Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* cold and dry through early next week, but milder days follow *

12:30 p.m. Wednesday, December 10, 2025:

The first 10 days of December have been persistently and unseasonably cold across the Central and Eastern States, but there has been very little snow to show for it across the Lower Susquehanna Valley. Other than the December 2nd system, the storm tracks of subsequent systems simply haven't aligned correctly for winter-weather events across our region. Instead, snow-lovers in Virginia and the southern mid-Atlantic States have "cashed in" on the pattern with predominantly dry conditions farther north between I-70 and I-76. Why are we stuck in a "cold but dry" pattern? It's actually quite simple. The northern and southern branches of the Jet Stream have not linked up or joined forces. Additionally, the southern branch has and will continue to be very weak. Strong, northwesterly flow from Canada associated with the much stronger northern branch has directed multiple shots of Arctic air into the Northeast, but these air masses have also been incredibly dry. As a result, we've dealt with frequent, clipper-type systems diving southeastward out of Alberta, Canada, and into the Midwest and Great Lakes region (see below). From there, they have either tracked into New England or continued on their southeastward journey into the Carolinas and southern mid-Atlantic States. Essentially, northern MD and southeastern PA have been caught in the middle and, therefore, received very little snow in the past week. Clipper-type systems are a staple of La Niña winters, and that's not ideal for snowhounds. They move very quickly, are often starved for moisture, and rarely produce more than "minor" or "light" snowfalls. In addition, accumulating snow typically only falls in a narrow swath along and just north/east of their track, so they have to take the "perfect path" to produce accumulating snow in a given region. Over the next 7 days, this large-scale pattern should change very little, and the only meaningful chance for measurable snow will come on Saturday night.

Last Thursday, I expected no more than a few rain/snow showers to impact northern MD and southeastern PA this week. Despite plenty of hype from the "usual suspects" on Twitter(X) and other social media platforms, that forecast is certainly coming to fruition due to the factors outlined above. One such clipper system is currently zipping from the Great Lakes region into southern New England. It will bring rain showers to the Lower Susquehanna Valley this afternoon, but more significant precipitation will remain north of Interstate 80 due to the system's track. Southerly winds of 12-25 mph have ushered more seasonable air back into the region and already boosted temperatures well above 32°F as of midday, so only a few wet snowflakes or sleet pellets are expected to mix in with the rain at times across Perry, Lebanon and Dauphin Counties (see below). Showers should push south/east of the I-95 corridor by 7 PM, and most areas will only pick up 0.10-0.20" or less of rain from the event. Due to the abnormally dry and moderate drought conditions in place, any precipitation will be largely beneficial (see below). Just make sure to reduce speed on any wet roadways this afternoon.

A cold front trailing southward from the clipper system will sweep through the Commonwealth tonight. In its wake, west-northwesterly winds of 15-30 mph with gusts up to 40 mph will usher another colder and drier air mass into the mid-Atlantic States on Thursday. Under partly-to-mostly cloudy skies, temperatures will only rebound a few degrees throughout the day with highs in the upper 20s to mid 30s from northwest-to-southeast across the region. That's about 10°F below average for mid-December, and it will feel even worse outside with wind chills in the mid teens to low 20s. You may even see a few snow flurries in the air at times, as well. Fortunately, a bubble of high pressure will then settle into southern New England from Thursday night into Friday and cause winds to gradually subside. However, it will remain unseasonably cold on Friday with highs in the low-to-mid 30s and wind chills in the mid-to-upper 20s under mostly cloudy skies. Needless to say, you'll need to dress in layers and bundle up before heading out the door late this week! The cold won't be of the "dangerous" variety, but it will still feel downright miserable outside. 

In terms of snow, I'm monitoring two additional clipper systems for a potential "light" event or two through the weekend. The first of these systems will likely track too far south (Tennessee River Valley to the Carolina coastline) to bring any measurable snow to the Lower Susquehanna Valley later Friday into Friday night. Instead, Friday night should just turn out mostly cloudy and seasonably cold with lows in the mid-to-upper 20s. I won't rule out some flurries or very intermittent, light snow south of the Turnpike, but I'm not expecting any accumulation north of the Mason-Dixon Line and perhaps even I-70 (see below). Behind the first system, the weekend will then kick off on a mostly cloudy but somewhat more seasonable note with highs around 40°F Saturday afternoon. However, the next system will then track through the mid-Atlantic States Saturday night and bring the chance for snow back into the forecast.

