textproduct: Philadelphia/Mt Holly

SYNOPSIS

Canadian high pressure will build into the region through today. High pressure will move offshore tonight, then a clipper system will impact the region early Tuesday with a warm front lifting north. A cold front will push offshore Tuesday night, followed by weak high pressure building on Wednesday. Another weak clipper may impact the area with light precipitation on Christmas Day, then a stronger system looks to impact the region on Friday. Unsettled weather may persist into the weekend.

NEAR TERM /UNTIL 6 PM THIS EVENING/

Today: High pressure across the region today will slowly move east by evening. Sunny skies are expected today with seasonably cool temps expected. Highs will top out in the upper 30s for North NJ, the Lehigh Valley and the southern Poconos. Other areas will reach the low 40s. Winds will be West to Northwest at 5 to 10 mph this morning then Southwest around 10 mph for the afternoon.

SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY NIGHT/

A period of light snow, and potentially some wintry mix and rain are anticipated to impact portions of the region late tonight through Tuesday morning. There have been no significant changes to the forecast for this system. Thereafter, temperatures will remain relatively mild and seasonable with breezy conditions expected for Christmas Eve (Wednesday).

Cold and dry Canadian high pressure will shift gradually offshore into tonight. The cold and dry Canadian airmass will remain in place, at least initially. We should see radiational cooling during the evening, with temperatures dropping into the mid 20s to low 30s before thicker clouds start moving in from the approaching system. Warm advection will increase aloft overnight as a mid level wave, clipper-like system approaches. With the absence of any strong surface pressure features, low level wind fields, including surface winds, will remain quite light. So the warm advection will be very weak, and the cold, dry antecedent surface airmass should remain anchored in place through much of the overnight period as mid level dynamics begin to produce some light precipitation despite a warm front trying to nudge its way in. Even if surface temperatures increase a few degrees overnight, wetbulb temperatures will remain below freezing across much of interior New Jersey and Pennsylvania thanks to dewpoints near 20 degrees.

Guidance remains in decent consensus that we will see at least a few hundredths of QPF as far south as the Philly metro after midnight tonight into Tuesday morning, and as much as 0.1-0.25" near and especially north of I-78. The CAMs and hi resolution guidance seems a little low given the mid level dynamics at play, so not totally buying their drier solutions (compared to synoptic models) just yet. Near and north of I-78, PoPs remain in the 70-80% range. South of I- 78, PoPs are in the 40-60% range. It still appears that precip will begin before dawn Tuesday for many areas, as this is when the main push of warm advection and mid level dynamics increase. One limiting factor is that, while there will be sufficient moisture and forcing for precip, the precip will have to overcome the initially dry air in the low levels. This will work to limit QPF and snowfall amounts farther south into the Philly metro, where forcing and moisture lessens the farther south you go. THus, there is still a degree of uncertainty on how far south any measurable snowfall will occur into the Philly metro, southern New Jersey, and northern Delaware. The warm advection nature of this system should also 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 near and 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). For the I- 95 corridor and Philly metro, thinking a full 1" would be a reasonable worst case scenario we could see from this system. Areas north of the I-78 corridor have the best chances of receiving a plowable 1-2" of snowfall. The usual higher elevations of northern New Jersey and eastern Pennsylvania could see isolated amounts as much as 3-4" of snow in a reasonable worst case scenario, but the probability of this is low at the moment (~20%), and will be conditional on exactly how much QPF we can squeeze out of this weak system.

There also remains some potential for a light glaze of ice, mainly for the southern Poconos, 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 last weekend's snow storm. With that said, snow falling Tuesday morning could certainly cause travel issues and slippery/icy roads despite the light snowfall amounts. In any case, this is still not expected to be a significant snow event for our area. A Winter Weather Advisory could be warranted for Carbon and Monroe Counties in Pennsylvania if confidence increases enough for snowfall amounts nearing 3" and/or a light glaze of ice to occur in these areas.

