Philadelphia Weather: Sunny Veterans Day, Rain Thursday, Much Colder Friday, Wednesday November 11

Day	Sky	High / Low	Rain
Wed	Mostly sunny	65 / 49	Dry
Thu	Rain after 9 a.m.	63 / 42	80%
Fri	Morning rain chance	49 / 36	50%
Sat	Partly sunny	48 / 36	Dry
Sun	Sunny	50 / 35	Dry
Mon	Sunny	51 / 33	Dry
Tue	Mostly sunny	52	Dry

Today

Veterans Day is the pick of the week. After patchy early fog, expect sunshine and a high near 65. The air is calm this morning, with a southwest wind around 5 mph this afternoon.

Tonight

Clouds fill in and the low holds near 49. Calm air gives way to a light east wind around 5 mph after midnight. No rain is in the forecast overnight.

Tomorrow

Rain arrives Thursday, mainly after 9 a.m., with an 80 percent chance and a tenth to a quarter inch possible. The high is near 63, and a light east wind turns west in the afternoon. Thursday night carries a 40 percent chance of more rain, a low around 42 and a northwest wind of 5 to 10 mph.

The week ahead

Friday is the turn to cold: a high near 49, a 50 percent chance of rain before 1 p.m., and northwest gusts to 20 mph. A 20 percent chance of rain returns after 1 a.m. Friday night. The weekend is dry with highs of 48 to 50 and lows in the mid 30s, and Mount Holly forecasters expect Saturday to be the windiest day, with gusts across the region peaking near 35 mph. Highs stay near 50 into Tuesday, and Monday night dips to about 33.

Behind the forecast

A strong upper-level trough is digging into the eastern United States. Mount Holly forecasters expect a surface low to organize off the New Jersey coast Thursday, then deepen as it heads toward the Canadian Maritimes Friday into Saturday, with northwest winds pulling colder air in behind it. Confidence is lowest on rain totals: they hinge on the exact track and on whether a band of rain wraps back around the low, and guidance currently leans toward a more northern track, which would mean less rain here.

Two larger drivers stand out. The first is the Arctic Oscillation (AO), a measure of pressure over the Arctic. When it is negative, the jet stream tends to sag south and cold air reaches the mid-latitudes more easily (NOAA Climate.gov). The Climate Prediction Center’s daily index, built from GFS analyses, fell from about +1.3 on October 27 to -1.2 on November 10, and the November 10 GFS run takes it near -2 on Thursday and Friday before it returns to neutral around November 17. CPC’s Week 3-4 discussion of November 6 ties the dip to an unusually early sudden stratospheric warming, a disruption of the polar vortex high above the Arctic, and notes that models keep it short-lived. That timing fits a sharp but brief cold shot this weekend more than a lasting pattern change. The second driver is El Niño. CPC’s October 8 discussion calls for a strong to very strong event through winter, and CPC expects it to dominate late November into early December, with a ridge from central Canada into the Northeast that limits Arctic outbreaks. The outlooks agree: CPC’s November 8 discussion favored above-normal temperatures and below-normal precipitation in Pennsylvania for November 16 to 22, and the Week 3-4 outlook leans warm in the Northeast with a more active East Coast storm track for November 21 to December 4. These are tendencies, not forecasts.

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Forecast data: National Weather Service, Mount Holly. Forecast issued 1:23 a.m. Wednesday, November 11; forecast discussion issued 10:09 p.m. Tuesday, November 10. Pattern sources: CPC Week 3-4 outlook discussion, CPC daily AO index data, CPC 6-10 and 8-14 day discussion, CPC ENSO diagnostic discussion, NOAA Climate.gov on the Arctic Oscillation.