textproduct: Philadelphia/Mt Holly

SYNOPSIS

Canadian high pressure will build into the region into tonight following a cold front moving offshore this morning. High pressure will move offshore Monday night, then a clipper system will push a warm front into the area early Tuesday. The system's cold front is then forecast to move across the area Tuesday night into early Wednesday, followed briefly by high pressure late Wednesday. A pair of weak disturbances could then affect the area for the end of next week.

NEAR TERM /THROUGH TONIGHT/

Today: A cold front, attached to low pressure across Canada, will cross the region this morning. The front is accompanied by little in the way of low level moisture, so except for some mid/high level clouds, it will be uneventful. Once the front passes, colder air arrives on a strengthening West to Northwest wind. Highs today will range from the low 40s for the N/W areas to the upper 40s S/E. Winds will make the air temps feel cooler however. Wind gusts 20 to 30 mph developing by late morning.

For tonight: High pressure across the Ohio Valley build east towards our region. Clear skies are expected tonight and with decreasing winds, good radiational cooling is expected. Lows will drop into the teens across the far N/W areas and low 20s elsewhere. Wind chills will be in the single digits across the Pocono Plateau and teens most elsewhere.

SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/

Initially cold and dry to start the holiday week. A period of light snow and some rain appearing increasingly likely to impact portions of the region late Monday night through Tuesday morning.

Cold and dry Canadian high pressure will be in place for Monday, which will shift gradually offshore into Monday night. Highs will range mostly from the mid 30s to low 40s under mostly sunny skies, though cirrus will begin to stream into the area late in the afternoon. WNW winds near 5-10 mph.

For Monday night, 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 as mid level dynamics begin to produce some light precipitation. 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 is coming into better consensus that we will see at least a few hundredths of QPF as far south as the Philly metro after midnight Monday night into Tuesday morning, and as much as 0.1-0.25" near and especially north of I-78. In these areas, PoPs are now mostly in the 50-70% range, and I would anticipate these to increase further assuming current trends hold. It should be noted too that confidence is increasing that this precip will begin before dawn Tuesday, early enough for all or mostly all snow in these areas. 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. 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.

The bottom line with this update is that confidence is increasing for the aforementioned interior southern New Jersey, Philly metro, and points north into eastern Pennsylvania and northern New Jersey will see light accumulating snowfall, but this will not be a significant event for our area. 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.

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). 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 along and 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 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.

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. It will also be warmer here, with wetbulb temperatures mainly above freezing. 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 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 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, which should help to dry things out a bit more too.

LONG TERM /WEDNESDAY THROUGH SATURDAY/

Some sun with a diminishing breeze will be had on Christmas Eve (Wednesday) as high pressure builds into the region, with highs again in the 40s for most, and 30s for the Poconos and NW NJ.

This high slides overhead and then east Christmas Eve night (Wednesday night), with another weak clipper-like system approaching from the northwest. Confidence is much lower on this system, but its not impossible some light precip accompanies it, especially in northern areas, which could even be in the form of a little snow if surface temps drop enough during the evening before clouds arrive. Lows in the 20s to low 30s.

Better chance this weak clipper arrives during the day Thursday (Christmas Day), by which time temps should be solidly above freezing and any precip would be rain. Confidence is still low however, and PoPs remain near 30-40%. Temps remain similar to the prior two days, with highs mainly in the 40s, 30s Poconos.

Initial system may depart by Thursday night (Christmas night), but another weak wave could arrive in its wake, providing a continued chance of some precip. Temps likely stay above freezing with the southerly to southwesterly flow across the region by this point, resulting in any precip likely just being rain.

Friday still looks pretty mild, but uncertainty continues regarding any precip with another impulse possibly crossing the region. Highs still look like upper 40s and 50s for most of the region, possibly topping 60 Delmarva but only low 40s for Poconos/NW NJ.

By Saturday, the forecast remains much more uncertain regarding whether we're still mild or cooling off, or dry vs wet, as the speed and progress of disturbances and fronts gets pretty uncertain by this point.

AVIATION /08Z SUNDAY THROUGH THURSDAY/

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

Today...VFR expected with mostly just high clouds. Winds will shift from SW to W this morning and then further turn NW by afternoon. Winds speeds increase to around 10 knots shortly after sunrise then continue to increase by mid-morning. Gusts 20 to 30 kts expected thru the afternoon. High confid.

Tonight...VFR expected. Decreasing NW winds overnight. High confid.

Outlook...

Monday...VFR. No significant weather.

Monday night and Tuesday...Restrictions likely in light snow and some rain. Conditions improving Tuesday afternoon.

Tuesday night through Wednesday...VFR. Gusty winds possible.

Wednesday night through Thursday...Restrictions possible with a chance of rain.

MARINE

High pressure remain offshore this morning. A cold front crosses the waters early today and winds will shift from SW to NW thru the afternoon. Gusts 25 to 30 kts expected. SCA flag will remain in place today.The winds diminish later tonight and SCA flags come down overnight. Fair weather today and tonight.

Outlook...

Small Craft Advisory conditions will diminish early Monday. Sub advisory conditions then prevail rest of Monday into Monday night, but advisory conditions may redevelop late Monday night and persist through Wednesday. Light snow or rain may restrict visibility late Monday night into Tuesday morning. There might even be a period of gales on Tuesday into Tuesday night. Sub advisory likely prevails Wednesday night before advisory conditions possibly redevelop Thursday.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 3 AM EST Monday for ANZ430-431. Small Craft Advisory until 6 AM EST Monday for ANZ450>455.