textproduct: Philadelphia/Mt Holly

SYNOPSIS

A warm front associated with a clipper system will lift through the region by this afternoon, with a cold front following it this evening. Weak high pressure will follow in its wake, building in through Wednesday night before sliding offshore Christmas Day with another weak low pressure system moving through. An active pattern will take shape for the weekend with the potential for a series of storm systems impacting the area into early next week.

NEAR TERM /THROUGH TONIGHT/

A period of light snow, some wintry mix and rain will impact portions of the region through this morning before ending into the early afternoon hours. There have been no significant changes to the forecast amounts or anticipated impacts for the system through today. Breezy conditions will develop overnight as a cold front pushes offshore, including potential for gusts near 40-50 mph for higher elevations of the Poconos and northwest New Jersey.

Canadian high pressure has shifted offshore. Latest analysis as of 6 AM shows temperatures in the low to mid 30s across much of the area with dewpoints in the mid 20s to low 30s. Thus, wetbulbs are near to just below freezing, and slowly rising as weak warm advection is underway. Radar indicates steadier precipitation is gradually working its way toward the surface, eroding and cooling the dry low level air. This trend will continue through mid morning as the developing clipper system approaches from the Great Lakes region, with precipitation reaching peak intensity and coverage during the mid morning hours before beginning to taper off into the afternoon.

Guidance remains in decent consensus that we will see at least a few hundredths of QPF as far south as the Philly through this morning, and as much as 0.1-0.25" near and north of I-78. There is still some uncertainty on how far south measurable snowfall will occur into the Philly metro, southern New Jersey, and northern Delaware. The warm advection nature of this system should yield poor snow ratios. We are forecasting snow ratios near 5:1 to 10:1 at best.

As for forecast snowfall accumulations, generally expecting less than 1" for areas south of I-78 down into the Philly metro and adjacent areas of interior southern New Jersey, far northern Delaware, and the northern Jersey Shore (LBI and north). Areas near and north of the I-78 corridor still have the best chances of receiving a plowable 1-3" of snowfall. The usual higher elevations above 1,200 feet of northern New Jersey and eastern Pennsylvania could see snowfall totals as much as 3-4" of snow, but the probability of this is moderate at best (~20-40%), and will be conditional on exactly how much QPF we can squeeze out of this weak system. Our forecast is for mostly 1-3" near/north of I-78.

There also remains potential for a light glaze of ice, mainly for elevations above 1,000 feet north of I-78, as precipitation could change to rain there before temperatures warm above freezing.

While we do have sufficient forcing and moisture for snow, the dynamics at play are vastly weaker than the snow storm two weekends ago. So the over performing potential we saw with that system is very low. With that said, snow falling this morning could certainly cause travel issues and slippery/icy roads for all areas north and west of I-95, despite the light snowfall amounts for some places.

The Winter Weather Advisory was expanded to include Morris, Warren, Northampton, and Lehigh Counties based on the latest guidance and trends. The more notable impacts are anticipated to occur within the advisory area through early this afternoon, where snowfall totals ranging mainly from 1-3" and a light glaze of ice is possible.

Chances for precip and QPF are lower south of the Philly metro into far southern New Jersey and Delmarva. This region will have much less moisture and forcing, and thus lower measurable precip if any. It will also be warmer here, with wetbulb temperatures mainly above freezing by the time any precip of the measurable QPF variety arrives after dawn. So any snow that does fall here is most likely to be "white rain" (snow melting as it falls).

Any lingering precipitation will be ending by early afternoon, then temperatures should start warming up by a few degrees. High temperatures by late afternoon will range from the mid 30s north of I-78 to the mid to upper 40s for far southern New Jersey and Delmarva. Skies should remain mostly cloudy for the remainder of the day, and warm advection gradient winds will not be very strong at all. Most areas should eventually warm above freezing into the afternoon, which should help to start melting away our light snowfall accumulations and improve lousy road conditions for the holiday travelers.

Low temperatures tonight into Wednesday morning are mostly in the mid to upper 30s (above freezing) south of I-78, and close to 30 degrees north of there. So any icy roads from refreezing snow melt or slushy should be confined to untreated roads north of I-78. Winds will also increase tonight as a cold front pushes offshore, which should help to dry things out a bit more too. Guidance indicates potential for wind gusts near 40-50 mph across the Pocono Plateau and higher elevations of northwest New Jersey developing after midnight tonight into Wednesday morning. This will be due to strong low level winds developing beneath an inversion and the Bernoulli effect accelerating winds across the ridge lines and mountain peaks, and significant wind gusts are not expected outside of these areas. A Wind Advisory may need to be considered. Elsewhere, gusts around 25-35 mph are forecast.

SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/

Northwesterly flow aloft will continue to overspread the area through the period, with one embedded shortwave departing to the east on Wednesday and another moving through the region on Thursday. At the surface, high pressure centered over the Great Lakes vicinity will shift eastward into the region through the day Wednesday. The high's influence will be short lived, with a weak area of low pressure passing through on Christmas day.

