textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

There has been a trend for slightly slower cold air arrival Thursday morning, reducing snow chances and amounts.

For the long-term, PoPs were introduced for Sunday near the coast as there is a low chance for snow showers. Confidence is increasing in a prolonged cold stretch beginning on Sunday, continuing through at least Wednesday, where wind chills could drop into the single digits or even below zero.

KEY MESSAGES

1. A low pressure system will continue to bring some light rain to the area through tonight. Rain may mix with or change to snow for parts of the area before ending Thursday morning. Rapidly falling temperatures Thursday could lead to patchy ice and travel impacts.

2. Blustery and cold Thursday Night through Friday morning, with wind chills in the single digits or even below zero.

3. Multiple rounds of precipitation possible this weekend, some of which could fall as snow. Watching an area of low pressure slide by off the coast.

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

DISCUSSION

KEY MESSAGE 1...A low pressure system will continue to bring some light rain to the area through tonight. Rain may mix with or change to snow for parts of the area before ending Thursday morning. Rapidly falling temperatures Thursday could lead to patchy ice and travel impacts.

A potent upper-low will shift eastward from the Great Lakes region and towards the area tonight and into the day Thursday. An associated surface low will pass to the north of the area, with a cold front moving through the region late tonight into Thursday morning. Scattered areas of light rain will continue to overspread the region into this evening, though it should be light with minimal impacts.

An area of rain directly associated with the frontal passage is expected to pass through the region overnight. As the cold front moves through, there remains a possibility that rain could mix with or even change to snow, though this potential has decreased from the previous assessment. This is due in large part to a slowing trend in the arrival of the coldest air, with guidance now indicating it arriving near or after daybreak. A few snow showers will be possible across portions of eastern PA and northern NJ after the primary wave of precipitation moves through, primarily through mid-morning Thursday. Still forecasting around a dusting of snow for portions of the Lehigh Valley and into adjacent areas of northern NJ. Isolated amounts could still approach 1 inch in the Poconos should cold air come in more quickly, but this is no longer explicitly in the forecast. After the cold front moves through, temperatures will quickly fall into the day Thursday, and patchy ice development could still lead to hazardous travel in spots. This is most likely northwest of I-95.

In addition to the potential for hazardous travel due to patchy ice, breezy west-northwesterly winds are expected throughout the day Thursday as cold air advection continues across the region. Wind gusts through the day are expected to be 25-35 mph. This will lead to wind chills falling into the teens and 20s by afternoon for most, and into the single digits across the Poconos.

KEY MESSAGE 2...Blustery and cold Thursday Night through Friday morning, with wind chills in the single digits or even below zero.

The lowest 850 mb temperatures (-14 to -19 C) move over our region on Thursday Night, with breezy conditions continuing under the cold- air advection regime. This will result in temperatures dropping into the upper teens and low 20s with lower teens in the upper elevations of the Pocono Plateau and northwestern NJ. A well mixed boundary layer due to very cold air aloft will result in wind gusts around 20- 30 MPH through the night and 25-35 MPH for the higher elevations.

The outcome will be wind chills in the single digits for most, with areas in the Poconos likely getting below 0. Regionwide, we are a few degrees above needing any cold weather headlines, but after several days of above normal temperatures, this shot of cold air won't be very pleasant waking up to on Friday morning.

KEY MESSAGE 3...Multiple rounds of precipitation possible this weekend, some of which could fall as snow. Watching an area of low pressure slide by off the coast.

A longwave trough sets up over the Eastern US, with a series of shortwaves moving over the area, resulting in some unsettled weather this weekend. The first system looks rather weak and should move through on Saturday. Precipitation looks to start as mainly snow with onset occurring near or just after daybreak when temperatures are near their nadir. Weak warm air advection and daytime heating will quickly result in temperatures increasing though, transitioning to mainly rain for everyone except the northern Lehigh Valley and southern Poconos, where snow could hang on longer. Precipitation should be rather light though. Snowfall amounts up to an inch are possible in the Poconos, with up to half an inch possible north and west of the I-95 corridor. Nothing more than a trace for areas along and south/east of I-95. QPF amounts are less than a tenth of an inch across the region.

