textproduct: Philadelphia/Mt Holly

SYNOPSIS

Low pressure tracks to our north during today with its cold front crossing our region this evening. Another cold front moves through Friday night, then weak low pressure may develop along it as it tracks nearby Saturday night into Sunday. Arctic high pressure centered well to our west builds into our region during early next week.

NEAR TERM /THROUGH TONIGHT/

Dry air ahead of the main precipitation shield has inhibited the advancement of much rain/snow from reaching the ground early today. Heavier precipitation rates are just arriving across the Poconos and just east of I-81 which will gradually make its way east across the region this afternoon. Snow will remain the primary precip type across the Poconos. Areas around the Lehigh Valley and northern NJ are still at risk of seeing a wintry mix despite surface temperatures creeping above freezing. Once precipitation begins to fall, temperatures could wet bulb back down near or below freezing for some time.

Arctic high pressure has moved offshore, and temperatures are markedly warmer now that they were 24 hours ago. A clipper system over the Great Lakes later this morning lifts to the north and east through central New York and into northern New York and northern New England, along the Canadian border this afternoon and evening. This low then moves into the Canadian Maritimes late tonight.

Warm air advection will be underway today as south to southwest flow increases with the passage of this low. Precipitation develops later early this afternoon, initially over the southern Poconos and western portions of the Lehigh Valley. Precipitation starts out as snow, and will remain as snow across Carbon and Monroe counties for the rest of the afternoon and evening. For Berks county, as well as Lehigh and Northampton counties, more of a wintry mix, with a brief period of freezing rain. With a light icing becoming likely, will go ahead and expand the Winter Weather Advisory to include these counties. For Carbon and Monroe counties, snow may become a wintry mix late with some light icing. Overall, 1 to 3 inches of snow, with perhaps locally as high as 4 inches above 1000 feet, will accumulate.

For the rest of the region, although a brief period of rain and snow is possible for portions of northern New Jersey and the Lehigh Valley, precipitation will will be plain rain. A warmer day on tap with highs in the upper 30s to low 40s north and west of the Fall Line, and and in the mid to upper 40s across southern New Jersey, the Delaware Valley, and Delmarva.

Precipitation tapers off this evening, and conditions dry out tonight. A cold front passes through the region behind the departing clipper system. Winds turn to the west to northwest and increase to 10 to 15 mph with gusts up to 25 mph late tonight. Lows mostly in the 20s to low 30s.

SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/

Plenty of cyclonic flow during Thursday and Thursday night given the presence of an upper-level trough. A cold west- northwest wind will be in place and areas of stratocumulus clouds should be around. Model forecast soundings suggest steepening low-level lapse rates during the day Thursday, and with cold air advection occurring within strong flow, breezy to windy conditions look to occur. Peak gusts should be up to 35 mph, although a little higher mainly in the higher terrain of the Poconos. There will be a Great lakes response and it is quite possible that some snow showers get into portions of our Pocono region and northern New Jersey at times in the afternoon and evening. Some of the guidance suggest that a narrow lake effect streamer or two remains organized all the way into the Poconos and even northern New Jersey. A streamer may be connected all the way back to Lake Huron. Given the increased potential, started by adding some slight chance to chance PoPs (20-30 percent) across the northern areas. Some flurries cannot be ruled out even farther south. Temperatures continue below average, and with the wind factored in it will feel noticeably colder. The winds should diminish some at night, however wind chills look to get down into the teens by daybreak Friday (single digits in the Poconos and northwest New Jersey).

Daytime Friday looks to be mainly dry as an axis of weak high pressure slides across our area. Air temperatures look to be similar to Thursday, which is several degrees below average. However, it will not be as harsh as the winds will be turning lighter ahead of the next system arriving Friday night. The trend in much of the guidance has been to weaken the clipper as it crosses the Appalachians, however there remains many moving parts in the active flow and therefore kept what the NBM has (chance PoPs).

LONG TERM /SATURDAY THROUGH TUESDAY/

Summary...Below to well below average temperatures with another blast of arctic air late in the weekend and early next week.