Wednesday will likely start with mostly clear skies, with increasing high clouds during the afternoon as the next system begins to approach. High temperatures look to be in the low to mid 40s for most. For the Poconos, temperatures could struggle to get out of the 30s, while they could approach 50 across the Delmarva and perhaps far southern NJ. While it will be dry, it does look to be breezy, with northwest winds gusting in the 25-35 mph range areawide. A few gusts of 40-45 mph can't be ruled out across the Poconos. Wednesday night appears to be mostly cloudy, with diminishing winds. Lows look to be in the low 20s across the Poconos and into far northwest NJ, upper 20s for most of eastern PA and NJ, and low 30s for the Delmarva and the urban corridor.

With the weak low pressure system moving through on Thursday, there will be a chance for a little light rain or snow for the southern half or so of the area during the morning hours. Amounts appear negligible at this time, and any light precipitation that does occur should depart by afternoon. Skies will remain mostly cloudy, however. High temperatures look to be in the low-mid 40s for most, slightly cooler in the Poconos and slightly warmer in the Delmarva. Skies look to remain mostly cloudy Thursday night, with lows in the upper teens to lower 20s across much of eastern PA and northern NJ, and mid to upper 20s for southeastern PA, southern NJ, and the Delmarva.

LONG TERM /FRIDAY THROUGH MONDAY/

Significant uncertainty remains regarding the extended forecast, but there is high confidence that multiple storm systems will impact the area as a very progressive pattern takes shape.

It appears that a shortwave embedded within northwesterly flow aloft will pass through the area Friday afternoon into Saturday morning. At the surface, low pressure looks to track out of the Ohio Valley and towards our region, with additional surface cyclogenesis possible somewhere off the Eastern Seaboard Friday night. There remains significant spread amongst GEFS and EPS members in where this will occur, ranging from just east of North Carolina to east of the New Jersey coastline. Where this occurs will depend in large part on a strong area of high pressure centered over Quebec, and how quickly it retreats northeastward. Latest trends have been favoring a stronger high, and therefore low placement farther south. Widespread precipitation is expected across our area with this system, and with the trends towards a stronger and slower retreating high, the forecast has also continued to trend colder. It is too soon and model spread remains too high to speculate on specifics, but the odds of impactful wintry precipitation are increasing.

The pattern looks to remain very unsettled into early next week, with model guidance indicating another storm system impacting the area Sunday into Monday. This could once again bring a variety of weather impacts and precipitation types to the region. However, as previously stated, confidence is currently quite low at this range.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

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

Today...MVFR ceilings developing through 15Z as light snow and light rain spread into the region. MVFR visibility likely, with IFR most likely for RDG/ABE. Ceilings lowering to IFR for I-95 terminals and north for at least a brief period by 18Z or so. ACY/MIV likely stay MVFR. Precipitation ending between 18-21Z at the latest, though some light drizzle or fog/mist may linger following precip cessation. South to southwest winds near 5-10 kts. Low confidence on the exact timing of restrictions developing and improving as the day progresses, but high confidence in at least MVFR conditions occurring everywhere.

Tonight...Any lingering MVFR ceilings lifting and scattering out to VFR by 03Z. Westerly winds increasing to 10-15 kts and shifting northwest by 06Z with gusts 20-25 kts. High confidence.

Outlook...

Wednesday through Wednesday night...VFR. Breezy northwest winds during the day on Wednesday, with gusts 25-30 kt possible.

Thursday...Primarily VFR. Slight chance (around 20%) of brief restrictions in light rain or snow.

Thursday night...VFR. No significant weather.

Friday through Saturday...Restrictions likely with widespread precipitation.

MARINE

Marginal advisory conditions anticipated to develop today across the Atlantic coastal waters as southerly wind gusts increase to around 20-25 kts and seas build to 3-5 feet. Conditions will improve for a few hours this evening, then winds and seas will build again after midnight as a cold front moves offshore. Northwest winds gusting 25- 30 kts forecast by dawn Wednesday with seas 3-5 feet.

The Small Craft Advisory was extended through tonight into Wednesday for the Atlantic waters, and expanded to include Delaware Bay for early Wednesday morning.

Rain likely this morning, then fair weather with no visibility restrictions through tonight.

Outlook...

Wednesday...SCA conditions expected with northwest winds 20-25 kt and gusts around 30 kt. There is a low chance (around 20%) of gales. Seas 3-5 feet.

Wednesday night...No marine headlines are currently anticipated with winds remaining below 25 kt and seas below 5 feet.

Thursday through Thursday night...SCA conditions possible with southwest winds increasing to 15-25 kt on Thursday and shifting to northwesterly Thursday night. Seas 3-5 feet.

Friday through Saturday...SCA conditions likely with northerly winds 15-25 kt shifting to east-northeasterly on Saturday. Seas 3-5 feet on Friday increasing to 4-6 feet Saturday.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Weather Advisory until 4 PM EST this afternoon for PAZ054-055. Winter Weather Advisory until 1 PM EST this afternoon for PAZ061-062. NJ...Winter Weather Advisory until 4 PM EST this afternoon for NJZ001-007-008. DE...None. MD...None. MARINE...Small Craft Advisory from 4 AM to 4 PM EST Wednesday for ANZ430-431. Small Craft Advisory until 6 PM EST Wednesday for ANZ450>455.