textproduct: Philadelphia/Mt Holly

SYNOPSIS

High pressure over eastern Canada builds down through the Northeast this morning before sliding offshore by midday. Low pressure approaches from the west and passes through the region later today and tonight before departing Saturday morning. High pressure then passes through the region Saturday afternoon through Sunday afternoon. Another clipper system passes through the region late Sunday through Monday. High pressure returns for the start of the new work week. Another disturbance may affect the area Wednesday through Thursday.

NEAR TERM /THROUGH SATURDAY/

An impactful winter storm will impact the area beginning later today and continue through Saturday night. The change in the forecast was to upgrade the Winter Weather Advisories for Somerset, Middlesex, and Monmouth to Winter Storm Warnings. This was largely done due to the potential for mesobanding to developing just south of the NYC metro area which would heavily impact travel along the NJTP and I- 78. Southern portions of these three counties will likely not see the higher snow totals as sleet mixing in will significantly reduce snowfall totals.

The overarching theme still remains that the key threat will be widespread travel disruptions during the evening hours and heading into the overnight hours.

The setup for this storm is an area of low pressure currently over the Great Lakes will dive toward the Mid-Atlantic and moving offshore by Saturday morning. Cold air will be firmly entrenched at the surface as a Canadian high pressure system locks in a cold air damming pattern across the Mid Atlantic. As the surface low digs through the Mid Atlantic it will bring a widespread mix of wintry precipitation across our forecast area.

Warm air advection in the mid levels will promote lift this afternoon and lead to an advancing precipitation shield during the early to mid afternoon. Generally guidance is showing this leading edge to starting out as snow for most areas with predominantly sleet starting to mix in as a low level warm nose starts pushing into the region. There still is some considerable spread in the guidance at this point with respect to the strength of the warm nose and the extent it pushes northward. Looking at ensemble soundings in DESI suggest that the warm noise will push close to I-78 but stays just south. The only guidance that pushes the warm air further north is the NAM and it certainly can't be discounted given the historical record when it comes to boundary layer temps in previous winter storms across the Mid Atlantic.

Further south across the Philly Metro area, the ensemble guidance confirms deterministic guidance with a modest 0.5-1.5C warm noise setting up over the southern portion of the forecast area.

What all this means is that in areas north of I78, we'll likely see mostly snow throughout the event, from roughly Philly to I-78, we'll see a widespread mix of snow and sleet generally favoring sleet as there's warm air starting the melting process aloft, and for areas south and east of Philly it will be mostly a rain event.

As for specific totals, the challenge in addition to ptype is the presence of a good 850mb deformation band expected to develop as a result of strong 700mb fgen. This points to the development of a mesoscale band that sets up somewhere between I78 and the Catskills. Within that band, we'll see orographic lift and snowfall rates of 1-2" an hour. Ensemble guidance suggests that the band will most likely be pushing through between 7-11pm but could arrive as early as 5pm.

This leads to snowfall totals in the warned area ranging from 4-6 for the lower elevation's of Carbon/Monroe in PA and Warren/Sussex in NJ with local amounts up to 8-12 in the higher elevations. All of this remains subject to the warm nose staying south however so the failure mode for this forecast is somewhat high. Where the advisories were upgraded this morning represents mainly an impact based upgrade. There is potential for 5-6 inches of snow across the northern tier of Somerset, Middlesex, and Monmouth, but regards of the higher end amounts, the presence of inch an hour snow rates in the night time rush hour on a Friday suggest high potential for travel disruptions over the holiday weekend.

Looking further south, its really all about how strong the warm nose is. Soundings and guidance have honed in on the idea that for most of this area, its going to be predominately a sleet storm. Current forecast has about 1 to 5" or so of snow and sleet with a glaze of ice possible in the Allentown area.

Around the Philly metro area and into South Jersey, generally a tenth to 2 inches are expected. Areas along and west of the Delaware River also see some light icing, up to a 0.05" or so. Thinking the stronger dynamics and colder air is more off to the north, limiting snow totals with a brief period of snow and a prolonged period of sleet. While less snow is expected, it will be quite messy with the combination of snow, sleet, and freezing rain. The Winter Storm Watch was converted to a Winter Weather Advisory in this area as well.

Going further south to far South Jersey, and northern Delmarva there is increased confidence in potentially some more icing compared to other locations. Low-level cold air will struggle to be removed as warm-air advection at the surface is not particularly strong. The result will be a period of freezing rain and a Winter Weather Advisory for icing rather than snow in Salem/Cumberland County in NJ, New Castle County in DE, and Kent County in DE. HREF Probability is around 60-80% to see measurable ice accumulation in these spots tomorrow night. While there will be some snow at onset, significant accumulations are unexpected, with amounts only up to an inch.

For the South Jersey coast and lower Delmarva, this will primarily be a rain event. While some flakes are possible at onset, a quick change to rain is expected.

Precipitation tapers off Saturday morning as temperatures struggle to warm into the 30s. For areas with a fresh snowpack anticipate sub freezing max temps while areas further south should be able to warm into mid to upper 30s.

SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY/

High pressure centered over eastern Canada builds down into the Northeast Saturday night, and the center of the high will pass through New Jersey before sliding off the Mid-Atlantic coast Sunday afternoon. A cold night will be on tap, especially across northern zones where a fresh snowpack will lie, with lows in the teens across the southern Poconos and northern New Jersey, and in the low 20s from across western portions of the Lehigh Valley through around Trenton. Lows will be in the low to mid 20s over the Delaware Valley and southern New Jersey, and in the upper 20s in Delmarva. While winds will be light, skies should be cloudy enough to keep optimal radiational cooling conditions from developing.

