textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Snowfall totals were nudged up slightly, maybe an inch or so in most spots. Impacts remain unchanged. A major winter storm continues to unfold.

KEY MESSAGES

1. A major winter storm will impact our entire area through tonight 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 through tonight This storm will bring significant snow and ice accumulations to the entire area.

A winter storm that has resulted in winter weather alerts from Arizona to Maine is finally moving into our region this morning. A massive ribbon of large scale ascent associated with jet dynamics and low pressure will overspread through the morning. As this event continues to get underway, 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 as the morning goes on. Incoming impressive 700-850 mb frontogenesis will bring increasing snowfall rates from south to north this 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 today with arctic high pressure near northern New England, a new low is forecast to develop off the Mid-Atlantic coast. The track should end up inside the benchmark (40N, 70 W), which usually brings a mixed precipitation storm (and a forecasting headache). The track closer to the coast will result 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 this afternoon and evening. Above freezing surface temperatures should get right up I- 95/I-295/NJ Turnpike. 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 this 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. Mesoanalysis will be key to watch this in real time and make any late adjustments in the forecast as needed.

No significant changes were made to the forecast snow and ice totals other than a slight uptick in snow by about an inch or so. This change is minor and will result in no change to the impacts. We are still hitting warning criteria for all areas at this point and therefore will maintain the Winter Storm Warnings through tomorrow morning.

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 today into tonight. 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, and 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 /07Z SUNDAY THROUGH THURSDAY/

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

Today... Widespread IFR/LIFR expected as snow overspreads the area before dawn. Warmer air aloft will change snow to sleet then freezing rain as the morning/afternoon progresses. The latest TAFS (06Z) saw only minor timing changes to the previous fcst. Slow improvement possible after 00Z from SW to NE as precip ends. Low clouds will linger however. East to Northeast winds 10 to 20 kts. Confid in category changes is low/medium.

Tonight... Precip (-SN or -DZ) ends but low clouds remain. IFR probable but LIFR not out of the question. Slow improvement in conditions after midnight from SW to NE possible. Winds switch to NW overnight. Low/medium confid.

Outlook...

Monday... Except for some morning clouds, mainly VFR expected. A few snow showers KMPO vcnty.

Monday night thru Friday...Generally VFR expected thru the period.

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 night...Gales possible all waters in the evening, diminishing late to SCA level gusts. Seas of 6-9 feet. Freezing spray possible.

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

Wednesday through Wednesday night...SCA conditions early Wednesday, then no headlines anticipated. Winds up to 25 kts early, then 20 kt or less and seas of 2-4 feet.

Thursday through Thursday night...SCA conditions probable Thursday, possibly reaching gales Thursday night. Wind gusts up to 25-30 kt during the day, increasing up to 35 kts at night with seas of 2-4 feet, increasing to 3-6 feet.

CLIMATE

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

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