textproduct: Philadelphia/Mt Holly

SYNOPSIS

A low pressure system and its associated cold front continue to approach the region, with the cold front passing through tonight. Low pressure will develop over the Southeast Monday night before tracking northeast towards our area Tuesday into Tuesday night. 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.

NEAR TERM /THROUGH MONDAY/

A mid-level shortwave axis embedded in a larger scale trough is currently pivoting northeastward into portions of far southern Ontario and Quebec. The trough will depart to the northeast tonight, with subtle height rises through Monday as a subtle ridge axis shifts through the area ahead of the next major trough. At the surface, a cold front to the west of the area will approach and move through this evening. Thereafter, high pressure will build in from the Ohio Valley region and remain in place through Monday evening.

Light rainfall continues to move eastward across the area this afternoon, and will come to an end during the early evening hours. Remaining rainfall amounts look to be in the neighborhood of 0.10" southeast of I-95, with very little northwest. Low clouds should remain in place through the evening hours. As the cold front approaches beginning this afternoon, breezy south- southwesterly wind with gusts 20-25 mph can be expected.

Tonight, clouds will clear with the passage of the cold front. There may be a brief lull in the wind during the early hours of the night, but as the cold front moves through it will again get breezy out of the northwest, with gusts 20 to perhaps 30 mph. With strong cold air advection taking place, lows tonight will be in the upper 20s to near 30 across the entire area.

Monday should start mostly clear, with some increase in clouds during the afternoon hours. Gusty northwest wind in the morning will diminish by the afternoon as the high moves overhead and the pressure gradient relaxes. High temperatures will be near freezing in the Poconos, in the upper 30s to lower 40s for eastern PA and much of NJ, and mid 40s for the Delmarva into far southern NJ.

SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/

The main story for the short term will be the strengthening low pressure system moving northeast Monday night and off the coast into Tuesday, which will bring widespread precipitation to the region. There will be a low developing over the southeastern US and our high pressure system will retreat off to the northeast Monday night. 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, no significant changes have been made to the forecast or messaging for Tuesday. There are chance to likely PoPs (50-70%) starting Monday night, with the highest PoPs (90-100%) being during the daytime Tuesday. There has been a trend toward a somewhat faster solution in the deterministic guidance, hence the slight increase in PoPs in the predawn hours Tuesday morning. If this trend holds, it could have implications on initial precip type into the I-95 corridor. The precipitation will end from west to east pretty quickly by Tuesday evening into Tuesday night as the low moves off to the northeast.

For the precipitation accumulation, QPF has remained fairly steady around 0.75-1.25" across the board, greatest across the coastal plain and least farther inland north of I-78. Due to the earlier trend in precip onset, there is the potential for light snow initially for much of the area, as far south as northeastern Maryland, central and northern Delaware, and portions of interior southern New Jersey. This initial light snow could result in minimal accumulation (a dusting less than 1"), and would not last long as the low draws closer to the area changing the precip to rain. Again, any snowfall occuring this far south will be dependent on exactly how quickly the precip moves into the area. A slower onset would probably mean mostly all rain southeast of I-95.

Through the day Tuesday, the onshore flow and warm marine influence will impact the temperature profiles. By daybreak Tuesday, the rain/snow line should be near or perhaps just NW of the I-95 corridor. As the warmer air continues to filter in, the rain/snow line may get as far NW as I-78 and northwestern New Jersey by Tuesday afternoon. For perspective on this, the temperatures for highs on Tuesday are near freezing in the Poconos, upper 30s northwest of the fall line, near 40 degrees along the I-95 corridor, and in the mid 40s to near 50 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. Right now, the Poconos have the best potential at an all snow event from this system.

As for snowfall accumulation, the potential continues to increase for plowable snow (2" or more) 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. Our latest forecast includes snowfall amounts less than 1" for the immediate I-95 corridor, 1-3" for areas just northwest of there, and in the 3-5" 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 4-8" range. Due to increasing confidence of warning level snow (6 inches or more) in the Poconos and in Sussex county NJ, we have issued a Winter Storm Watch for Carbon and Monroe counties in PA and Sussex county in NJ from late Monday night into Tuesday evening.

