textproduct: Philadelphia/Mt Holly

SYNOPSIS

High pressure initially centered over portions of the Mid-Atlantic and New England will begin to retreat northeastward away from the region tonight. Low pressure will develop over the Southeast tonight before tracking northeast towards our area Tuesday into Tuesday night, bringing widespread precipitation to the area. High pressure and below normal temperatures return for the middle of the week. Another cold front will track through the region Thursday night, with high pressure building back in thereafter. Another system may impact the region Friday night into the weekend, before high pressure builds back in for early next week.

NEAR TERM /THROUGH TUESDAY NIGHT/

For the remainder of this afternoon into the evening hours, we will continue to see clouds filter in ahead of our next system that arrives late tonight into Tuesday morning. The rest of our Monday remains dry with most of tonight being dry as well. A high pressure system moved in today and is fast moving, so it starts to depart off to the northeast into tonight. This will then be followed by our low pressure system that will move out of the southeast US and off to the northeast. Lows tonight are in the 20s for most. However, these low temperatures actually happen earlier in the night and we warm up heading into daybreak on Tuesday as warm air filters in with our system arriving.

This low pressure system is the main story for the near term period as it approaches the area tonight and then off the coast into Tuesday, which will bring widespread precipitation to the region, including the first accumulating snowfall to portions of the area. This low will make its closest approach to the area Tuesday afternoon, and begin departing to the northeast by Tuesday night.

In terms of the details with this system, there have been some changes to the forecast. One of these changes is the arrival of the system being a little later and precipitation may not even arrive to the area until as late as daybreak on Tuesday. We have chance PoPs (30-50%) currently beginning right around the early morning hours Tuesday. Quickly after daybreak, the precipitation becomes widespread across the area with the highest PoPs (90-100%) being during the daytime hours tomorrow.

Due to a later arrival, and more warm air in place at the onset of the precipitation, this has cut back on snow totals and how far southeast they extend. At the onset, especially if it is near daybreak Tuesday, the rain/snow line may already be near or northwest of the I-95 corridor. By mid-morning, the rain/snow line is inching north of I-78. During the afternoon, the rain/snow line has the potential to reach into the Poconos and parts of northwest NJ. There is even the potential for mixing in the higher elevations of these areas above 1,500 feet.

Regarding more details about the precipitation accumulation, QPF has trended lower which has also impacted snow totals with this system. Totals are now ranging from 0.50-1.00" with localized amounts to 1.25" with the greatest across the coastal plain and least farther inland north of I-78. Due to the later trend in precip onset, the potential for snow looks to be little to none for the I-95 corridor. South and east of the I-95 corridor is likely to be all rain given the latest trends.

Through the day Tuesday, the onshore flow and warm marine influence with the surface low drawing near will impact the temperature profiles. Through the day Tuesday, the rain/snow line will continue to move northwest. For perspective on this, high temperatures on Tuesday are near or a couple degrees above freezing in the Poconos, mid 30s to near 40 degrees northwest of the fall line, low 40s along the I-95 corridor, and in the mid 40s to low 50s in Delmarva and along the coastal areas. Although areas may start as snow, it does not appear that it will last for much of the area. The southern Poconos have the best potential at an all snow event from this system, but even they could see some mixing with sleet, freezing rain, or rain Tuesday afternoon.

As for snowfall accumulation, the forecast has trended lower with this update. The probability for plowable snow (2" or more) remains high (60-90%) in the Poconos and areas along and north of I-78. As you head well south and east of I-78, the potential for plowable snow quickly decreases to near 0% for the I-95 corridor. Our latest forecast includes snowfall amounts of a few tenths of an inch for the very NW portion of the I-95 corridor, up to an inch for areas just northwest of there, and in the 1-3" range for the I-78 corridor and NW NJ. Higher elevations (especially 1,500 feet and higher) in NW NJ and across the Pocono Plateau have potential for snowfall in the 3-6" range. The Winter Storm Watch is now a Winter Weather Advisory for the southern Poconos and Sussex County NJ, where probability for warning criteria snowfall (6" or greater) has trended too low to justify a warning for these areas.

A Winter Weather Advisory remains for much of the counties in northern NJ and the I-78 corridor and down into the far northwestern reaches of the Philly metro. These are the areas most likely to experience a troublesome Tuesday morning commute, with snowfall in the 1-3" range and there is the potential for freezing rain.

