textproduct: Philadelphia/Mt Holly

SYNOPSIS

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 /THROUGH TUESDAY/

Current analysis places cold high pressure just south of our region over the southern Chesapeake and Delmarva. Meanwhile, a clipper system is pushing southeast across the Canadian prairies, heading towards the Great Lakes tonight and towards Montreal on Tuesday. While the tap isn't great, there is just enough of a feed off the Gulf ahead of the system to bring our region a bit of wintry weather as it passes.

For this evening, clouds will increase, but temps should manage to drop below freezing beforehand in just about the whole region, save possibly the immediate coast and warmest urban centers. Clouds should prevent temps from dropping much below freezing, with most areas ending up in the mid-upper 20s, possibly even starting to rebound overnight as the clouds thicken and wind starts shifting more southerly. However, warm advection aloft will beat the surface warming for much of the region, resulting in the outbreak of precipitation across northern and central areas before dawn and across the south after daybreak. For northern areas, this will be mostly a wintry system, with snow late tonight and tomorrow morning transitioning to either a little freezing rain/drizzle or just ending as mist towards midday/early afternoon. Further south, from Philly southward, any snow early likely shifts to rain before ending. With more marginal temps and a briefer period of snow, accumulations near and south of Philly should be minimal if any. However, near and especially north of I-78, an inch or two of snow is more likely, with up to 3 or even perhaps 4 inches north of I-80. Areas north of I-80 also have a better shot of getting that little glaze of ice at the end. Based on this, have opted to issue winter weather advisories mainly near and north of I-80 for tonight/Tuesday. The trend has been a little wetter/snowier today, so if future guidance continues the trend, its possible this may need a last-minute extension further south, particularly for Morris County, but not feeling the confidence here just yet. In any case, anyone planning to commute or otherwise travel Tuesday morning near and especially north of Philly should be prepared for slippery travel, and delay until afternoon if possible. Temps will try to warm after precip tapers off during the afternoon, with temps pushing into the low 40s near Philly, but may struggle to reach freezing in the Poconos. By contrast, southern Delaware may top 50.

SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/

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 MONDAY/

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 /18Z MONDAY THROUGH SATURDAY/

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

Rest of today...VFR expected. Increasing high clouds. West to southwest winds 5 to 10 knots. High confid.

Tonight...VFR early with light/variable winds, then snow moves in from the NW after 6Z. Cigs likely drop towards MVFR and VSBY may drop to IFR by dawn at ABE/RDG/TTN. For PNE/PHL and south, less odds of restrictions. Low confidence.

Tuesday...MVFR/IFR much of the day with snow transitioning to rain and then ending. Low confidence on details regarding how rapidly precip finally reaches the more southerly terminals and also on how quickly snow transitions to rain before ending. However, MVFR should predominate by sunset. Low-moderate confidence.

Outlook...

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

Sub-Small Craft Advisory through tonight, but south to southwest winds expected to reach marginal SCA levels over the ocean zones on Tuesday. Thus, issued for those zones only.

Outlook...

Tuesday night 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...Winter Weather Advisory from 1 AM to 4 PM EST Tuesday for PAZ054-055. NJ...Winter Weather Advisory from 1 AM to 4 PM EST Tuesday for NJZ001. DE...None. MD...None. MARINE...Small Craft Advisory from 6 AM to 6 PM EST Tuesday for ANZ450>455.