Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Bitterly Cold Prior to the Holiday, But Odds of a 'White Christmas' Are Low *

12:15 p.m. Wednesday, December 18, 2024:

Sunday's storm system gave residents of Franklin and Perry Counties a "snowy surprise" with a widespread 1-4" of accumulation prior to the changeover to rain Sunday night. Elsewhere, the forecast generally played out according to expectations. A slushy coating to 1" of snow fell across much of Dauphin, Lebanon, western Adams and northern York Counties, but areas east of I-83 and south of Route 30 received no accumulation. Temperatures slowly rose into the low-to-mid 40s by Monday afternoon and continued rising Monday night. A cold front brought more showers to the region later Monday afternoon and evening, but the rain had already pushed east of the I-95 corridor by early Tuesday morning. In the wake of the system, clouds gave way to sunshine Tuesday afternoon, and high temperatures soared into the mid-to-upper 50s. Needless to say, any snow that fell on Sunday is long gone, and the pleasantly mild conditions yesterday afternoon were most welcome and very refreshing! As anticipated, a widespread 0.50-1.00" of liquid equivalent precipitation fell across northern MD, southeastern PA and the Lower Susquehanna Valley spanning Sunday-Tuesday (see below). Due to the ongoing drought, any precipitation remains highly beneficial.

We were greeted by sunshine again this morning, but clouds have since overspread the region in advance of another disturbance. The weak storm system will zip from West Virginia to the southern New England coastline over the next 12-18 hours and bring periods of generally light rain to northern MD, southeastern PA and the Lower Susquehanna Valley this afternoon and evening. However, the wet weather will be short lived with rain having already exited the I-95 corridor by 1-2 AM Thursday (see below). Rainfall amounts won't exceed one third of an inch in most areas, but wet roadways and areas of reduced visibility may lead to slow travel during the PM commute. Remember to drive within the speed limit and increase following distance to minimize the risk of hydroplaning. Despite the clouds and wet weather, temperatures have reached the mid 40s north/west of I-81 to mid 50s along the I-95 corridor early this afternoon.. quite mild by mid-December standards. They'll drop by several degrees or more through the rest of the afternoon but remain well above freezing through the evening hours.

Behind today's system, skies should gradually clear overnight, and northwesterly winds of 8-16 mph will usher cooler and more seasonable air into the mid-Atlantic States on Thursday. Temperatures may briefly fall below freezing early Thursday morning, especially in areas north of the Turnpike and west of Route 15, so there may be a few icy spots on bridges and overpasses between 5-9 AM. However, most roadways should just be wet or even dry by that point. Under partly sunny skies, highs will reach the low-to-mid 40s Thursday afternoon with wind chills in the upper 30s. While chillier than today, tomorrow's weather is essentially "par for the course" in December. Clouds are expected to increase again Thursday night ahead of a complex but disorganized storm system. An upper-level disturbance within the northern branch of the Jet Stream will zip through the Ohio Valley and mid-Atlantic States from Thursday night into Saturday. However, the Gulf of Mexico won't be "open for business," thus limiting the amount of moisture available for the system. Additionally, the primary low pressure system will die out over the Ohio Valley on Friday, and secondary redevelopment should occur hundreds of miles offshore Friday night. Due to an absence of high-latitude blocking over Greenland and northeastern Canada, there will be no mechanism by which to slow the storm system down. As a result, the secondary low will likely remain too weak and form too far offshore to bring any significant precipitation to the region. Instead, I only expect some rain/snow showers across southeastern PA and the Lower Susquehanna Valley Friday afternoon through Friday night with no snowfall accumulation for most (see below). Temperatures will bottom out in the upper 20s Thursday night but reach the mid-to-upper 30s prior to the arrival of precipitation on Friday. I won't rule out a slushy coating to 2" of snow on grassy surfaces and untreated, secondary roadways in the South Mountain Range and higher elevations north/west of I-81 and in Berks, Chester, Lehigh and Montgomery Counties, but there should be no issues on primary roads, highways and interstates. Moreover, any of those secondary roads likely won't turn slick or slushy until well after the PM commute. Generally speaking, the "snow drought" should continue this week in the northern MD and southeastern PA "snow hole"..

In the wake of Friday night's system, the coldest air mass in 11 months will settle into the eastern United States through early next week. Strong, north-to-northwesterly winds of 12-25 mph with gusts up to 35 mph will usher bitterly cold air into the Northeast and mid-Atlantic States on Saturday. Despite lows in the mid-to-upper 20s Friday night and partial sunshine on Saturday, temperatures will only rebound back into the low-to-mid 30s. Factor in the blustery conditions, and wind chills will be stuck in the teens and 20s all afternoon. A Canadian high pressure will shift from the Upper Great Lakes region to southern New England over the weekend and remain in place over the Northeast on Monday. Due to the large-scale subsidence, or sinking motion, associated with the high, winds will gradually subside Saturday night into Sunday, but that will also promote mainly clear skies and an ideal setting for efficient, radiational cooling. As a result, lows will plunge into the teens Saturday night and may hit "rock bottom" in the single digits Sunday night. With the Arctic air mass directly overhead, high temperatures will likely be around 15°F below average Sunday afternoon, or in the mid-to-upper 20s, and only a few degrees higher on Monday. No snow or ice will accompany the Arctic air, and wind speeds should decrease to 10-20 mph on Sunday and 6-12 mph by Monday. Thus, wind chills will probably remain in the single digits above zero early Sunday morning and hover in the teens Sunday afternoon. Due to calm conditions early Monday morning, wind chills will essentially be the same as the air temperature. Fortunately, there won't be many clouds in the sky on Sunday or Monday, so the sunshine should help ease the chill a bit. However, the sun is at its lowest point in the sky across the Northern Hemisphere, so it's hard to feel much warmth from it in late December. Astronomical winter officially begins at 4:21 AM EDT on Saturday, December 21, and the first few days of the season are going to feel like it. Bundle up and dress in layers from Saturday-Monday, and limit outdoor exposure to the cold. Temperatures and wind chills won't be low enough for frostbite to develop in less than 1-2 hours, but you should still dress warmly and cover as much skin as possible. 

Despite the frigid conditions in the days leading up to Christmas, the odds of a "white" one remain very low. The aforementioned high pressure system will lift into eastern Canada or the Canadian Maritimes during the middle of next week, and south-to-southeasterly flow on its backside will direct slightly milder and more seasonable air into the region. Christmas Eve still won't be "warm" or "pleasant," but high temperatures should at least be closer to normal and back in the mid-to-upper 30s. There is a small chance that a weak disturbance could drop from the eastern Great Lakes region through the Commonwealth from Tuesday into Tuesday night, but the existence, strength and track of this potential disturbance are highly uncertain at the present time. If precipitation does impact the Lower Susquehanna Valley on Christmas Eve, it would likely fall as a wintry mix of sleet, snow and rain. However, recent storm tracks suggest a greater likelihood of wintry precipitation across central and northern parts of the state. The forecast on Christmas Day itself will be highly dependent on the timing and track of Tuesday's system, as well as the existence or absence of a potential second system over the Tennessee River Valley during the middle of next week. I'll be away from the office from Friday-Sunday and won't be able to finesse these details until early next week, so check back Monday for an update! As of now, the odds of there being at least 1" of snow on the ground at 7 AM Christmas morning are about 5% or less. Sorry, snow-lovers, but the prospect of a White Christmas looks bleak yet again this year.. -- Elliott