Outside of potentially the Poconos and perhaps the higher elevations in NW NJ, the snow that falls will then be melting through the day with the change to all rain and warm air moving further inland. During the transition period from snow to rain, some sleet is possible briefly. Due to the stronger signal of warmer air advecting in, the transition zone from snow to rain may not be as clean. There is now the potential for a light accumulation of ice from freezing rain in the areas where a Winter Weather Advisory is in place. The ice accumulation looks to be a light glaze. The areas that see all rain or a change to rain will have a good soaking all-day type of rain. A few precipitation and/or snowfall records could be challenged. See the Climate section below for more details on the records.

Once the low departs Tuesday night, a high pressure system will build in from the southwest. This will lead to drier conditions through Tuesday night. Lows Tuesday night are in the 20s for most, which may lead to some black ice issues where any slushy snow melt refreezes on roadways (especially north of I-78).

SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/

By Wednesday morning, the axis of the H5 trough that will lead to Tuesday's widespread precipitation will be in the Gulf of Maine vicinity, rapidly lifting northeast away from the area. A subtle upper-level ridge axis will shift eastward across the area through the day Wednesday. Wednesday night into Thursday, enhanced southwesterly flow aloft will begin to overspread the area as a trough digs southeastward over portions of Ontario and eventually Quebec. The trough axis will pass to the north/glance the region Thursday. Subtle height rises will follow with flow aloft becoming zonal Thursday night. At the surface, high pressure will build in from the west on Wednesday. A surface low will pass well to the north of the area on Thursday with a strong trailing cold front moving through by Thursday evening.

Wednesday should be a partly to mostly clear day. Highs will range from the mid 30s across eastern PA and northern NJ to the low 40s across southern NJ and the Delmarva. Wednesday night will feature increasing clouds from northwest to southeast as the cold front begins to approach the region. Lows will be in the low-mid 20s across most of eastern PA and NJ, with upper 20s to near 30 for the coastal plain, urban corridor, and Delmarva.

Thursday will be partly to mostly cloudy ahead of the front. While not explicitly mentioned in the forecast at this time, a couple of snow showers could occur with the frontal passage across the Poconos. If this were to occur, amounts would be light. Otherwise, it looks to remain dry. Will defer to future shifts on potential inclusion of PoPs for this region. Highs will generally be in the 30s northwest of I-95 and 40s southeast. Skies should clear behind the front, but breezy northwest winds will ensue, so it will be quite chilly.

With the cold post-frontal airmass in place, decreasing winds, and partly to mostly clear skies, the stage will be set for a very cold Thursday night. Lows are expected to be in the single digits across the Poconos and into far northwestern NJ, the teens across the remainder of eastern PA and NJ, and the low 20s across the Delmarva.

LONG TERM /FRIDAY THROUGH MONDAY/

Friday looks to start with high pressure centered over our area under zonal flow aloft. Deterministic guidance still shows the potential for a weak impulse aloft to track through the region Friday night into Saturday, but the overall trend has been for a much less amplified pattern. The surface high will shift eastward and off the coast Friday night into Saturday. Ensemble guidance still suggests the possibility of surface cyclone formation, though the trend has been weaker and farther south. As a result of these trends, the potential for a significant storm system this weekend appears to be decreasing. With that said, still have chance (30-50%) PoPs across most of the area for Friday night, with PoPs near 60% for southern DE and far southeastern NJ. These ultimately may continue to come down, but trends will be monitored in the coming days as this potential storm system gets closer in time. The airmass in place will be cold enough that any precipitation that does fall across the area with this system would have the potential to be of the wintry variety.

Regardless of precipitation chances for Friday night into Saturday, the pattern favors well below average temperatures. In fact, temperatures may remain below freezing on Friday for most of eastern PA and northern NJ, and only reach the mid 30s across the Delmarva and into southern NJ. Saturday and Sunday look to remain below average, though slightly warmer than Friday. Another cold front could bring at least low precipitation chances and a renewed shot of cold air Sunday night into Monday.

AVIATION /20Z MONDAY THROUGH SATURDAY/

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

Rest of Today (through 00z)...VFR. North/northwest winds around 5-10 kt with gusts near 20 kt through 19z-20z and diminishing quickly thereafter. High confidence.

