MU's Current Conditions: (at 00:57) Temp: 23 F | Dew Point: 14 | Wind: - calm mph
SPECIAL WEATHER DISCUSSION (Updated twice weekly, but more often when storms threaten.)

* Light Wintry Mix Sunday -- More Arctic Air Next Week*

2:00pm Friday, December 9:
The season's first Arctic outbreak is upon us, and today's high temperature was just 36 degrees (8 below average) and the wind chill made it feel like the 20s. Saturday and Sunday will be a degree or two colder yet, while overnight lows dip to near 20 degrees. A weak disturbance, just coming in off the Pacific into the Northwest states this afternoon, is the crux of the short-term forecast. While not strong, nor all that moist, this system will bring us a wintry mix of precip types Sunday into Sunday night. With a track to our north, the system will ultimately end up bringing a little plain rain and drizzle early Monday. With Arctic air in the way of this system, however, some intermittent light snow and flurries is likely Sunday and then a few hours of snow/sleet mix and/or freezing rain Sunday night. Temps Sunday and Sunday night will hold near freezing, then rise into the 40s Monday. Accumulations are tricky to call with confidence at this point, since a small change in track and timing will influence the outcome....but right now, I expect that much of central and northern PA will pick up 1 - 4 inches of snow, while Lancaster is more likely to see just a coating to perhaps an icy inch or so. Again, this system is a bit nebulous, and I'll do my best to pin down more details on Saturday--please follow my Twitter Feed for updates @MUweather.

Monday into Tuesday will turn briefly milder (highs in the 40s) before the next Arctic blast arrives Wednesday into Thursday. This looks like a deeper push, than the current Arctic air mass, as the Polar Vortex pivots down across Ontario. So while the current cold shot has yielded daytime highs in the 30s, the deeper/stronger blast late next week could yield a couple days with highs only in the 20s (either THU & FRI or FRI & SAT). As for precip chances next week...there's a chance for some rain and/or snow showers ahead of and behind the Arctic front, respectively. There's also a chance that a sharpening jet stream system off the Pacific will bring a low pressure system our way between December 17th - 20th. With deep Arctic air in the way, this system--if there is one--has a better chance of bringing some measurable snowfall to our area. Then again, I continue to see a bias for low pressure systems to track to the west and north of PA; thus, a changeover to plain rain can't be ruled out, despite the prominence of unseasonably cold air into mid-month. --Horst


2:00pm Wednesday, December 7:
December began the same as November ended—with temperatures running several degrees above average through the month’s first six days. A dramatic jet stream pattern change is now underway, however, and I see at least two Arctic blasts aimed at the United States into mid-December.

The initial surge of Arctic air is directed at the Northern Rockies (with sub-zero temps), and this air mass will fortunately moderate as it spreads towards the East coast. The leading edge of this Arctic push will arrive here on December 8th, and the period December 9 – 11 will average 5 to 8 degrees below normal with high temperatures in the mid/upper 30s and nighttime lows into the 20s. A second colder blast will arrive around December 13th or 14th, and so temperatures on December 15th – 17th are likely to be even colder (perhaps as much as 10 to 12 degrees below normal). Beyond that, the vortex of coldest air may pivot back towards the North Pole with gradually moderating temps. There’s even a chance—though I’m not sold on this idea—that Pacific air may regain control across much of the country towards month’s end; if so, December would close out on the mild side. Alternatively, an additional blast (or two) of Arctic air is possible into the holiday period. We’ll see.

What about snow? Yes, I do think we’re likely to receive our first measureable snow sometime in the next two weeks. Heavy lake effect snows will certainly fall in the usual spots (northern and western PA) as the two Arctic blasts move through, but little more than a flurry or snow shower is likely to extend into southeastern PA. I do see signs of two low pressure systems that might threaten us during mid-December. The first (weak) system looks on a track across PA around December 11 or 12th. Such a system could bring us a few hours of light snow or light wintry mix before changing to light rain. More intriguing is the potential for a disturbance to attack the second Arctic blast as it retreats off the Northeast coast between December 17 – 20th. This just might be the system that turns our area white with the first measurable snow of the season. For folks dreaming of a white Christmas…it’s much too early to get excited; but given a continuation of the aforementioned pattern, it’s fair to say there’s a chance! --Horst


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