in partial clearing and slightly cooler conditions. Still, temperatures shouldn't fall below 40°F Wednesday night and may very well reach 50°F again Thursday afternoon, but the gentle breeze will certainly make it feel a bit chillier outside. The colder and drier air mass over New England will continue to build farther south into the mid-Atlantic States on Thursday night and Friday, resulting in lows within a few degrees of freezing (32°F) and highs in the low-to-mid 40s. Despite the drier air mass in place, it's likely that skies remain mostly cloudy to round out the week.

My attention then turns to a small but well-defined, upper-level disturbance moving toward us from the Midwest and Ohio Valley. A large, Jet Stream ridge over Greenland will remain in place and even expand across eastern Canada through the upcoming weekend. This pronounced, high-latitude blocking.. associated with the negative phase of the North Atlantic Oscillation (NAO).. often prevents storm systems from "cutting" into the Great Lakes region. Instead, they're forced to take a west-to-east path underneath the block. If cold enough air is present to the north of the system's track, then a narrow swath of accumulating snow can occur. There should be no exception to this rule in the Midwest late this week, with a band of snow breaking out across northern Iowa Thursday night and extending into southern Michigan for Friday. However, the system will become "strung out" and lose much of its energy before reaching areas east of the Appalachians. In addition, northeasterly winds will direct a very dry air mass down the eastern slopes of the Appalachians on Friday, and that air mass should remain in place Friday night. Thus, it will take some time for the low-levels of the atmosphere to become moist enough for any precipitation to reach the ground. Ultimately, how much (if any) and what type of precipitation falls in northern MD and southern PA also depends greatly on the system's precise track (see below). At this point, I expect any precipitation that does occur late Friday and Friday night to be light and have minimal impacts on travel. Further details, such as arrival time of rain/snow and location of any minor snowfall accumulations, won't be possible to pin down until later tomorrow or on Thursday.. so follow @MUweather on Twitter for updates! ❄️❄️

Regardless of the outcome Friday night, I'm fairly confident that dry conditions will return on Saturday in the wake of the disturbance. Unfortunately, moisture should remain high in the low-levels of the atmosphere and prevent much, if any, sunshine from breaking through the clouds. As a result, high temperatures will be "near-normal," or generally in the low-to-mid 40s. Another disturbance may then take a similar track to the late-week system during the second half of the weekend, but I expect it to be even less intense and more disorganized. As a result, only light rain, or mixed rain and snow, showers should impact southeastern PA on Sunday and Sunday night. Once again, I expect the precipitation to have little, if any, impact on travel with temperatures remaining at least several degrees above freezing. A large high pressure system over eastern Canada associated with the -NAO will remain nearly stationary this weekend into the start of next week, and this blocking high pressure system will cause the weather pattern to slow down over the eastern U.S. As a consequence, it's certainly possible that rain/snow showers linger into Monday, it's equally likely that partly sunny, seasonable, and dry conditions return. Predictability and accuracy beyond 3-5 days decrease and are greatly reduced during strong -NAO periods. Generally speaking, the evolution (strength, location, and orientation) of the high-latitude blocking plays a crucial role in determining how slowly storm system progress to the east and which days turn out active or dry. Unfortunately, numerical computer models are notoriously bad at handling the evolution of this blocking, leading to a greater amount of uncertainty in the forecast during the 5-10 day period. So, you'll have to check back in on Friday for updates on the Sunday/Monday system and to find out what the weather has in store for the rest of next week.

Hype is rife on social media regarding a potential East Coast snowstorm on December 14th or 15th, but the more likely outcome is for an all-rain event. The NAO blocking should temporarily break down during the middle-and-latter part of next week, allowing the storm system to track into the Great Lakes region or Upper Midwest and putting the mid-Atlantic on its "warm side." For snow-lovers, I can certainly say that the pattern thereafter has potential, especially in the week leading up to Christmas (Dec. 18-24). However, it can easily turn into "heartbreak" if that potential is never realized. Stay tuned! -- Elliott