textproduct: Philadelphia/Mt Holly

SYNOPSIS

High pressure over the Mid-Atlantic and Southeast drifts out to sea tonight. A weak cold front passes through the region late Wednesday through Wednesday night. High pressure briefly develops over the area Thursday before moving out to sea Thursday night. A strong cold front approaches late Thursday night and moves across the region on Friday. High pressure returns on Saturday, then another cold front passes through on Sunday. High pressure returns for the start of the new week.

NEAR TERM /THROUGH WEDNESDAY/

Northwesterly flow aloft will become more zonal tonight as a shortwave shifts east away from the area. A shortwave will shift east out of the Northern Great Lakes region Wednesday, passing to the north of the area during the afternoon. At the surface, high pressure centered over portions of the Southeast and Mid- Atlantic will shift eastward away from the region tonight, with a cold front moving through Wednesday afternoon.

An area of high clouds this evening should give way to a mostly clear sky across the area for the majority of tonight, and winds will be light to start. Therefore, effective radiational cooling is expected once again. Snowpack remains across much of the area, which should aid in cooling as well. Towards daybreak, winds will begin to increase out of the southwest ahead of the approaching cold front and as a low-level jet arrives. Temperatures may bottom out during the overnight hours as opposed to near dawn as is typical. Still, minimum temperatures will generally be in the upper teens to low 20s for most of the area. Near the coast, lows closer to 30 are expected.

Wednesday may start out with some clouds, otherwise mid and upper-level clouds move in with the approaching cold front. Winds should increase further out of the southwest, with gusts 20-25 mph. It will be a much warmer day than the past several, with highs in the low-mid 40s for eastern PA and most of NJ, and upper 40s for southern portions of the Delmarva into far southeastern NJ. The cold front should pass through the region by evening. Precipitation chances with this frontal passage appear negligible.

SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/

High pressure will move into the area Wednesday evening before passing overhead late Wednesday night. The high moves offshore on Thursday and continues to move into the western Atlantic. Quiet conditions on tap for Wednesday night and Thursday. Skies clear out Wednesday night, and winds become nearly calm late. Radiational cooling conditions develop, and lows Wednesday night will drop into the teens and 20s. Return flow sets up behind the departing high and ahead of an approaching cold front on Thursday. Warm air advection will be underway, and highs on Thursday climb into the mid and upper 40s north and west of the Fall Line and in and in the low to mid 50s along the I-95 corridor, the Delaware Valley, and southern New Jersey. Highs will be in the mid to upper 50s in southern Delmarva. These temperatures will be some 10 degrees warmer compared to Wednesday. Clouds will increase throughout the day, but there should be enough sunshine and temperatures will be warm enough to allow for the snowpack to at least begin to melt off.

A developing storm system will then impact the area starting Thursday night. Low pressure over the Northern Plains Thursday morning will move into the Great Lakes during the day, then lifts into the Provinces of Ontario and Quebec Thursday night through Friday morning. As is does, a strong cold front will develop as Arctic high pressure builds into the central United States. Meanwhile, a deepening H5 trough with strong shortwave energy will build down through the Great Lakes and Ohio Valley and into the Gulf Coast states and Southeast. The cold front will pass through the region Friday afternoon, and then cold air advection will be underway as Arctic high pressure builds in from the west.

A south to southwest pressure gradient will tighten Thursday night. In addition, a 60 to 80 kt low level jet will pass over the area late Thursday night through Friday morning. However, given southwest flow as well as warm air at the surface and aloft, model soundings are not indicating efficient enough mixing for the strongest of those winds to mix down. Given the gradient, though, winds should increase to 20 to 30 mph with 30 to 40 mph gusts. There is not enough instability for thunderstorms, but will have to monitor how this develops, as the potential for a rumble of thunder or two cannot be ruled out, and if that happens, some locally stronger wind gusts could develop.

Moderate to locally heavy rain develops late Thursday night and continues through Friday morning before beginning to taper off. With cold air advection underway, some snow may mix with the rain as precipitation ends with no significant impacts.