Of the two systems, the second one has a much greater likelihood of producing accumulating snow across northern MD and southeastern PA. The weak low pressure system will originate in a similar location to Friday's system, but it will then veer east-northeastward from the Lower Ohio Valley/central Appalachians into the Delmarva Region Saturday night. A clipper track through northern VA or MD/DE is "ideal" for a light snowfall event between I-70/76 and I-81/95, and this case should be no exception. However, the system will be starved for moisture and moving very quickly, so there will only be about a 4-6-hour window for snow Saturday night. Thus, I expect no more than a general coating to 2" of snow across the region with the highest totals near I-95. The clipper is still ~3.5 days away, so there can be some changes over the next 24-36 hours. Either way, it should be a low-impact event and not commence until the overnight hours. Better yet, any snow will have already exited southeastern PA prior to Sunday morning church services. Paved surfaces may still be slick and slippery, however, so use extra caution on the roadways Sunday morning. 

Behind the clipper, northwesterly winds of 15-30 mph with gusts up to 40 mph will usher one more reinforcing shot of Arctic air into the mid-Atlantic States from Sunday into early next week. The weather looks downright awful Sunday afternoon with temperatures in the mid-to-upper 20s and wind chills in the single digits and teens. Partial cloud cover and a persistent breeze should keep temperatures in the mid teens to perhaps 20°F Sunday night, but minimum wind chills could certainly bottom out near 0°F early Monday morning. A Canadian high pressure system should then shift from the Midwest into the central Appalachians and Southeast early next week. As the system gets closer to our region, winds will certainly slacken Sunday night into Monday. However, next week will still kick off on a brisk and frigid note under partly-to-mostly cloudy skies. High temperatures will only reach the mid-to-upper 20s again Monday afternoon, and a westerly breeze of 12-25 mph will keep wind chills in the single digits and teens throughout the day. From Sunday night through Monday morning, the cold may actually fall into the "dangerous" category, so it will be imperative to limit time outside and cover up exposed skin to limit the risk of frostbite or hypothermia. 

Fortunately, the large-scale weather pattern should finally ease and "reshuffle" by the middle-to-latter part of next week, and aggregate temperatures will likely be much closer to or slightly above average in the week leading up to Christmas. The Madden-Julian Oscillation (MJO), or eastward-moving wave of tropical convection between the Indian and central Pacific Oceans, was active in "Phase 8" between December 2-8. Phase 8 is a notorious "cold phase" for the eastern U.S. during the winter months, so our frigid start to December comes as no surprise. However, the wave of convection died out over the past 24-48 hours, and the MJO is now in the "null phase." It is projected to remain very weak or simply stay in the null phase over the next few weeks. When in the null phase, the MJO typically has very little influence on the large-scale pattern across the mid-latitudes, and other factors take precedence. One such factor is the Stratospheric Polar Vortex (SPV). After being remarkably weak through the middle of the month, it is projected to rapidly strengthen between December 16-23 and then become abnormally strong in January. A strong SPV lessens the likelihood of Arctic outbreaks across the mid-latitudes. In addition, the Pacific Jet Stream is expected to undergo a major retraction between December 17-22. As a result, a massive ridge, or northward bulge in the Jet Stream, is already developing over the Bering Sea, and this feature will only amplify and become more pronounced over the next 2 weeks. The Bering Sea ridge often precedes an eastern U.S. warmup and flareup of the "Southeast Ridge" by 6-10 days, and this case should be no exception (see below). Thus, I expect a big moderation in temperatures during the middle of next week.

At the surface, the high pressure system mentioned above will shift into the mid-Atlantic States on Tuesday and western Atlantic by Wednesday, ultimately establishing itself as said "Southeast Ridge." On its backside, south-to-southwesterly flow should return on Tuesday and persist through at least Thursday. A gradual warming trend will ensue, and I expect high temperatures to be back in the 30s Tuesday afternoon under partly sunny skies. They should then reach the 40s next Wednesday-Friday and may even sneak into the low 50s on on or two of those days. A cold front may follow later Friday into Saturday and temporarily "beat down" the Southeast ridge for a day or two, but milder conditions should make a quick comeback by the start of Christmas week. If so, then odds of a White Christmas would dwindle significantly, and the weather could end up being quite mild and benign over the holidays. Time will tell! I'll be out of town and away from Twitter(X) and e-mail later tomorrow afternoon through Sunday, so check back early next week for an update! -- Elliott