textproduct: Philadelphia/Mt Holly

SYNOPSIS

The center of arctic high pressure gradually weakens and shifts to our east today. Low pressure tracks to our north during Wednesday with its cold front crossing our region Wednesday evening. A cold front moves through Friday night or Saturday, then low pressure along it tracks nearby Saturday into Sunday. Arctic high pressure centered well to our west builds into our region during early next week.

NEAR TERM /THROUGH WEDNESDAY/

High pressure over the area slides offshore later today. Arctic airmass over the area results in high temperatures some 10 to 15 degrees below normal, generally in the 20s in the southern Poconos to the low to mid 30s for most of the region, and in the mid to upper 30s for southern Delmarva and along the Jersey Shore. Skies clear out this morning, then high clouds build back into the region as the afternoon progresses.

Return flow sets up behind the departing high, and after temperatures drop in the evening, temperatures will mostly hold steady through the overnight hours tonight generally in the mid to upper 20s.

A clipper system over central Canada just north of the Northern Plains will intensify as it dives into the Great Lakes tonight through Wednesday morning. This system will move into western and central New York on Wednesday before lifting into northern New York by Wednesday evening.

Precipitation develops Wednesday morning initially over the southern Poconos and the Lehigh Valley before spreading into New Jersey and most of Delmarva. In terms of p-type, generally expecting snow over the southern Poconos and far northwest New Jersey, and snow precipitation will mostly be snow in the southern Poconos and far northwest New Jersey throughout the duration of the event. For the Lehigh Valley, precipitation will start out as snow, followed by a brief period of wintry mix which may include freezing rain for some spots, then rain by Wednesday afternoon. For southern New Jersey, the I-95 corridor including Philadelphia, and into northern Delmarva, a rain/snow mix is possible initially before changing to plain rain late Wednesday morning. Rain then continues through the day.

The highest snow accumulations are expected to be in the southern Poconos, and especially the higher elevations, with as much as 2 to 4 inches. Up to an inch is likely for northern New Jersey and the Lehigh Valley, while trace amounts are possible elsewhere. In the wintry mix/freezing rain areas, generally a glaze of ice is possible.

Southerly flow ramps up ahead of the approaching low and with the low passing north of the area, and this allows for strong warm air advection. Highs warm back into the low to mid 30s for the southern Poconos, and otherwise in the low 40s for northern New Jersey and the Lehigh Valley. For southern New Jersey, the Delaware Valley (including Philadelphia), and Delmarva, highs will be in the mid to upper 40s.

SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/

As as stronger upper-level through evolves across the East during this time frame, strong low pressure will lift up across northern New England to the Canadian Maritimes. A cold front shifts to our east Wednesday evening and any precipitation is expected to quickly come to an end. A notable breeze may occur as cold air advection increases combined with enough vertical mixing.

Plenty of cyclonic flow during Thursday and Thursday night given the presence of the upper-level trough. A cold west-northwest breeze will also be gusty and areas of stratocumulus clouds may result. There will be a lake response and it is possible that some flurries and snow showers get into portions of our Pocono region and far northwestern New Jersey at times. Temperatures continue below average, and with the wind factored in the wind chill during the day will be mostly in the 20s across the region. The winds should diminish at least some at night, however wind chills look to get down into the teens for most by daybreak Friday (single digits in the Poconos).

LONG TERM /FRIDAY THROUGH MONDAY/

Summary...Below average temperatures with another blast of arctic air late in the weekend and/or 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 surface low developing along a trailing cold front with this low tracking near our area.

For Friday...As our region is on the southern side of a large upper- level trough in central to eastern Canada, the axis of weak high pressure slides across our area. A low pressure system however tracks well to our north Friday night into Saturday morning with a cold front crossing our area during Saturday. This system looks to be moisture starved overall and therefore may not produce much in the way of precipitation. Some differences in the guidance on this therefore ran with the NBM guidance which has 20-40 PoPs across the area.

For Saturday and Sunday...The cold front associated with the aformentioned system may tend to stall 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 an upper-level trough across the Northeast. This may result in a surface low developing along the trailing cold front, which then tracks near our area. Some guidance is weaker with this feature and therefore just has a front crossing our area, while others have a more pronounced wave along the front. 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 stray away from the NBM which paints chance PoPs (around 30 percent) Saturday night into Sunday morning. Any snowfall will depend on the wave developing and also tracking close enough as a rather cold air mass heads southward. Temperatures still below average, however Sunday looks to be several degrees colder than Saturday.

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

AVIATION /12Z TUESDAY THROUGH SATURDAY/

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

Today...VFR. LGT/VRB winds in the morning, becoming S 5 to 10 kt by 18Z. High confidence.

Tonight...VFR. S-SW winds 5 to 10 kt. High confidence.

Wednesday...Sub-VFR in RA and/or SN, with SN most likely at KRDG/KABE. SN changes to RA in the afternoon. S winds 10 to 15 kt with 20 to 25 kt gusts. Moderate confidence.

Outlook...

Wednesday night...Conditions improve to VFR. Southwest to wind gusts may gust to 20 knots at times.

Thursday...VFR. West-northwest wind gusts to around 25 knots.

Friday and Saturday...A period of sub-VFR conditions possible with snow or rain, especially Friday night and Saturday morning. Westerly winds may gust to about 20 knots on Saturday.

MARINE

Will cancel the Small Craft Advisories that were in effect from Cape Henlopen DE to to Fenwick Island DE until 7 am for winds gusting to 25 kt and seas up to 5 feet as conditions continue to subside. Sub-SCA conditions expected thereafter, and sub-SCA conditions on all other waters for today for north winds 10 to 15 kt, becoming south late this afternoon.

South to southwest winds increase to 15 to 25 kt with gusts up to 30 kt tonight. A SCA is now in effect for all waters for tonight.

On Wednesday, VSBY restrictions in rain expected. A Gale Watch is now in effect for the ocean waters for Saturday as southwest winds will range from 20 to 30 kt with gusts up to 35 kt. For Delaware Bay, winds will range from 15 to 25 kt with gusts up to 30 kt.

Outlook...

Wednesday night...Small Craft Advisory probable. Gale force wind gusts possible, especially during the evening, and a Gale Watch has been issued.

Thursday...Small Craft Advisory conditions gradually subside at night.

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

Saturday...Small Craft Advisory probable, with gale force wind gusts possible.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Weather Advisory from 6 AM to 6 PM EST Wednesday for PAZ054-055. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 6 PM this evening to 6 PM EST Wednesday for ANZ430-431. Small Craft Advisory from 6 PM this evening to 6 AM EST Wednesday for ANZ450>455. Gale Watch from Wednesday morning through Wednesday evening for ANZ450>455.