textproduct: Philadelphia/Mt Holly

SYNOPSIS

Low pressure and its associated strong cold front moves through the area and offshore overnight. High pressure centered near the Gulf Coast begins to build in through Tuesday, before another cold front crosses our area later Wednesday. High pressure then starts to arrive later Thursday and remains nearby through the start of next weekend.

NEAR TERM /THROUGH MONDAY/

A tricky and complex forecast is in store over the next 12 hours or so across the region with several boundaries over the area. A weak low located near the mouth of the Delaware Bay is slowly lifting north along a warm front which is the culprit for showers across much of New Jersey and Pennsylvania this afternoon. Across the Delmarva, showers have tapered off for the moment, as the warm front has already cleared this area. Temperatures in its wake have warmed into the low to mid 60s. As the front continues to lift north, should see a break in shower activity for the remainder of the area later this afternoon and into this evening. In fact, temps may continue to slowly rise a couple more degrees, even after sunset.

In any event, another round of showers is expected later this evening as the primary low and cold front will be approaching and crossing through the area. Timing with this round looks to be generally after 8-11 PM from southwest to northeast and continuing through much of the overnight hours. There also remains a risk of an embedded thunderstorm south of the warm front, however, this potential looks to be really isolated as the better convective environment is well south of our forecast area. In terms of rainfall amounts, an additional tenth to quarter of an inch is possible, with locally higher amounts possible. Although it'll be warm early in the night, temps will fall mainly into the 40s, with 30s in the Poconos following the frontal passage.

By Monday morning, the cold front will be located off the coast and low pressure will be tracking up through New England. Any remaining showers are expected to cease shortly after daybreak. Cannot rule out an isolated flurry up across the Pocono Plateau as some colder air filters in. As the day progress, the deepening upper-level trough will begin to pivot into the region delivering a shot of modified, arctic air. With cold air advection increasing under cyclonic flow, this should support a pretty hefty strato-cumulus cloud deck, resulting in mostly cloudy to at times overcast skies. Thus, high temperatures are expected to peak by mid-day with highs only in the mid 40s to mid 50s. Northwest, post-frontal winds will increase as well, occasionally gusting around 20-25 mph. This will result in wind chill values falling into the 20s and 30s, especially by late afternoon.

SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/

A deepening upper-level trough will delivery a shot of modified arctic air into the region, resulting in the coldest temperatures so far this season Monday night through Tuesday.

The deep upper-level trough begins to move into the region on Monday night with the trough axis located over the East Coast by Tuesday morning. Some of the coldest air of the season, so far, arrives as 850 mb temperatures will be as cold as -10C to -12C. Despite partly to mostly cloudy skies, low temperatures will be below freezing for all on Monday night, except perhaps for areas along the immediate coast. Wind chill values should fall into the teens and 20s. Some guidance also indicates some snow showers, especially in the Poconos and Lehigh Valley overnight. Looks like this is tied to lake-effect upstream, so wouldn't be surprised if some flurries survive the trip with such strong flow at the surface and the trough overhead.

Temperatures on Tuesday will feel more like December than November, with highs only in the low to mid 40s (30s in the higher terrain). The other item of note, is that the Mid- Atlantic region will be situated between strong low pressure over the Canadian Maritimes and strong high pressure over northern Gulf coast. This results in a tight pressure gradient over the area. West-northwest winds near 15-25 mph are anticipated with wind gusts 30-40 mph before diminishing later in the day and especially Tuesday night. The area will remain nestled within the upper-level trough through the afternoon before departing the region by Tuesday night. This will allow the airmass to moderate aloft. So, while it will still be quite chilly, low temperatures for Tuesday night will not be as cold compared to the night before.

LONG TERM /WEDNESDAY THROUGH SATURDAY/

Summary...Airmass moderates some before cooling some to start the weekend. Any precipitation chances look rather limited as of now.

Synoptic Overview...Shortwave energy should maintain a trough from the Great Lakes and the Northeast Wednesday and Thursday. This may amplify across the northeast Friday into Saturday before shifting eastward with some ridging approaching from the west into Saturday. At the surface, a cold front crosses our area later Wednesday, then high pressure gradually approaches from the west later Thursday before settling more into our area Friday and Saturday.

For Wednesday and Thursday...While the main upper-level trough has already departed, additional shortwave energy diving southeastward from the Midwest and Great Lakes should maintain a trough into the Northeast and Mid-Atlantic regions through Thursday. As this occurs, surface high pressure centered across the Southeast U.S. will track eastward. A notable breeze looks to be present for both Wednesday and Thursday as a modest pressure gradient persists. A weak clipper system will also track across eastern Canada on Wednesday, with a weak cold front crossing our area later Wednesday. Overall, this front looks to be moisture-starved. Aside of a stray shower up into the higher terrain, much of the area should remain dry. Temperatures will be warmer compared to earlier in the week, but largely will remain a few degrees below average.

For Friday and Saturday...The Mid-Atlantic region looks to be situated on the backside of the aformentioned upper-level trough as it may initially amplify Friday, then move away by later Saturday. High pressure still looks like it will be centered off to the west, so cool temperatures continue into the start of next weekend. As of now, it looks mainly dry given the arrival of surface high pressure.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

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

Rest of Today...Mainly MVFR conditions expected, although temporary improvements to VFR are possible later this afternoon. Showers likely through 21-22Z, before becoming more sporadic in nature. East- southeast winds around 5-10 kt. Moderate confidence.

Tonight...Prevailing MVFR conditions likely, with temporary improvements to VFR early -- included a TEMPO group at some terminals, where confidence is higher. Otherwise, showers likely overnight with perhaps maybe a rumble of thunder near KACY/KMIV but occurrence is not great enough to include in TAF at this time. Southerly winds around 5-10 kt, becoming W-NW after 06Z as a cold front passes. Low confidence, overall.

Monday...Primarily VFR ceilings, although a MVFR stratocumulus cloud deck will move into the area, especially by mid-day. A slight chance of a rain shower. West-northwest winds around 10 kt with gusts up to 20 kt possible. Moderate confidence.

Outlook...

Tuesday...VFR. West-northwest wind gusts 25-35 knots during the day, diminishing some at night.

Wednesday through Friday...VFR. Wind gusts 20-25 knots, diminishing each night.

MARINE

No marine hazards are in effect through tonight. A Small Craft Advisory has been issued beginning at 1 PM Monday for both the Atlantic Coastal Waters and Delaware Bay.

Through tonight, southerly winds around 10-15 kt will quickly transition to a west-northwest wind in wake of a cold frontal passage. Seas of 3-4 feet. Showers likely with perhaps and isolated thunderstorm. Dense marine fog possible.

On Monday, west-northwest winds will increase to around 10-20 kt with gusts up to 25 kt possible during the afternoon. Seas of 3-5 feet. Outside of developing SCA conditions, fair weather is expected.

Outlook...

Monday night...Small Craft Advisory in effect. Northwest winds increasing to 20-30 kts and seas building to 3-5 feet.

Tuesday...Westerly winds increasing to 25-35 kts with gusts up to 40 kts possible. Seas 4-7 feet. Gale Watch is in effect Tuesday through Tuesday evening.

Wednesday through Friday...Small Craft Advisory conditions probable.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 1 PM Monday to 6 AM EST Tuesday for ANZ430-431-450>455. Gale Watch from Tuesday morning through Tuesday evening for ANZ430-431. Gale Watch from Tuesday morning through Tuesday evening for ANZ450>455.