MU's Current Conditions: (at 17:22) Temp: 63 F | Dew Point: 53 | Wind: NNW - 5 mph
SPECIAL WEATHER DISCUSSION (Updated twice weekly, but more often when storms threaten.)

* Mild, Mainly Dry Weekend...Cooler Pattern Arrives for Month's End *

September 20, 2018:
The pattern since July has supported consistently warm and wet conditions with, of course, record rainfall in a few unlucky spots. Temperatures have been tracking in the range of "Top 10 warmest" and September has continued this trend, per Tweet below. But Autumn officially begins Saturday evening (9:54pm)...and I see signs of a modest pattern change later next week. Let's break down the details.

First up, we have a relatively seasonable weekend (which will soften the month-to-date temperature anomaly shown below) with no threat of heavy rain. Booyah! Two flies in the ointment, that may prevent a really nice weekend, are low-level easterly flow (today and Friday) and a front that will stall not far to our south Saturday into Sunday. The easterly flow will make for lots of low clouds today and Friday, although peeks of sunshine will make for warm/muggy afternoons (especially west of the Susq River). A passing cold front Friday night may yield a shower overnight, before it stalls to our south early Saturday. Depending on where this front stalls, we may have some lingering clouds throughout the weekend.

Sunday into Monday is somewhat uncertain as a large high pressure builds across New England while the front to our south begins to move slowly back northward with warm, moist air advancing our way. This is a good set up for a period rain...but when? One possibility is a fast return of the front with rain developing Sunday...though perhaps a bit more likely is a slower return with light rain developing Monday midday into Monday night. Either way, I do think the front will lift to our north by Tuesday midday, so that that warm air takes hold with a few showers for the high temps return to the 80s by Wednesday.

Beyond that, a jet stream ridge building over British Columbia will dig a long-wave trough across the Midwest and Mid-Atlantic around September 29 through October 3...and this should infuse the first Autumn air mass into the region. The turn toward cooler conditions will likely come via a pair of cold fronts...the first arriving late on the 26th or 27th...and the second around the 29th. A few showers are possible with each front...and perhaps the first flakes of wet snow will fly on the north shore of Lake Superior by next weekend. Here in Lancaster temps will pull back to at least normal range by next weekend (high ~70 and low ~50)...and perhaps even drop a few degrees below normal for the opening days of October. --Horst

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