textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Today's system is winding down a bit sooner than expected. No additional snowfall is expected this afternoon and Winter Weather Advisories for today have been cancelled.

Low pressure for Sunday continues to trend north and west with increasing confidence for most of the area to see 1 to 4 inches of snow. New Winter Weather Advisories have been issued for our urban corridor along with adjacent zones just north and south.

KEY MESSAGES

1. Dense fog will reduce visibilities to as low as 1/4 mile or less through this evening.

2. A coastal low is now expected to bring widespread snow accumulation to the area for Sunday with 1 to 4 inches generally expected.

3. A prolonged period of below normal temperatures are expected this upcoming week, with single digit and below zero wind chills possible both Monday Night and Tuesday Night.

DISCUSSION

KEY MESSAGE 1. Dense fog will reduce visibilities to as low as 1/4 mile or less through this evening.

Skies cleared out, and the combination of low level moisture over the area due to snowfall earlier today with weak cold air advection has resulted in areas of dense fog across portions of southeast Pennsylvania and New Jersey. A Dense Fog Advisory is now in effect until midnight. That fog should begin to dissipate when the clouds and rain and/or snow moves in from the west.

KEY MESSAGE 2...A coastal low is now expected to bring widespread snow accumulation to the area for Sunday with 1 to 4 inches generally expected.

The latest indications are that the coastal low expected for Sunday will track a bit farther north and west than earlier thinking bringing a more widespread accumulating snowfall event to the area. Overnight tonight into early Sunday, a shot of upper level energy will be rounding the based of the long wave upper level trough over the east helping to initiate surface cyclogenesis near the SE coast along the baroclinic zone. Expect that associated precipitation with with this developing low will start to break out over the area around dawn from SW to NE. Given the lack of any "fresh" arctic air over the area along with the cloud cover, temperatures by late tonight will only be down into the 20s to around 30 north of the I- 95 corridor and in the low to mid 30s for areas along and SE here. As a result, current thinking is precip will likely start as rain or a rain/snow mix near the coast with even some mixing as far N/W as the I-95 corridor possible. However as we progress through the day Sunday the low will continue to deepen and move north and east off the coast with the center tracking by well to our east. As this occurs, precip should intensify somewhat through the late morning into the early afternoon with the system also infusing with an arctic boundary coming in from the west and shifting winds to more of a N/NW direction. The upshot of this as the precip should transition to mainly just snow through the late morning into the afternoon as temperatures hold fairly steady or even start to fall. With the shift in the system a bit farther north and west it now appears areas near and S/E of the urban corridor will see around .25 to .50 inches of liquid. The initial rain mix will keep snow amounts down near the coast where amounts of around .5 to 1.5 inches are expected. However this setup now looks to put the jackpot near the I- 95 corridor and adjacent zones on either side just NW and SE. As a result, we've issued a Winter Weather Advisory now in effect from 6AM to 8 PM Sunday for these zones across the heart of our CWA where 2 to 4 inches of snow can generally be expected. North and west of here across the Lehigh Valley into the southern Poconos and extreme NW NJ generally expect around 1 to 2 inches of snow.

The snow looks to eventually taper off late Sunday afternoon into the evening from west to east as the deepening low pulls away.

KEY MESSAGE 3...A prolonged period of below normal temperatures are expected this upcoming week, with single digit and below zero wind chills possible both Monday Night and Tuesday Night.

A deep trough sets up over the eastern US on Sunday Night, ushering in an arctic airmass. The stretch of below normal temperatures commences that night, with lows in the mid to upper teens for most. The height of the cold airmass will be over our area on Monday Night through Tuesday, where 850 mb temperatures will be around -15 to -20 C, resulting in low temperatures at the surface into the teens and single digits. Combined with breezy winds expected Monday night, this will result in wind chills between -10 and -15 in the southern Poconos, below zero over the Lehigh Valley, and in the single digits over the rest of the area. We almost certainly need some cold weather headlines, especially in the southern Poconos, but cannot rule them out for the rest of the region.

With the arctic airmass directly overhead on Tuesday, highs won't climb out of the 20s. With a steady westerly breeze, wind chills will be in the single digits and the low teens. Areas in the southern Poconos likely won't get above 0 even during the daytime.

High pressure will center itself overhead Tuesday night, allowing winds to diminish and skies to become mostly clear. We could see decent radiational cooling as a result, potentially leading to more widespread overnight lows in the single digits than the previous night. While temperatures may be colder, the calmer winds will make the wind chills less severe, though still in the single digits for most and below zero in the southern Poconos. These values will not warrant any cold weather headlines, but it will be bitterly cold nonetheless.

We break the cold snap on Wednesday as temperatures get toward more seasonable levels, and even potentially above normal on Thursday. However, a cold front moving through sometime late week will drop temperatures back to near or slightly below normal levels to end the work week.

AVIATION /01Z SUNDAY THROUGH THURSDAY/

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

Tonight...Fog across the terminals has reduced VSBYs into the IFR/LIFR range except for KPHL/KILG. Increasing clouds from the south may limit additional development and may cause VSBYs to rise once it overspreads the terminals. Snow arrives towards dawn with lower VSBYS and CIGS then. Low confid overall.

Sunday...MVFR/IFR conditions as snow develops during the morning, especially for the I-95 and South Jersey terminals, with even some rain/snow mix possible at the onset especially at KMIV/KACY. Some light snow should occur at KRDG/KABE. Winds becoming north-northwest and increasing to around 10 knots. Low confidence in the details including the timing.

Outlook...

Sunday Night...Primarily VFR, though KMIV/KACY could see some lingering restrictions as a coastal low pulls away.

Monday and Monday Night...VFR. West/southwest wind gusts 20-30 kt during the day, 15-20 kt at night.

Tuesday and Tuesday Night...VFR. West/northwest wind gusts 20-30 kt, becoming 10 kts or less at night.

Wednesday and Wednesday Night...VFR. South/southwest wind gusts 15- 25 kt, becoming 10-15 kt at night.

MARINE

The Small Craft Advisory is no longer in effect as conditions have subsided to sub-SCA criteria.

Low pressure will bring rain mixed at times with snow over the waters for Sunday with increasing NW winds in the system's wake for Sunday night. Another Small Craft Advisory will likely be needed.

Outlook...

Sunday Night...SCA conditions possible (20-40%). Rain/snow expected on the coastal waters.

Monday through Tuesday...SCA conditions likely (60-70%) with wind gusts out of the west/southwest around 25-30 kt and seas near 5 feet.

Tuesday Night...No marine headlines expected.

Wednesday through Wednesday night...SCA conditions possible (50%) as gusts out of the south/southwest get near 25 kt.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Weather Advisory from 6 AM to 8 PM EST Sunday for PAZ070-071-101>106. Dense Fog Advisory until midnight EST tonight for PAZ060>062- 070-071-101>106. NJ...Winter Weather Advisory from 6 AM to 8 PM EST Sunday for NJZ008>010-012-013-015>019. Dense Fog Advisory until midnight EST tonight for NJZ007>010- 012-013-015-017>020-022-027. DE...Winter Weather Advisory from 6 AM to 8 PM EST Sunday for DEZ001. MD...None. MARINE...None.