textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Low pressure continues to be on track to impact the area with the potential for significant snow impacts. While there are some trends that show a slightly more northern track which could result in sleet and freezing rain that would result in slightly lower snowfall totals, impacts will still be significant. Confidence remains low on exact snow totals and where the highest snow totals will fall.

KEY MESSAGES

1. A warm-up is expected through Thursday ahead of an arctic cold front that crosses through the area late in the day.

2. An Arctic airmass spreads into the region Friday night. Bitterly cold temperatures and wind chills return to the area Friday night through Saturday.

3. Low pressure will impact the area with the potential for significant snow impacts, as well as a wintry mix, Saturday night through Monday morning.

DISCUSSION

KEY MESSAGE 1...A warm-up is expected through Thursday ahead of an arctic cold front that crosses through the area late in the day.

High pressure has shifted offshore. Low pressure tracking through the northern Great Lakes will continue to slide east, lifting a warm front through the area early this evening. Despite very dry low levels, and the better lift/moisture displaced further west, a few flurries and/or snow showers may clip the Lehigh Valley, Poconos and northern New Jersey as the front lifts through this evening. No accumulations are expected. A warm air advection regime will begin once the front passes, so low temperatures tonight will likely occur early this evening, before ultimately rising a few degrees overnight. Temps tonight will largely range in the upper 20s to upper 30s.

The WAA advection regime will continue on Thursday as southerly flow continues to pump milder air northward. As a result, high temperatures are expected to rise into the low 40s to low 50s across the area. However, the warmth will be short-lived as yet another arctic cold front associated with the aforementioned low pressure crosses through the area late in the afternoon. As of now, the front lacks any moisture this far south, so other than an increase in clouds during the day, no precipitation is expected.

KEY MESSAGE 2...An Arctic airmass spreads into the region Friday night. Bitterly cold temperatures and wind chills return to the area Friday night through Saturday.

A cold front moves through the region Friday night, and then strong cold air advection will be underway late Friday night through Saturday. A tight pressure gradient forms between low pressure over the Canadian Maritimes and high pressure building into the Great Lakes and Ohio Valley. Northwest winds will increase to 15 to 20 mph with 25 to 35 mph gusts Friday night, diminishing through the day Saturday. Low temperatures will be down into the single digits for the southern Poconos, northern New Jersey, and Lehigh Valley down to the Fall Line and from 10 to 15 degrees for southern New Jersey, the Delaware Valley, including Philadelphia, and Delmarva. Minimum wind chills values will range from 0 to 10 below for the majority of the area, and from 10 to 20 below for the Southern Poconos and far northwest New Jersey, where a Cold Weather Advisory may be needed.

KEY MESSAGE 3...Low pressure will impact the area with the potential for significant snow impacts, as well as a wintry mix, Saturday night through Monday morning.

High pressure building into the Great Lakes and Ohio Valley on Saturday lifts into eastern Canada on Sunday. The positioning of the high will greatly impact how the storm affects the local area.

A deep upper trough with strong shortwave energy exits out of the West Coast and Rocky Mountains Saturday night, and then surface low pressure organizes and develops ahead of it over the Texas Gulf Coast. This low then tracks through the Gulf Coast states and the Tennessee Valley Sunday before moving off the Mid-Atlantic coast Sunday evening, then moving out to sea Sunday night.

Snow develops Saturday evening first over Delmarva, then lifting north into the southern Poconos and far northwest New Jersey by daybreak Sunday. What continues to be an issue is the position of the surface low. There are some signals that the high will be a bit farther north, allowing the low to take a slightly more northern track. This allows for at least some sleet and freezing rain for Delmarva and southern New Jersey, and possibly even a period of all rain for southern Delaware and extreme southeast New Jersey. Snow continues through Sunday night before tapering off Monday morning.

13Z/21 NBM has a 90 to 100 percent probability for more than 6 inches of snow for most of Delmarva, the Delaware Valley including Philadelphia and into southern New Jersey for the 48 hour period ending at 7 am Monday and an 80 to 90 percent probability for more than 6 inches of snow for the southern Poconos, Lehigh Valley, and northern New Jersey. In addition, 13Z/21 NBM has a 70 to 80 percent probability for more than 12 inches of snow for most of Delmarva, the Delaware Valley including Philadelphia and into southern New Jersey for the 48 hour period ending at 7 am Monday and a 60 to 70 percent probability for more than 12 inches of snow for the rest of the forecast area.

In addition, the 13Z/21 NBM has a 25 to 35 percent probability for more than 0.1 inches of ice for Delmarva and 10 to 20 percent probability for more than 0.1 inches of ice for the I-95 corridor in southeast Pennsylvania and New Jersey, highlighting the potential for that mixing.

In terms of models, the 12Z/21 GFS has the track farthest south of the models, and results in the highest snowfall, while both the Canadian and the ECMWF have tracks farther north, which allows for more warmer air aloft and cuts back on snowfall totals, especially for southern New Jersey and Delmarva.

While confidence remains high that widespread impacts will be significant from this storm, confidence remains low in exact snowfall totals and where those highest totals will occur.

AVIATION /19Z WEDNESDAY THROUGH MONDAY/

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

Rest of Today (through 23Z)...VFR with increasing mid-high level clouds. South-southwest winds around 10 kt with occasional gusts up to 15-20 kt possible. High confidence.

Tonight...VFR with mostly cloudy skies. South-southwest winds around 5-10 kt. Low-level wind shear likely up to 45 kt mainly between 00- 10Z. Slight chance of a snow shower at KRDG/KABE. Moderate-high confidence.

Thursday...VFR. Mostly clear skies in the morning with scattered afternoon clouds. West-southwest winds around 10-15 kt with occasional gusts up to 20-25 kt possible in the afternoon. Moderate- high confidence.

Outlook...

Thursday night through Saturday...VFR. NSW.

Saturday night through Sunday night...Sub-VFR conditions in SN, which may be heavy at times. Wintry mix possible on Sunday for at least KILG/KMIV/KACY.

Monday...Lingering SN in the morning, then improving conditions.

MARINE

Small Craft Advisories from on the Atlantic Coastal Waters from Cape May to Cape Henlopen and Sandy Hook to Cape May are now in effect until 10 AM Thursday and 1 PM Thursday, respectively. A Small Craft Advisory has been added for the Lower Delaware Bay from 7 PM tonight until 5 AM Thursday. No hazards are currently issued for the Upper Delaware Bay.

South-southwest winds increase this afternoon to around 15-20 kt where gusts up to 25-30 kt are possible through this evening. Winds and gusts will then gradually diminish through the day on Thursday. Seas of 2-4 feet this afternoon, build to 4-7 feet tonight, before gradually lowering to 3-5 feet on Thursday. Fair weather expected outside of SCA conditions.

Outlook...

Thursday night through Friday...Sub-SCA conditions.

Friday night through Saturday...SCA conditions expected through Saturday morning. Freezing spray possible.

Saturday night through Sunday night...SCA conditions expected with gale force winds possible. VSBY restrictions in snow and wintry mix.

Monday...SCA conditions. VSBYs improve through the day.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 7 PM this evening to 5 AM EST Thursday for ANZ431. Small Craft Advisory until 1 PM EST Thursday for ANZ450>453. Small Craft Advisory until 10 AM EST Thursday for ANZ454-455.