* Mild Start to October - Cooler Mid-month *
* Warmer (and Wet) Early Next Week -- Cooler Thanksgiving Weekend *

2:00pm Friday, November 21:
Just a brief update, as the Wednesday discussion (below) nails most of the key points. After a frigid Friday night (lows within a few degrees of the record of 15), the weekend brings a warming trend to the low 40s Saturday and low 50s Sunday. Outside of an isolated sprinkle or shower Sunday, the weekend will be dry...ahead of a period of steady rain Sunday night into early Monday. Deep southwest flow will blast drier air into the region Monday and temps will soar 10 to 15 degrees above normal (low to mid 60s) and a few spots to our south and east may hit 70! Monday night will remain mild, but a passing cold front (weak) will usher in a gradual shift back to seasonably cool conditions for midweek.

Thanksgiving day brings the chance of coastal low pressure system which could miss us out to sea (cool and dry), cut up over us (cool and wet), or just maybe take a barely-off-shore track which could bring us some wet snow or rain ending as snow (<20% chance). It's much too far away to finesse, especially given the complex pattern across North America. Beyond that, a glancing shot of cold air will make for a chilly weekend, but I still see the potential for a mild spell to start December. --Hörst


2:00pm Wednesday, November 19:
This week's record Arctic outbreak is one of the coldest on record so early in season. While we haven't broken any record lows (here), yesterday afternoon and today are about as cold as daytimes get in Lancaster in mid-November with highs in the upper 20s to low 30s (normal is 53). Nationwide, Tuesday morning was the coldest November morning since 1976, and here in Lancaster this is looking like the coldest mid-November Arctic Outbreak since 1933. Impressive, yes! A sign of things to come? Not necessarily.

The cause of this anomalous pre-Thanksgiving cold snap was a large jet stream ridge that developed over Alaska and the Yukon region last week. This ridge dislodged and helped direct three blasts of Arctic air (from the North Pole and arguably Siberia) down across Canada and into the US, between November 12th (into the Rockies) and this week (eastern US). This upstream high-latitude ridge is the same feature responsible for last winter's extreme cold. Fortunately, the AK ridge is now fading and so after this week we should be done with the extreme cold for this month. Therefore, the current Arctic blast is not a sign of an imminent descent into persistent wintry conditions...but it is likely that we'll see it again, at times, in the coming weeks/months. [My Winter Outlook in a nutshell: periods of unseasonable cold and above average snowfall (30" to 45" in LanCo), but NOT as snowy nor persistently cold as last winter.]

After a frigid Saturday morning, the temperature trend is upward with a high temps near 50 on Sunday and in the 60s on Monday! A passing warm front will bring a period of rain late Sunday into early Monday, before Monday afternoon turns partly sunny and unseasonably mild (70s not far to our south in Virginia?). A cold front (NOT Arctic) will cross the Commonwealth in the Tuesday-Wednesday time frame, but highs both days may still reach near 50.

Looking ahead to Thanksgiving, we may have a clipper crossing the region with a cold front and Canadian air mass in tow. If so, I'd expect some snow showers in the mountains and perhaps a flurry or snow shower visiting Lancaster on Turkey Day with highs in the 30s (mountains) to low 40s in Lancaster. A shallow Arctic air mass might glance across the Great Lakes and Northeast next Saturday or Sunday, however I can see how another brief warm up develops in early December. The bottom line: Wintry conditions are not here to stay, but I don't foresee an extended warmth spell either. --Hörst


Noon Monday, November 17:
Today's chilly rain will be followed by a tremendous blast of Arctic air that will challenge 55-year-old records tomorrow. While the record low temperature (11) will not to be broken, due to lack of snow cover, the record for "lowest max temp" (36) and "lowest mean temp" (27) are both in play. It's going to be a close call, since the Arctic front will only pass through LanCo this evening and we'll need temps to drop below 36 by midnight in order to break the record for "lowest max temp" for the 18th. Regardless, it's a brutal blast for so early in the season, and wind chills Tuesday will be in the teens and 20s, as afternoon temps struggle just too reach 30 or 32 degrees. A passing flurry is possible, but the big story is the gusty winds and extreme cold. This incredible blast was set up over the past week by a large jet stream ridge over Alaska and the Yukon which has been funneling Arctic air from near the North Pole southward. This is the exact same pattern we saw much of last winter....

A passing clipper may bring a few flurries or a snow shower Wednesday night followed by a secondary surge of Arctic air Thursday into Friday. As a result, late-week temps will remain well below normal with highs only in the mid to upper 30s. A moderating trend will finally develop Saturday into Sunday (highs back in the 40s), but still the period Tuesday through Saturday will go down as one of the most extreme five-day November cold spells on record.

A pattern de-amplification this weekend into the middle of next week will bring welcome relief as temperatures will return to the 50s for a couple of days. The return of milder air Sunday will result in clouds and a chance of showers--a low pressure system may then cut to our west with a mild rain day in store for next Monday. Right now, I see relatively mild conditions lingering into the middle of next week, however a cold front may be on track to cross the Commonwealth around Thanksgiving day. I'll speculate that Thanksgiving and Black Friday may be variably cloudy, breezy, and cool with mountain snow flurries. However, I do NOT see another Arctic blast--only less harsh Canadian air--coming next week...and so temperatures should be closer to "seasonably cool" for Thanksgiving weekend (highs somewhere in the 40s). --Hörst

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