MU's Current Conditions: (at 06:42) Temp: 38 F | Dew Point: 37 | Wind: ENE - 5 mph
SPECIAL WEATHER DISCUSSION (Updated twice weekly, but more often when storms threaten.)

* Milder Pattern, But Colder for Christmas? *

Friday, December 14, 2018:
Today's drizzle and developing rain event brings an end to the 11 day dry spell that began on December 3rd--this ties for longest dry spell of 2018! The last 11 days have also averaged more than 4 degrees below normal. But the pattern is changing...as high latitude blocking breaks down and the Pacific jet takes control across the country. This new pattern favors milder weather across the central and eastern states, but frequent mountain snows across the western US.

This weekend's rain storm is one such Pacific jet stream disturbance, and it will spawn a pair of weak low pressure systems that will track by to our south this weekend. On and off rain/drizzle will prevail into Saturday with slowly rising temps...to near 50 Saturday afternoon. Periods of rain on Sunday might end with a few wet snow flakes mixed in, as cooler air wraps in from the North behind the exiting disturbance (a few inches of snow is possible in the mountains of central and northern PA). Here in Lancaster, total rainfall will likely be around 1"...with up to 2" in spots.

Monday will begin mild, ahead of a cold front that will drop in during the afternoon. Winds will increase out of the North into Monday night as temps drop below freezing here for the first time in 3 or 4 days. Tuesday will be breezy and quite cold with highs only in the mid/upper 30s and below-freezing wind chills throughout.

The period Wednesday through Friday will then bring another Pacific disturbance and warming trend. Some rain is possible late Thursday or Friday, and temps may pop to the 50s again for a day or two (next Friday and/or Saturday). However, I do see signs of a more amplified northern jet stream late next weekend--if so, a blast of cold air could arrive here between December 23rd and 25th. It may not be a long-lasting cold spell, but it may feel the season (breezy & cold) for the start of the holidays! --Horst



Monday, December 10, 2018:
It's been quite cold and dry that past 7 days with temps running 4 degrees below normal and not a drop or flake of precipitation (at MU Weather Center). While these same conditions will prevail into midweek, a warming trend will unfold into the weekend with a high temp in the low 50s possible on Saturday.

Precipitation-wise, we'll break the current streak of dry days on Friday night as rain develops with a slow-moving southern storm. One thing to watch, is a possible front-running band of light snow that might form across central PA on Thursday...as the leading edge of warm air overruns lingering shallow cold air. This is not a major snow threat, but a coating is possible in parts of the state on Thursday. Otherwise it will turn milder with periods of rain for the first half of the weekend. Sunday will turn breezy with a leftover shower possible and temps in the mid/upper 40s.

Next week is tricky to forecast as mild Pacific air will make a couple attempts to spread across the country, while a glancing cold front or two track across the Great Lakes and the Northeast. Pennsylvania will be on the "air mass fence" throughout this period...so I can see how a shallow shot of colder air arrives next Tuesday, but then a blast of milder air arrives on Thursday into next weekend. A meaningful rain or wintry mixed storm is possible in the final week leading up to Christmas, but I don't see us locking in cold air for an extended stay...in fact, a mild bias may very well prevail for the second half of the month.


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