Tonight...Primarily VFR. Chance (20-40%) of MVFR conditions after 10Z as rain and snow begin to move in though things come down more towards daybreak and after. East-northeast winds around 5 knots or less. Moderate confidence.

Tuesday...IFR/LIFR conditions develop between 11z-15z. High confidence in all rain at KACY/KMIV. The trend as of this morning has been warmer and slower with precipitation moving in. This will result in pretty much all rain for KILG/KPHL and have taken snow out of the TAF for now. Cannot rule out a few flakes/mix at onset, but would expect a quick flip to rain once precipitation begins. For KPNE/KTTN, expecting a longer period of snow and/or mix, but a flip to all rain should occur by 12z-14z. A period of all snow expected at KRDG/KABE from about 10z-16z before mixing occurs and eventual flip to all rain after 18z. High confidence in at least IFR through the morning with the potential (25-35%) in prevailing LIFR. Moderate confidence in timing of precipitation type changes. Winds will be out of the east/northeast around 5-10 kt, expecting to shift to north/northwest between 18z-21z. Moderate confidence in timing of wind shifts.

Outlook...

Tuesday Night...Conditions gradually improving though MVFR/IFR expected to start, with VFR anticipated later in the night. Rain showers taper off as the night goes on.

Wednesday through Wednesday Night...VFR. No significant weather.

Thursday...VFR. Gusts near 20 kt expected out of the west/northwest

Thursday Night through Friday...VFR. No significant weather.

Friday Night through Saturday...Restrictions possible (40-60%) with rain and snow showers expected to move through.

MARINE

No marine hazards are anticipated through tonight with seas 2-3 feet and winds out of the east to then southeast gusting to 10-20 knots.

Winds and seas will increase Tuesday morning with the Gale Watch being converted to an SCA for the ocean zones starting at 10AM. For the Delaware Bay, the SCA starts at 3PM. Winds will gust to 25-30 knots out of the north to northwest. Occasional gusts to 35 knots are possible on the ocean zones. Seas will reach 6-8 feet Tuesday night. The Small Craft Advisory continues for Tuesday night.

Outlook...

Wednesday...SCA conditions could linger into the first part of the day. While winds are expected to have subsided, seas of 4-7 feet are possible in the morning, decreasing to 2-4 feet through the day.

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

Thursday through Thursday night...SCA conditions are possible with northwest winds 15-25 kt and seas 3-5 feet.

Friday through Saturday...No marine headlines are anticipated with winds below 25 kt and seas below 5 feet.

CLIMATE

Widespread moderate to heavy precipitation is expected on Tuesday, December 2nd. A few daily precipitation and/or snowfall records could be challenged. Records for our climate sites are listed below:

Record Precipitation (Rainfall) December 2 Site Record/Year Allentown (ABE) 0.86"/1934 AC Airport (ACY) 1.03"/1996 AC Marina (55N) 1.18"/1929 Georgetown (GED) 1.11"/1974 Mount Pocono (MPO) 3.56"/1996 Philadelphia (PHL) 1.48"/1986 Reading (RDG) 1.29"/1981 Trenton (TTN) 2.13"/1986 Wilmington (ILG) 1.27"/1991

Record Snowfall December 2 Site Record/Year Allentown (ABE) 3.6"/1952 AC Airport (ACY) T/2019* Philadelphia (PHL) 2.0"/1903 Reading (RDG) 6.0"/1929 Trenton (TTN) 3.0"/1903 Wilmington (ILG) 1.0"/1952

*Multiple years with a Trace.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Weather Advisory from 4 AM to 1 PM EST Tuesday for PAZ060>062. Winter Weather Advisory from 4 AM to 10 AM EST Tuesday for PAZ101-103-105. Winter Weather Advisory from 4 AM to 6 PM EST Tuesday for PAZ054-055. NJ...Winter Weather Advisory from 4 AM to 7 PM EST Tuesday for NJZ001. Winter Weather Advisory from 4 AM to 1 PM EST Tuesday for NJZ007-008. Winter Weather Advisory from 4 AM to 10 AM EST Tuesday for NJZ009. DE...None. MD...None. MARINE...Small Craft Advisory from 3 PM Tuesday to 4 AM EST Wednesday for ANZ430-431. Small Craft Advisory from 10 AM Tuesday to 4 AM EST Wednesday for ANZ450>455.