Confidence and chances for precip are lower (30-50%) 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 greater precip chances arrive. So any snow that does fall is most likely to be "white rain" (snow melting as it falls).

Any lingering precipitation will be ending by noon or so on Tuesday, then temperatures should start warming up by a few degrees. Based on the latest deterministic guidance, our current high temperature forecast for Tuesday could still be a bit too high, especially for any areas that experience snow cover north of the Philly metro. Skies should remain mostly cloudy for the remainder of the day, and warm advection gradient winds will not be very strong at all. Long story short, a low confidence high temperature forecast for Tuesday, but 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 Tuesday night 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 Tuesday night as a cold front pushes offshore, which should help to dry things out a bit more too.

Weak high pressure will build into the area from the Great Lakes region Christmas Eve and center itself over the area Christmas Eve night into Christmas morning. We look to stay out of the strong cold advection this time around, so temperatures should remain close to normal for late December. However, it'll be a bit breezy during the day with NW winds gusting near 30 mph possible, then becoming light to calm overnight. High temperatures will be mainly in the 40s on Christmas Eve, and lows mainly in the mid 20s to mid 30s for Santa during Christmas Eve night. Mostly sunny skies for Christmas Eve, then increasing clouds overnight.

LONG TERM /THURSDAY THROUGH SUNDAY/

Christmas Day should be a decent day overall with no significant weather, but a weak clipper system passing through could bring some brief light rain or sprinkles (flurries possible up north) in the morning. PoPs remain around 20-40%. Otherwise, it'll be mostly cloudy to overcast with seasonable temperatures (highs in the upper 30s and 40s). Christmas night will remain cloudy, but this will keep temperatures relatively mild with lows in the mid 30s to low 40s.

Beyond Christmas Day, forecast uncertainty becomes very high. Guidance indicates there will be a high over eastern or southeastern Canada with low pressure moving eastward across the Great Lakes and toward the Northeast or Mid Atlantic. This setup will likely bring precipitation to our area. The 00Z suite of guidance had a significant change in course, indicating a much stronger high and weaker, more suppressed low. This solution would introduce potential for wintry precip for our region and much colder temperatures, quite the change from widespread 50s and rain from before. The shift in guidance was consistent between different models. GFS, ECMWF, and CMC all showing this much colder, wintry solution now, however given this *very* recent change in course, have stuck with NBM for now until there is more consistency in a colder, more wintry solution. Precipitation is still likely, with 50-80% PoPs, however we could be dealing with all rain, a significant wintry system, or anything in between. Remain alert for changes to the forecast for the Friday timeframe.

The weekend forecast is also highly uncertain, but could remain unsettled. Stuck with NBM.

AVIATION /12Z MONDAY THROUGH FRIDAY/

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

Today...VFR expected. Occasional high clouds at times. Northwest winds 5 to 10 knots shifting to West then Southwest. High confid.

Tonight...VFR early then increasing and lowering CIGS. Light S or SE winds. Precipitation arrives PHL probably around dawn with MVFR possible. High confid early then low confid around dawn with flight category and if/when precip arrives.

Outlook...

Tuesday...Restrictions likely in snow/rain and low clouds.

Tuesday night through Wednesday night...VFR. Gusty NW winds on Wednesday.

Thursday through Friday...Restrictions likely in low clouds and potential for rain/snow. Gusty winds possible. Very low confidence forecast for Friday.

MARINE

High pressure builds in today. Winds will decrease early today and seas will continue to diminish too. The SCA flag for the ocean has been cancelled as of 545 AM. Winds and seas will remain below SCA today and tonight. Fair today and a chance for rain or snow starting late tonight.

Outlook...

Tuesday through Wednesday...Potential for advisory conditions as wind gusts increase to 20-25 kts with seas near 4-6 feet possible.

Thursday...No marine hazards expected. A chance of light rain or sprinkles.

Friday...Advisory conditions probable. Precipitation likely restricting visibility.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...None.