Outside of mainly 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. In terms of freezing rain, the overall threat looks minimal given the current set up. 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 near and north of I-78).

For Wednesday, the high pressure system is in place which keeps us dry. It is also a colder day with highs in the mid 30s to low 40s.

LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/

In the long term period, we stay active with our weather pattern. A cold front looks to cross the area Thursday into Thursday night. As the cold front passes through, there is the potential for some snow showers, especially near/north of I-78. The main factor with this cold front will be the notably colder airmass in its wake. Thursday night looks increasingly likely to be the coldest night so far this season, with widespread low temperatures in the teens for most, and even single digits for the higher terrain. Wind chill values Thursday night will be in the single digits to teens. By Friday, highs are in the upper 20s to upper 30s for most areas. Beyond that, our next system looks to arrive late Friday into the weekend, bringing potential for another round of widespread precipitation, including wintry precipitation. However, details on this system remain very unclear at the moment.

AVIATION /19Z SUNDAY THROUGH FRIDAY/

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

Through 00Z...MVFR primarily. A few terminals are low end VFR but should see ceilings come down to MVFR for all terminals by 29-21Z. Light rain coming to an end from west to east by 19-21Z. KRDG and KABE may see ceilings lift to VFR prior to 00Z. South- southwest wind around 10 kt with gusts 15-20 kt. Moderate confidence.

Tonight...VFR with clouds lifting and scattering out. South- southwesterly wind around 10 kt will shift to northwesterly 02-06Z as a cold front moves through. Winds will increase behind the front to around 10 kt with gusts around 20 kt. High confidence.

Monday...VFR. Northwest wind around 10 kt with gusts around 20 kt. High confidence.

Outlook...

Monday night...Primarily VFR. Chance (40-60%) of sub-VFR conditions in rain or snow late Monday night.

Tuesday through Tuesday night...Restrictions expected in low clouds and precipitation. Snow changing to rain at RDG/ABE. For the I-95 terminals, precipitation may begin as snow for a couple hours before a change to rain. All rain for MIV/ACY. Conditions should improve by late Tuesday night, though gusty NW winds will be possible.

Wednesday...VFR. No significant weather.

Thursday...Mainly VFR. Gusty WNW winds possible.

Friday...Mainly VFR for Friday. Restrictions possible heading towards Friday night with a chance (40-60%) of rain/snow.

MARINE

A Small Craft Advisory is in effect for all ocean zones, as southerly wind of 15-20 kt with frequent gusts to 25 kt is expected this afternoon. Low visibilities will be possible in rain through the evening hours. Seas 4-6 feet.

Overnight, wind will become northwesterly at 20-25 kt with frequent gusts near 30 kt for all ocean zones and the Delaware Bay. As a result, the Small Craft Advisory will be in effect for the Delaware Bay as well starting at 10 PM. Seas over the ocean should be around 5 feet to start the night, decreasing to around 4 feet by sunrise.

Wind will begin to diminish on Monday morning, falling below 25 kt and with seas less than 5 feet. The Small Craft Advisory for the Delaware Bay will expire at 6 AM, and for the ocean waters at 9 AM.

Outlook...

Monday night...No marine hazards anticipated. Fair weather.

Tuesday through Wednesday...SCA conditions likely with east winds increasing to near 25-30 kts and then becoming northwest Tuesday night into Wednesday. The potential for wind gusts near 35 knots has increased, so a Gale Watch was issued for the ocean zones Tuesday night. Winds look to diminish through the day Wednesday. Seas 5-7 feet, also diminishing through the day Wednesday.

Thursday through Thursday night...SCA conditions likely Thursday afternoon into early Thursday night as NW winds increase to near 25-30 kts and seas 4-6 feet.

Friday through Friday night...No marine hazards anticipated.

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 Storm Watch from late Monday night through Tuesday evening for PAZ054-055. NJ...Winter Storm Watch from late Monday night through Tuesday evening for NJZ001. DE...None. MD...None. MARINE...Small Craft Advisory from 10 PM this evening to 6 AM EST Monday for ANZ430-431. Small Craft Advisory until 9 AM EST Monday for ANZ450>455. Gale Watch from Tuesday afternoon through late Tuesday night for ANZ450>455.