textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Snowfall totals for late tonight through Sunday night lowered about 2-4 inches across the region.

Gale Warning issued for all marine areas for Sunday into Sunday night.

KEY MESSAGES

1. A major winter storm will impact our entire area from late tonight/early Sunday morning through Sunday night. This storm will bring significant snow and ice accumulations to the entire area.

2. Arctic air remains in place with temperatures below freezing through next weekend.

DISCUSSION

KEY MESSAGE 1...A major winter storm will impact our entire area from late tonight/early Sunday morning through Sunday night. This storm will bring significant snow and ice accumulations to the entire area.

Arctic air is firmly entrenched across the entire region this afternoon and that will continue right into Sunday. This will set the stage for a winter storm that will start to arrive late tonight.

A massive ribbon of large scale ascent associated with jet dynamics and low pressure will overspread our area late tonight and especially during Sunday. As this event gets underway later tonight, warm air advection around the 700 mb level will drive increasing ascent from south to north. This will result in a quickly expanding snow shield from south to north and east to west overnight tonight into the early morning hours on Sunday. The air mass is very cold and also very dry to start, therefore it will take some time for the lowest levels to saturate for snow to reach the surface. Once this occurs, incoming impressive 700-850 mb frontogenesis will result in increasing snowfall rates from south to north Sunday morning into early Sunday afternoon. Model cross sections show deep lift crossing the prime snow growth area for about a 4-7 hour window from south to north. It is during this period of time where the snow will be falling at least 1-2 inches per hour. This will result in quick accumulations during the morning hours, and lingering into the afternoon hours especially near and north/west of I-95.

As one surface low tracks up the Ohio Valley during Sunday with arctic high pressure near northern New England, a new low is forecast to develop off the Mid-Atlantic coast. The model trend continues to develop this low closer to the coast. This results in a strong easterly jet near 700 mb advecting in milder air. This milder air mass will initially be above the arctic air, however closer to the coast the milder air should win out and edge surface temperatures above freezing for a time Sunday afternoon and early evening. This will also translate to snow mixing with and changing to sleet and then freezing rain, especially from I-95 south and east. However, some sleet looks like it will make it all the way to near the I-80 corridor during the peak of the storm Sunday afternoon or evening. Closer to the coast, as long as surface temperatures rise above freezing then just plain rain occurs. The forecast challenge continues to be the evolution of this warm layer aloft and thus the changing precipitation types. This timing will significantly determine how much snow accumulates prior to any mixing or changeover. The model soundings show the warm layer aloft initially being not all that deep, therefore if the precipitation intensity is heavy enough and longer then it would be enough to cool that layer to keep snow going longer. This may occur especially the farther northwest one goes across our area, however this is of lower confidence. There will also be a zone of freezing rain where the transition occurs, and this looks to be centered right near the I-95 corridor.

The main change with this update is lowering the snow amounts by about 2-4 inches across the entire area. The NBM snowfall totals have also been trending downward given the guidance showing more of a mix or changeover for many areas. We are still hitting warning criteria for all areas at this point and therefore will maintain the Winter Storm Warnings. The start times of the warning has been delayed by several hours, but left the end times for now which may be to long. Snow ratios will be high to start given the arctic air mass in place, however these will then decrease quite a bit from south to north as the event continues on during Sunday and Sunday night. This will change the consistency of the snow from a dry and fluffy type to a more moist/wet type. In areas that have a mix or change to sleet and/or freezing rain will have an ice layer on top of the snow which will make conditions more difficult. Not a lot of change regarding ice amounts from freezing rain. The amount of freezing rain realized will depend on small temperature changes aloft whether it is sleet or freezing rain. The combination of snow, sleet and freezing rain will result in MAJOR impacts across the region. There is an increasing concern that some areas will have a prolonged period of accumulating sleet.

The precipitation tapers off Sunday evening with some drizzle or even freezing drizzle possible. Some guidance has hinted at some light snow occurring again Sunday night into early Monday morning before ending. This is of lower confidence, but if it were to occur any additional amounts look to be on the light side. Northeast winds will be increasing some during Sunday and where the snow remains drier could lead to some blowing and drifting snow.

KEY MESSAGE 2...Arctic air remains in place with temperatures below freezing through the start of next weekend.

A deep longwave trough will remain situated over the eastern CONUS through the the start of next weekend where several shortwaves embedded within it will bring several reinforcing shots of cold arctic air to the region. At the surface, strong arctic high pressure settles over the northern Gulf Coast through Wednesday, whereas a couple of cold fronts are also progged to track through the area as well.