The second more organized system has more high-end potential but also could end up being nothing, depending on the track of an area of low pressure offshore Sunday into Sunday Night. The operational GFS and its AI counterpart have been going hard in the paint, bringing plowable snow to the NJ and DE coast. This appears to be an outlier though as the large majority of deterministic, ensemble, and probabilistic guidance support either just light snow near the coast, or shunting this system out to sea entirely and Sunday being totally dry. Bumped up PoPs slightly along the coast to low end chance near the coast, but nothing higher than 30% at this point. Overall, thinking the majority of guidance is correct in that this will be a minor event or even nothing at all. This is a result of a more strung-out upper air pattern. The clusters of ensembles that do support more snow show a more amplified pattern, with a deeper trough advancing into the eastern US, and stronger ridging over the western CONUS and north Atlantic. With the passage of a cold front on Saturday Night, there should be an abundance of cold air available, its just a matter if the surface low and precipitation shield get close enough, which looks unlikely at this point. NBM probabilities of 1"+ or more of snow is around 20-30% for the coast and 10% for 3" or more. It bears watching for sure, but the skepticism is there for any significant snowfall.

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 at least Wednesday. Highs struggle to get above freezing, and may even struggle to get out of the 20s on some days (especially Tuesday). Main concern will be overnight lows and wind chills though as most nights will feature apparent temperatures in the single digits. Areas in the Poconos likely (60-70% chance) have wind chills below zero. Cold Weather Headlines may be needed, especially on Tuesday Night.

AVIATION /00Z THURSDAY THROUGH MONDAY/

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

Tonight...Primarily VFR early, with MVFR ceilings expected to spread east across the terminals from 06-09Z ahead of a cold front. A period of light rain is also expected around this time at all terminals, with temporary vsby restrictions possible as well. There remains chance for rain to mix with or change with snow, particularly at KRDG and KABE. South- southwest wind 5-10 kt will become west late. Moderate confidence.

Thursday...MVFR ceilings will quickly lift to VFR by 14z. West- northwest wind increasing to 10-15 kt with gusts 20-30 kt. Slight chance (around 20%) of brief snow showers early for KRDG, KABE, and KTTN. Moderate confidence.

Outlook...

Thursday Night...VFR. Westerly winds 20-30 kt with gusts up to 35 kt.

Friday...VFR. Westerly wind gusts up to 20 kt.

Friday Night...VFR. No significant weather.

Saturday through Sunday...Primarily VFR, though sub-VFR conditions possible (30-50%) with periods of rain and snow moving through.

Sunday Night...Primarily VFR. Chance of snow (20-30%) around KACY/KMIV may result in restrictions.

Monday...VFR. No significant weather.

MARINE

A Small Craft Advisory is currently in effect for all NJ and DE Atlantic Coastal Waters. For the Delaware Bay, the Small Craft Advisory will go into effect at 3 AM Thursday.

Tonight, south-southwest winds 15-20 kt with gusts to near 25 kt. Winds will shift to west-northwest and increase to 20-25 kt with gusts around 30 kt on Thursday. Seas 3-5 feet through tonight, and 4- 6 feet on Thursday.

Outlook...

Thursday Night through Friday Morning...Gale Watch was converted to a Gale Warning for all marine zones. West/northwest winds around 20- 30 kt with gusts 34 to 38 kt. Chance of freezing spray (20-40%) over the open water.

Friday Afternoon...Small Craft Advisory conditions expected with seas 4 to 6 feet and wind gusts near 25 kt out of the west/northwest.

Friday Night through Saturday Night...SCA conditions likely (50-70%) with seas right around 5 feet.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 3 AM to 6 PM EST Thursday for ANZ430- 431. Gale Warning from 6 PM Thursday to 10 AM EST Friday for ANZ430- 431-450>455. Small Craft Advisory until 6 PM EST Thursday for ANZ450>455.