Philadelphia Weather: Showers and a Possible Thunderstorm Today, Rain Friday, Colder Weekend, Thursday November 12

Day	Sky	High / Low	Rain
Thu	Showers, possible thunderstorm	63 / 42	90%
Fri	Rain before 1 p.m.	47 / 36	100%
Sat	Partly sunny, breezy	48 / 35	Dry
Sun	Mostly sunny	51 / 36	Dry
Mon	Sunny	52 / 33	Dry
Tue	Sunny	53 / 33	Dry
Wed	Sunny	58	Dry

Today

Showers, with a thunderstorm possible, and a high near 63. The chance of rain is 90 percent. National Weather Service forecasters in Mount Holly expect most of it to hold off until the afternoon, when showers could be moderate to briefly heavy. Rainfall of 0.10 to 0.25 inch is forecast, with more under any thunderstorm. Wind is from the east at 5 to 10 mph, turning south in the afternoon.

The Storm Prediction Center has the area in a Marginal Risk, its lowest category, for this afternoon. The main concern is a locally damaging wind gust, with a small chance of hail. Forecasters describe confidence in any severe storms as lower than usual because the atmosphere is only weakly unstable.

Tonight

Cloudy with a low around 42. Showers are likely again, mainly after 4 a.m., with a 60 percent chance and less than a tenth of an inch. Wind turns north at 5 to 10 mph with gusts to 20 mph.

Tomorrow

Rain, mainly before 1 p.m., with the chance listed at 100 percent and another 0.10 to 0.25 inch. It will be much cooler, with a high near 47 and a northwest wind of 10 to 15 mph. Friday night stays mostly cloudy with a 30 percent chance of rain, mainly after 1 a.m., and a low around 36.

The week ahead

Saturday is dry but chilly, with a high near 48 and a northwest wind around 10 mph gusting to 20 mph in the city. Mount Holly expects Saturday to be the windiest day across the region, with peak gusts near 30 mph. Sunday through Tuesday stay sunny with highs of 51 to 53, and lows of 33 on Monday and Tuesday nights bring the city close to freezing. Forecasters say a first freeze is likely in parts of the region during this stretch. Wednesday turns milder, with a high near 58.

Behind the forecast

An upper-level trough is digging into the eastern United States. A surface low crossing the Mid-Atlantic hands its energy to a new low that organizes off the New Jersey coast later today, then deepens as it heads toward the Canadian Maritimes Friday into Saturday, pulling colder air in on northwest winds behind it. Mount Holly says the models now agree better on the track but not on the smaller disturbances inside the trough, so rain totals are the least certain part of the forecast. With the track trending north, forecasters trimmed amounts to under half an inch from Philadelphia southward, with 0.50 to 1.25 inches more likely north of the Pennsylvania Turnpike.

The driver to watch is the stratospheric polar vortex, the band of strong winds high over the Arctic. The Climate Prediction Center’s Week 3-4 discussion of November 6 reports an unusually early sudden stratospheric warming underway, which weakens the vortex. CPC’s daily Arctic Oscillation index, a measure of how tightly cold air is held near the pole, fell from +0.77 on October 31 to -0.88 on November 9. A disrupted vortex tends to be followed, a few weeks later, by a wavier jet stream and a negative AO that can let Arctic air spill south, though NOAA’s Climate.gov explainer stresses that these events do not always reach the surface. Working against it is a strong El Niño. CPC expects the warming to be short-lived, notes the GEFS model ensemble weakening the negative AO by the end of its second week, and sees El Niño remaining the main influence. Its outlooks agree: the 6-10 and 8-14 day outlooks issued November 11 favor above-normal temperatures in Pennsylvania from November 17 to 25, with confidence rated 5 and 4 out of 5, and the Week 3-4 outlook for November 21 to December 4 puts its best odds of warmth over the Great Lakes and Northeast. For Philadelphia, that leans toward this weekend’s chill being a brief shot and not the start of a lasting cold pattern.

Tools for today: Radar, Commute Index and Live Traffic Cameras.

Forecast data: National Weather Service, Mount Holly. Forecast issued 4:07 a.m. Thursday, November 12; forecast discussion issued 6:23 a.m. Pattern sources: CPC Week 3-4 outlook discussion, CPC 6-10 and 8-14 day prognostic discussion, CPC daily Arctic Oscillation index, CPC ENSO diagnostic discussion, NOAA Climate.gov, Understanding the Arctic polar vortex.