Low pressure will organize and develop over the Midwest and will lift north into the Great Lakes on Sunday. A warm front will extend from that low and will lift north through the region Sunday afternoon and evening. With warm air advection underway, high temperatures will rise into the mid and upper 40s for Delmarva and southern New Jersey, and in the upper 30s to low 40s from the Lehigh Valley and most of northern New Jersey down through the Delaware Valley. For the southern Poconos and far northwest New Jersey, highs will be in the low to mid 30s. Precipitation will be rain for the majority of the forecast area. However, a period of freezing rain is possible for the southern Poconos and possibly even the Lehigh Valley. 01Z NBM showing a 30-40 percent probability of greater than 0.05 inches of freezing rain ending 7 am Monday and a 20-30 percent probability of greater than 0.10 inches of freezing rain ending 7 am Monday. Rain picks up Sunday night, and in general, 1/4 to 1/2 inch or so of QPF is possible from around Philadelphia to areas north and less than 1/4 inch for southern New Jersey and Delmarva. Lows will be in the upper 20s to low 30s across the southern Poconos, northern New Jersey, and the Lehigh Valley and in the mid and upper 30s for the Delaware Valley, southern New Jersey, and Delmarva. Temperatures will gradually rise through the overnight hours.

LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/

Low pressure over the Great Lakes Monday morning will intensify as it tracks to the north and east into eastern Canada. This scenario would have the region in the warm sector on Monday with rain throughout the day and highs in the 50s to near 60. A strong cold front passes through the region late Monday afternoon and Monday evening, and then a tight northwest pressure gradient forms over the area Monday night through Tuesday. Gusty northwest winds will develop over the area, and strong cold air advection will be underway. Winds Monday night and Tuesday will increase to 20 to 30 mph with 30 to 40 mph gusts, and there is the potential for gusts up to 50 mph across the southern Poconos. Lows Monday night will mostly be in the 20s and highs on Tuesday will be in the 30s. Min wind chills Monday night look to be in the single digits to teens, should those winds develop.

Temperatures moderate a bit on Wednesday, but another weak disturbance passes through the region on Wednesday, and some cooler air may filter into the region Thursday.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

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

Today...VFR thru the morning then lowering CIGS during the afternoon. Precipitation expected to affect the terminals from west to east in the 18Z thru 00Z period. RDG/ABE may start as light snow in the afternoon before sleet mixes in. MVFR vis and cigs at the onset will begin approaching IFR across the I-95 terminals by 00Z. East winds up to 10 knots. High then medium confid.

Tonight...Widespread snow and sleet will likely drop all terminals to IFR with LIFR conditions possible. For the I-95 corridor, snow will change to a prolonged period of sleet, with possibly some brief freezing rain, especially at KPHL/KPNE/KILG. Precipitation should change to all rain at some point from KPNE on south after 06z. Confidence remains low on timing of precipitation types and precipitation changes. Winds generally out of the east around 5 to 10 kt.

Outlook...

Saturday...Wintry precip gradually diminishes through the morning and early afternoon, but light snow showers or drizzle (possibly freezing showers/drizzle) may linger through mid to late morning. IFR/LIFR ceilings slow to improve to MVFR and possibly VFR by the afternoon. Winds generally from the NE around 10 kts. Low confidence.

Saturday night...VFR.

Sunday through Monday...IFR in RA by Sunday afternoon, though a brief period of FZRA possible at KRDG/KABE Sunday afternoon and evening. S winds 5 to 10 kt, turning W-NW 10 to 20 kt with 25 to 30 kt gusts late.

Monday night through Tuesday...VFR. NW winds 15 to 25 kt with 30 to 35 kt gusts.

MARINE

SCA remains in effect with gusts up to 25kts expected. Seas will gradually subside through the day today with low-end SCA winds and seas continuing through the morning Friday morning. Precip arrives late in the day today with a mix of snow then sleet for the northern zone and mostly rain across the southern zone but no wind related hazards expected. Next chance for SCA flags will be on Saturday as seas start building again behind the departing system.

Outlook...

Saturday night...Sub-SCA conditions expected.

Sunday...Sub-SCA conditions expected. VSBY restrictions in rain starting in the afternoon.

Sunday night through Monday...SCA conditions expected. South winds with 25 to 30 kt gusts Sunday night, then winds turn NW late Monday. VSBY restrictions in rain.

Monday night through Tuesday...Gale force winds likely.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Weather Advisory from 3 PM this afternoon to 10 AM EST Saturday for PAZ070-101-102. Winter Storm Warning from 1 PM this afternoon to 10 AM EST Saturday for PAZ055. Winter Weather Advisory from 1 PM this afternoon to 10 AM EST Saturday for PAZ054-060>062-071-103>106. NJ...Winter Weather Advisory from 3 PM this afternoon to 10 AM EST Saturday for NJZ009-014>021-026-027. Winter Storm Warning from 1 PM this afternoon to 10 AM EST Saturday for NJZ001-007-008. Winter Storm Warning from 3 PM this afternoon to 10 AM EST Saturday for NJZ010-012-013. DE...Winter Weather Advisory from 3 PM this afternoon to 10 AM EST Saturday for DEZ001. MD...Winter Weather Advisory from 3 PM this afternoon to 10 AM EST Saturday for MDZ012. MARINE...Small Craft Advisory until 1 PM EST this afternoon for ANZ450>455.