In wake of departing low coastal storm and high pressure building over the Deep South, the return of very cold air is anticipated to begin Monday night into Tuesday morning. A cold front is then projected to cross through the area Tuesday night into Wednesday morning, followed by another cold front Wednesday night into Thursday morning. All in all, periods of windy conditions are expected throughout the week with gusts up to 30-35 mph at times. High temperatures will largely remain in the teens and 20s with lows in the single digits to even several degrees below zero throughout the course of the week.

As a result, wind chills will mainly range in the single digits to teens during the day. More importantly at night, wind chills will become dangerously cold generally ranging between 0 to -10 degrees in most areas. For the areas north and west of I-95 and especially in the higher terrain where snow pack is deeper, wind chills may range between -10 to -25 degrees at times. Overall, this will be a prolonged period of cold weather, where Cold Weather headlines will likely be warranted in the coming days.

With such a long duration of below freezing temps, do not expect the new snow pack to melt much. Aside for a slight chance of snow on Thursday with low developing offshore, dry weather is expected.

AVIATION /19Z SATURDAY THROUGH THURSDAY/

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

Rest of this afternoon...VFR with high level clouds. North-northwest winds around 10 knots. High confidence.

Tonight...VFR ceilings, which lower overnight. Sub-VFR conditions should quickly develop between about 08-10z as snow develops. Light and variable winds, becoming northeast 5-8 knots. Low confidence regarding the timing details.

Sunday and Sunday night...LIFR/IFR conditions. Snow, heavy at times with hourly snowfall rates of 1-2 inches, will mix with and change to sleet and freezing rain at KILG to KMIV and KACY first then work northward during the afternoon. KABE and KRDG remain all snow the longest. The precipitation looks to change to rain for a time at KACY and possibly at KMIV by later Sunday afternoon and evening. Northeasterly winds 10-15 knots with some gusts 20-25 knots. Low confidence regarding the timing details for the changing precipitation types.

Outlook...

Monday...Lingering sub-VFR conditions likely in the morning with light snow possible, improving to VFR by the afternoon. Gusty winds up to 25-30 kt in the afternoon.

Monday night through Thursday...VFR. Aside for a slight chance of snow on Thursday, no significant weather expected.

MARINE

The conditions continue to diminish this afternoon as arctic high pressure builds nearby. As low pressure develops near our marine area during Sunday, northeasterly winds will increase. A period of gale force wind gusts look probable especially for the New Jersey Atlantic coastal waters. It may be more marginal south of there including Delaware Bay. Given that it is close, went ahead and upgraded the Gale Watch to a Warning for Sunday into Sunday evening.

Outlook...

Monday through Monday night...SCA conditions continuing with gales possible on Monday night. Sustained winds of 20-30 kt with gusts up to 35 kt. Seas of 6-9 feet. Freezing spray possible.

Tuesday through Tuesday night...Lingering SCA conditions probable on Tuesday before diminishing Tuesday night. Wind gusts up to 20-25 kt and seas of 3-6 feet. Freezing spray possible.

Wednesday through Wednesday night...No marine headlines expected. Winds up to 20 kt and seas of 2-4 feet.

Thursday...SCA conditions probable. Wind gusts up to 25-30 kt with seas of 2-4 feet.

CLIMATE

An impactful winter storm bringing significant snow accumulations to the area is expected on Sunday. Here are the 1-day daily record totals for January 25th from our snow observing sites. Some of these could be challenged.

January 25th Daily Snowfall Records

Allentown (ABE) 10.4" (1988) AC Airport (ACY) 8.3" (2000) Philadelphia (PHL) 8.5" (2000) Reading (RDG) 9.0" (1905) Trenton (TTN) 14.0" (1905) Wilmington (ILG) 10.0" (2000)

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Storm Warning from 2 AM Sunday to 1 PM EST Monday for PAZ054-055-060>062-070-071-101>106. NJ...Winter Storm Warning from 2 AM Sunday to 1 PM EST Monday for NJZ001-007>010-012-015>019-021. Winter Storm Warning from 2 AM Sunday to 7 AM EST Monday for NJZ013-014-020-022>027. DE...Winter Storm Warning from midnight tonight to 1 PM EST Monday for DEZ001>004. MD...Winter Storm Warning from midnight tonight to 1 PM EST Monday for MDZ012-015-019-020. MARINE...Gale Warning from 6 AM to 6 PM EST Sunday for ANZ430-431- 452>455. Gale Warning from noon Sunday to midnight EST Sunday night for ANZ450-451.