textproduct: Philadelphia/Mt Holly

SYNOPSIS

Arctic high pressure shifts offshore tonight. 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 NIGHT/

Arctic high pressure will move quickly offshore tonight. 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 will intensify as it dives into the Great Lakes from the upper Midwest tonight through Wednesday morning. This system will move into the Saint Lawrence River Valley by Wednesday evening. This will lead to a period of "warm" advection during the day Wednesday, though we'll really only be advecting a modified arctic airmass. There will also still be some snowpack upstream across southern Virginia and northern North Carolina. Thus, temperatures will still end up a few degrees below normal across the board.

Precipitation develops Wednesday morning initially over the southern Poconos and the Lehigh Valley before spreading into New Jersey and northern Delmarva. In terms of p-type, generally expecting snow initially over the southern Poconos and far northwest New Jersey, and precipitation will remain mostly snow in the southern Poconos and far northwest New Jersey throughout the duration of the event. Some mixing with rain or freezing rain may occur before precipitation ends though. For the Lehigh Valley, precipitation will start out as a mix, or perhaps mostly rain south of I-78. Any wintry mix or snow here will quickly change to rain by late morning and into the afternoon. For southern New Jersey, the I-95 corridor including Philadelphia, and into northern Delmarva, all rain is now expected due to slightly later onset time of precipitation. For this area, the rain probably won't begin until the afternoon.

The highest snow accumulations are expected to be across the Pocono Plateau, especially elevations above 1,000 feet where roughly 1-3 inches is forecast. The Winter Weather Advisory remains in effect for Wednesday for this area. Elsewhere across the higher elevations of northwest New Jersey, Lehigh Valley, and northern Berks County, generally less than 1 inch of snow is expected, if any. In these same areas, including the Pocono Plateau, some brief freezing rain could result in a glaze of ice from late morning or afternoon. The probability of this is low, generally around 20-40%, and any glaze of ice would be confined to the highest elevations and likely not impact either the morning or evening commutes. Thus, we have not expanded the advisory to include any of these areas at this time.

Rainfall amounts will be roughly near 0.10" or less, perhaps closer to 0.25" north of I-78. The chance of measurable rainfall drops to about 30-50% for far southern New Jersey and southern portions of our Delmarva counties.

South to southwesterly flow ramps up ahead of the approaching low and with the low passing north of the area. SSW winds will increase to around 10-20 mph, with gusts near 30 mph possible at times. Highs warm back into the mid 30s for the southern Poconos, and otherwise in the upper 30s to 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, and perhaps close to 50 degrees near the coast.

The clipper's cold front will push through the area and offshore quickly overnight with breezy conditions persisting overnight. Temperatures will drop to around the mid 20s to low 30s by dawn Thursday, producing wind chills in the teens and 20s.

SHORT TERM /THURSDAY THROUGH FRIDAY/

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).

Daytime Friday looks to be largely dry as an axis of weak high pressure slides across our area. Air temperatures look to be similar to Thursday with highs in the mid 30s to near 40. However, it will feel a bit warmer as winds continue to relax and turn southwesterly ahead of the next approaching wave Friday night. Apparent temps are forecast to be in the upper 20s to low 30s. A slight chance (10-20%) that some light snow and/or rain begin to move into the area in the afternoon hours.

LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/

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 night...A low pressure system 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 40 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 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. Any northwesterly breeze should 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 shifts offshore.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

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

Tonight...VFR with increasing coverage and lowering of ceilings. SSW winds 5-10 kts. High confidence in prevailing conditions.

Wednesday...

For the Lehigh Valley terminals...a brief period of snow, mixed with rain will change to all rain between 15z-18z, with periods of MVFR restrictions expected by 18z. MVFR conditions expected to continue through the afternoon and evening. Winds will be out of the south/southwest around 10-15 kt with gusts 20-25 kt. Moderate confidence overall.

For the I-95 terminals...VFR to start with rain showers starting at 18z. Conditions may drop to MVFR around 22z-23z with lower CIGs moving in but confidence remains low on that, and some sites, especially further south ones like KPHL/KILG may stay VFR throughout. Winds out of the south/southwest around 15 kt with gusts hovering near 25 kt. Low confidence.

For the South Jersey terminals...primarily VFR conditions expected. Heavier rain and lower conditions should stay to the north at least through 00z. Rain showers move in between 19z-20z but should be light and CIGs/VSBYs should stay high enough to stay VFR. Winds out of the south/southwest around 15-20 kt with gusts 25-30 kt. Moderate confidence.

Outlook...

Wednesday night...VFR. Gusty WNW winds developing.

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

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

MARINE

Advisory conditions developing tonight with SSW winds increasing to 20-25 kts with gusts up to 30 kts. Seas building 4-6 feet. Small Craft Advisory remains in effect for all coastal waters for tonight.

SW winds increase further on Wednesday to 25-35 kts, with gusts up to 40 kts at times across the Atlantic coastal waters. The Gale Watch was upgraded to a Gale Warning, which is in effect Wednesday through Wednesday evening. However, an additional period of gale force wind gusts are possible during the pre-dawn hours early Thursday morning. Some brief gale force gusts are possible across Delaware Bay, but the probability and duration of this is too low to warrant a Gale Warning at this time. Advisory conditions will continue Wednesday night.

Outlook...

Thursday...Small Craft Advisory conditions continue through the day then gradually subside at night.

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

Saturday and Sunday...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 until 6 AM EST Thursday for ANZ430-431. Small Craft Advisory until 6 AM EST Wednesday for ANZ450>455. Gale Warning from 6 AM to 10 PM EST Wednesday for ANZ450>455.