With cold air advection underway, strong northwest winds will develop behind the departing cold front at 20 to 30 mph with 30 to 40 mph gusts Friday evening, diminishing after midnight.

Lows Thursday night will be in the upper 30s to low to mid 40s in the evening, then temperatures will rise through the overnight hours. Highs on Friday will be in the low to mid 50s north and west of the Fall Line and in the mid to upper 50s along the I-95 corridor, southern New Jersey, the Delaware Valley, and Delmarva. Highs will be in the morning, and then temperatures fall in the afternoon as the cold front passes through. Lows Friday night will be in the mid to upper 20s.

There is a 50 to 70 percent chance for over 1 inch of rain for Delmarva and southern New Jersey and a 70 to 80 percent chance for over 1 inch of rain from around Philadelphia north. There is also a 50 to 60 percent chance for over 1.5 inches of rain across the Lehigh Valley, southern Poconos, and northern New Jersey. While mainstem river flooding is not expected, cannot rule out urban and poor drainage flooding due to rainfall and snow melt.

LONG TERM /SATURDAY THROUGH TUESDAY/

High pressure passes through the Southeast and Mid-Atlantic Saturday through Sunday. Quiet, but cold, conditions on tap for Saturday with highs in the upper 30s to low 40s. Lows Saturday night will be in the upper 20s to low 30s. Milder on Sunday with highs in the mid to upper 40s. Some strong shortwave energy will pass north of the area on Sunday and may spark off some showers across the far northern portions of the forecast area, and then some showers across southern portions of the forecast area as a cold front works its way through the region Sunday night.

Highs will be some 10 degrees cooler on Monday compared to Sunday, and then temperatures rebound a bit on Tuesday. Another frontal system may impact the area by the middle of next week.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas.

Tonight...VFR. A period of high clouds, then some clouds between 4000-8000 feet AGL may arrive toward daybreak. West- southwest or southwest winds around 5 knots, becoming light and variable or calm. A southwest wind around 5 knots then should develop toward daybreak. Moderate confidence.

Wednesday...VFR with areas of mainly mid to high level clouds. Southwest winds increasing to 8-12 knots with some gusts to around 20 knots mainly during the afternoon. Low-level wind shear near 40 knots mainly at KRDG and KABE. Low confidence regarding how much wind mixes down to the surface and results in frequent gusts.

Outlook...

Wednesday night through Thursday...VFR/NSW.

Thursday night...IFR in RA and low stratus. SW winds increase to 15 to 20 kt with 25 to 30 kt gusts. A strong low level jet may result in 50 kt of LLWS.

Friday...IFR in RA and low stratus in the morning, then improving conditions through the afternoon. S-SW winds abruptly shift to the W-NW around 18Z at 15 to 20 kt with 35 to 35 kt gusts.

Friday night...VFR. W-NW winds 15 to 20 kt with 30 to 35 kt gusts in the evening, diminishing after 06Z.

Saturday through Sunday...VFR/NSW.

MARINE

A Small Craft Advisory for the New Jersey and Delaware Atlantic coastal waters from 7 AM to 7 PM Wednesday. The winds should remain below advisory criteria on Delaware Bay.

Southwest winds around 10 kt tonight will increase to 15-25 kt Wednesday morning, with gusts as high as 30 kt possible. Winds will become westerly by late Wednesday afternoon, and decrease to 10-15 kt by evening. Seas 1-2 feet tonight, increasing to 3-5 feet Wednesday.

Outlook...

Wednesday night through Thursday...Sub-SCA conditions.

Thursday night through Friday...Gale force winds developing Thursday night in SW flow, then will continue Friday afternoon in NW flow behind a departing cold front. VSBY restrictions in rain. Seas building to 6 to 8 feet Thursday night, then 8 to 12 feet on Friday.

Friday night...Gales will diminish after midnight, then SCA conditions.

Saturday...Sub-SCA conditions.

Saturday night through Sunday...SCA conditions possible.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 7 AM to 7 PM EST Wednesday for ANZ450>455.