textproduct: Philadelphia/Mt Holly

SYNOPSIS

A cold front will move through this evening with a weak high pressure system following in its wake. This high pressure system does not slide offshore until Christmas Day as another weak low pressure system moves through. An active pattern will take shape for the end of the week and weekend with the potential for a series of storm systems impacting the area.

NEAR TERM /THROUGH WEDNESDAY/

With the wintry precipitation having moved out of the area, attention now turns to a cold front which will move through the area into the early overnight hours bringing some cold air with it as winds turn out of the northwest.

Low temperatures tonight are mainly 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 should be confined to untreated roads north of I-78. Winds will also increase overnight as the cold front pushes offshore, which should help to dry things out a bit more too. There continues to be a signal 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 was issued for tonight into Wednesday morning for Carbon and Monroe counties. Wind gusts reaching near 40-50 mph in the higher elevations of NW NJ appear more localized, so a Wind Advisory was not issued for these areas at this time. Elsewhere, gusts around 25-35 mph are forecast.

Behind the cold front that moves through tonight, a high pressure system will approach from the west out of the Great Lakes region. This high pressure system will keep tomorrow on the more tranquil side. A majority of the day will be mostly clear before clouds increase during the evening hours. Highs on Wednesday will be in the 40s for most with the 30s for the Poconos. Winds in the morning will still range from 25-35 mph for many with upwards of 40-50 mph for the Poconos. Due to the high pressure system building in, the winds will lessen through the day. By Wednesday evening, gusts are 5-10 mph for most and upwards of 20 mph in the Poconos.

SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/

High pressure will be overhead on Wednesday Night and moving offshore ahead of a weak clipper system. Cloud cover increases as the night goes on but any precipitation should hold off until daybreak. Temperatures drop into the mid to upper 20s and low 30s.

For Christmas Day, a weak shortwave and associated area of low pressure dive to our southeast. This will clip the southern half of our region, bringing some light showers, though nothing of significance and rain accumulations below a tenth of an inch. Looking at around a 30-40% chance of rain for South Jersey and Delmarva, with a 15-25% for southeastern PA, and the rest of New Jersey. So, not a washout, but a mostly cloudy day with some showers around. Temperatures will be in the low to mid 40s with upper 40s over lower Delmarva.

Things dry out for Christmas night as an area of high pressure slides by to the north. Temperatures generally will be in the 20s with some teens north of I-80.

LONG TERM /FRIDAY THROUGH TUESDAY/

An active and unsettled period is ahead for the weekend and potentially into next week. Main period to watch will be Friday and Friday Night as well as Sunday and Sunday Night as two systems impact the region, likely bringing some form of wintry weather.

The first system comes in Friday into Friday Night. There is a lot of uncertainty with this system as high pressure off the north will result in some cold air in place, but warm air aloft will be trying to erode that cold air. There likely will be a transition zone setting up over our area between all rain, sleet/freezing rain, and snow. The track and timing of the surface low will make all the difference, and hopefully that will come into focus over the next few forecast cycles. The trend as of late has been a bit slower and further south with the track of the surface low, which would actually result in more impacts for our region. The potential is certainly there for accumulating snow, possibly significant, for areas north of Philadelphia. A wide variety of outcomes exist still among deterministic guidance, with the GFS bringing significant accumulations from Philadelphia on north, the GEM having minor snowfall limited to northern areas, and the ECWMF and its AI counterpart going more in the middle of those solutions. NBM Probability of 2" or more of snow is around 50-80% from Philly on north with probabilities decreasing drastically the further south you go. Probability of 6" or more of snow is around 15-30% for the Philly metro, Lehigh Valley, and Poconos, with around a 40-50% chance for North Jersey.

Another component with this storm is the potential for some freezing rain and measurable ice accumulation. Again, still too early to know for sure given low confidence, but current probabilistic guidance from the NBM has around a 30-50% chance of ice accumulations greater than a tenth of an inch from the I-95 corridor on west.

Overall, as mentioned previously, this is a low confidence forecast, but areas from about Philadelphia on north and west should keep a close eye on the forecast as this storm could be potentially disruptive and bring large impacts.

Saturday looks to be a period in between systems, however, some lingering rain or snow showers could hover over the region as an area of low pressure meanders offshore. Some guidance even shows a small chance for freezing rain/drizzle. However, things could also be completely dry if the low offshore goes further out to sea. PoPs are only around 20-30% for Saturday.

For Saturday Night, a warm front lifts northward with a stronger cold front coming through sometime Sunday or Sunday Night. Some cold air especially on Saturday Night and early Sunday could get trapped over the Lehigh Valley and Poconos if the warm front is slower and gets hung up. Some guidance shows another period of freezing rain possible. For most though, it will primarily be an all rain event with the cold front coming through on Sunday Night/Monday morning.

Temperatures likely take a tumble next week with breezy conditions in a cold advection pattern, though it should be dry as we get towards the new year.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

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

Tonight...Lingering MVFR ceilings lift and scatter out to VFR around 03-04Z. West wind increasing to 10-15 kt and shifting northwest after 06Z with gusts 20-25 kt. LLWS is expected for KRDG and KABE after 03Z, and the I-95 terminals after 05Z with the arrival of the cold front. Moderate confidence.

Wednesday...VFR. Northwest wind 10-15 kt with gusts generally 25-30 kt, perhaps as high as 35 kt for KRDG and KABE. Wind will start to decrease around 21Z to 5-10 kt. LLWS is again possible as surface winds diminish, but opted not to include in the TAFS at this time. Moderate confidence.

Outlook...

Wednesday Night...VFR. No significant weather.

Thursday...MVFR/IFR conditions possible, especially from KPHL on south as low clouds and showers move through.

Thursday Night...VFR. No significant weather.

Friday through Saturday...Sub-VFR conditions expected at all terminals is a winter storm impacts the region. All precipitation types on the table for all terminals, though the highest chance for snow and freezing rain will be from the I-95 corridor on west.

Saturday Night through Sunday...Sub-VFR conditions expected. Primarily rain at all terminals, though KABE/KRDG could see some freezing rain on Saturday Night.

MARINE

After marginal Small Craft Advisory conditions today across the Atlantic coastal waters due to southerly wind gusts around 20-25 kts and seas near 3-5 feet. There will be a lull for a few hours this evening as conditions fall below Small Craft Advisory criteria. After midnight, winds and seas will increase as another period of Small Craft Advisory conditions develop. Northwest winds gusting 25- 30 kts are forecast by dawn Wednesday with seas 3-5 feet. The Delaware Bay has a Small Craft Advisory that runs from 4 AM to 4 PM on Wednesday. There is the potential for occasional gale force gusts on the Atlantic coastal waters but due to the period being brief, no Gale Warning was issued.

Outlook...

Wednesday Night...No marine headlines expected.

Thursday through Thursday Night...SCA conditions possible (50%) as winds near 25 kt out of the northwest and seas near 5 feet.

Friday...No marine headlines expected.

Friday Night through Saturday...SCA conditions possible (50%) with seas nearing 5 feet.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Wind Advisory from 1 AM to 1 PM EST Wednesday for PAZ054-055. NJ...None. 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.