textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Confidence is increasing that there will be accumulating snow Saturday morning northwest of the I-95 corridor. Confidence remains low in the threat for significant snow on Sunday.

High confidence remains in a prolonged stretch of below normal temperatures from Sunday through Wednesday, with an increasing likelihood of needing cold weather headlines Monday Night and possibly Tuesday Night as wind chills could fall below zero and below -10 in the southern Poconos.

KEY MESSAGES

1. Blustery and cold through today. Wind chills in the single digits in many areas and below zero in the southern Poconos this morning. Wind chills will then remain in the 10s and 20s through much of the day.

2. Light snow likely early Saturday before transitioning to rain for most locations. Light accumulation expected near and northwest of I-95, with highest totals near and north of I-78.

3. A system developing offshore may bring snow to coastal areas on Sunday. Threat for a significant snowfall remains low at this time.

4. Prolonged period of below normal temperatures expected from Sunday into the middle of next week.

DISCUSSION

KEY MESSAGE 1...Blustery and cold through today. Wind chills in the single digits in many areas and below zero in the southern Poconos this morning. Wind chills will then remain in the 10s and 20s through much of the day.

The peak of the cold air advection has subsided, but cold and blustery conditions will remain, especially through this morning. Temperatures ranging from around 20-25 degrees with winds around 10-20 mph will keep wind chills into the single digits and teens for much of the area, and slightly below zero in the southern Poconos until temperatures begin to rebound later this morning. The gusty westerly winds will begin to subside into the afternoon hours while shifting southwesterly, which will help boost temperatures well into the 30s into the afternoon under party cloudy skies. The persistent breeze will still keep wind chills in the 20s for much of the day though. Not as cold tonight with lows in the mid to upper 20s and light south to southwest winds.

KEY MESSAGE 2...Light snow likely early Saturday before transitioning to rain for most locations. Light accumulation expected near and northwest of I-95, with highest totals near and north of I-78.

A longwave trough sets up over the Eastern US, with a series of shortwaves moving over the area. The first of this system looks to arrive in the pre-dawn hours Saturday morning. This coincides with an increase in synoptic lift with an area of 500 mb positive vorticity advection and low-level weak warm-air advection setting up. The main change with this latest forecast update is that the system continues to look slightly more dynamic. As a result, QPF totals have increased a bit with areas near and northwest of I-95 forecast to receive between 0.1 and 0.2 inches of liquid over the course of Saturday. Southeast of I-95, totals remain less than 0.1 inches. Precipitation still looks to start primarily as snow, but changeover to rain gradually occuring from SE to NW.

Little to no accumulation of snow is expected for areas southeast of I-95 corridor before the changeover to rain occurs. For areas near and immediately northwest of the I-95 corridor, snowfall amounts will be anywhere from a dusting up to an inch. North of I-78, in the Lehigh Valley, northwest New Jersey and southern Poconos, snow will hang on the longest. With the increase in QPF, the downstream effect is that in this area, snowfall totals are now in the 1-2" range, with up to 3" possible at the higher elevations in the southern Poconos, where minimal mixing of rain and snow may occur.

KEY MESSAGE 3...A system developing offshore may bring snow to coastal areas on Sunday. Threat for a significant snowfall remains low at this time.

Focus later in the weekend remains on a coastal low expected to develop off the coast of the Outer Banks and lift off to the northeast on Sunday. The bottom line remains that the threat for a significant snow event remains low at this time. The synoptic pattern and associated surface pattern just do not quite match up well for a big snow event. Low pressure slides by well offshore, with some light snow expected from the I-95 corridor on south and east during the day on Sunday. Deterministic model guidance continues with a consensus towards a more offshore solution. It should be noted that some ensemble guidance, notably the AI GFS/GEFS, brings the track of the low closer so this solution still cannot be completely ruled out but the likelihood of this occurring remains low overall. NBM Probabilities of Advisory level snow (2-3" or more) with this forecast cycle remain around 20-25% at the immediate coast, 10-20% from the NJ Turnpike on south and east, and 5-10% for the rest of the area. Current forecast has 0.5-1.5" for the coast and southern Delaware with a coating to an inch up to the I-95 corridor. Cannot rule out a few flakes north and west of I-95 on Sunday, but nothing more than trace accumulation. Precipitation will move out on Sunday night.

KEY MESSAGE 4...Prolonged period of below normal temperatures expected from Sunday into the middle of next week.

A deep trough sets up over the eastern US, ushering in an arctic airmass. High pressure at the surface will be in control keeping things dry. Several days of below normal temperatures begin on Sunday, continuing through Wednesday. The height of the cold airmass will be over our area Monday Night through Tuesday Night, with both nights featuring wind chills in the single digits and below zero. As it stands now, it is becoming increasingly likely that Monday Night will be the coldest night, with Tuesday Night just a touch warmer. Cold Weather Headlines may be needed on one or both nights. Highs on Tuesday continue to trend colder, with current highs projected to be in the upper teens and low 20s. By Thursday, the trough may begin to lift, allowing temperatures to moderate closer to normal.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

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

Today...VFR. Westerly winds 10-15 kts with gusts up to 20-25 kts through 15-18Z, gradually diminishing by 18Z or so and shifting more southwesterly. High confidence.

Tonight...Mainly VFR, though ceilings may lower to near MVFR after 06Z, especially at RDG/ABE. Light snow may develop (20-40% chance) near and northwest of I-95 between 06-12Z, which could cause brief reductions to MVFR visibility. South to southwest winds 5-10 kts. Southwesterly LLWS possible, with winds near 2kft increasing to near 35-40 kts. High confidence in prevailing conditions early, then lower confidence on any MVFR conditions developing late.

Outlook...

Saturday through Sunday...VFR should generally prevail over the weekend, though sub-VFR conditions possible (30-50%) with periods of rain and snow moving through during the daytime periods.

Sunday Night...VFR. No significant weather.

Monday through Tuesday...VFR. Wind gusts out of the west around 15-25 kt.

MARINE

Gale Warnings in effect for all coastal waters through 10 AM. Westerly winds remaining near 20-30 kts with gusts up to 35 kts at times. Seas 4-6 feet. Freezing spray is possible (20-40% chance) through this morning as air temperatures fall into the mid to upper 20s.

Thereafter, winds should diminish pretty quickly, ending up below advisory levels into the afternoon. Advisory conditions will likely develop again overnight as southwesterly winds increase to 20-25 kts and seas build to 4-5 feet.

Outlook...

Saturday...SCA conditions expected with seas 4 to 6 feet.

Sunday...Mainly sub-SCA conditions expected.

Monday and Tuesday...SCA conditions possible (40-60% chance) with seas of around 5 feet and wind gusts of 20-30 knots.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Warning until 10 AM EST this morning for ANZ430-431- 450>455.