Synoptic Overview...A series of stronger shortwaves sliding across southern Canada and the northern tier of the U.S. will continue to result in an upper-level trough as it pivots across central to eastern Canada. This will amplify an upper-level trough more than once across the Northeast and Mid-Atlantic regions. This will keep temperatures on the cold side. This looks to favor mostly clipper systems (quick moving) with their associated cold fronts crossing our region. A stronger trough however over the weekend may result in a weak surface low developing along a trailing cold front with this low tracking near our area.

For Saturday and Sunday...A front may be stalled to our south. The center of arctic high pressure across the Midwest and northern Plains will gradually build eastward. Ahead of this feature, a strong shortwave may end up amplifying into a strong upper-level trough across the Northeast. This may result in a weak surface low developing along the trailing cold front, which then tracks near our area. Some guidance continues to be weaker with this feature and therefore just has a front crossing our area, while others have a zone of some stronger forcing with a more pronounced mid level wave. Given the rather active upper air pattern with many moving pieces, the guidance will likely continue to show varying solutions. As a result, did not adjust from the NBM which paints chance PoPs (peaking up to 40 percent) Saturday night into Sunday morning. Any snowfall will depend on the track and strength of the wave and therefore the zone of ascent. Temperatures still below average, however Sunday is forecast to be several degrees colder than Saturday.

For Monday and Tuesday...As the parent upper-level low starts to exit, the center of arctic high pressure builds into our area during Monday and Tuesday. This will result in very cold days with temperatures well below average, particularly on Monday when a breeze will add to the cold/chill factor. The northwesterly breeze should then be easing as the high builds in more and therefore erodes the pressure gradient. Another clipper system may approach Monday night, however this feature will depend on the upper-level trough amplifying once again as the surface high starts to weaken and shift offshore.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

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

Rest of today (through 00Z)...VFR for the next hour or so before precip arrives with MVFR conditions west to east this afternoon. Mainly RA, but a brief period of -RASN across RDG/ABE cannot be ruled out. High chance of snow further north and west. For KTTN- KPNE- KPHL- KILG, VFR, lowering to MVFR after 20Z in RA. For KMIV/KACY...Conds should remain VFR throughout the day, though brief MVFR conds possible late in the day in RA. S-SW winds 5 to 10 kt in the morning, increasing to 10 to 15 kt with 20 to 25 kt gusts. Moderate confidence overall.

Tonight...Any MVFR conds lift to VFR from 03Z-06Z, then VFR. SW winds 5 to 10 kt, becoming W-NW 10 to 15 kt with gusts up to 20 kt after 06Z. Moderate confidence overall.

Outlook...

Thursday...VFR overall. A few snow showers possible near mainly KABE in the afternoon and evening. West-northwest winds 15-20 knots with gusts to around 30 knots, diminishing some at night.

Friday...Mostly VFR. A low chance for some light snow in the afternoon and at night. Westerly winds may gust to 20 knots during the day.

Saturday and Sunday...A period of sub-VFR conditions and some snow possible Saturday night into Sunday morning. West to west-northwest winds may gust to 20-25 knots.

MARINE

South to southwest winds generally 20 to 30 kt with 35 to 40 kt gusts on the ocean waters today through this evening. Gale warnings remain in effect for this time, then a Small Craft Advisory will be needed after the Gale Warning ends. SCA remains in effect for Delaware Bay for today and tonight as winds will generally range from 15 to 20 kt with 25 to 30 kt gusts.

VSBY restrictions in rain by this afternoon and through this evening.

Outlook...

Thursday...Gale force wind gusts possible. The winds should diminish some later at night.

Friday...The conditions may remain below Small Craft Advisory criteria.

Saturday and Sunday...Small Craft Advisory probable. Gale force wind gusts possible, mostly during Sunday.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Weather Advisory until 6 PM EST this evening for PAZ054- 055-060>062. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EST Thursday for ANZ430-431. Gale Warning until 10 PM EST this evening for ANZ450>455.