textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Potential has increased for ice accretion from freezing rain across the I-95 corridor and areas south and east. Otherwise, the forecast remains on track.

KEY MESSAGES

1. A major winter storm continues to impact our entire area through tonight.

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

DISCUSSION

KEY MESSAGE 1...A major winter storm continues to impact our entire area through tonight.

As of 430 PM, temperatures have risen above freezing across Sussex County, DE and Cape May County, NJ, and precipitation has changed to rain. Given these trends, the Winter Storm Warning has been cancelled for these areas. It is important to note that temperatures will fall below freezing again overnight tonight, and while significant additional accumulations of snow, sleet, or freezing rain appear unlikely, any wet roads in the area will have a high chance of experiencing refreezing. Caution is still advised if you must travel in these areas.

Areas of heavy snow and sleet continue to impact the region this afternoon. Locations where it is still snowing are experiencing snowfall rates of 1-2 inches per hour. As of 2PM, this area of heavy snowfall is along the I-80 corridor and points north. the rest of the region is now predominantly experiencing heavy sleet on top of the snow that has already fallen. Even the rate at which this sleet is falling is quite notable, with reports of around 0.5 inch of accumulation per hour! Additionally, given the atmospheric setup expected later this afternoon and into the evening, the potential for freezing rain has increased with a swath of 0.10-0.25 inch of flat ice accretion possible across a wider area along the I-95 corridor and areas just south and east.

The main parent low of this impactful system remains across the Blue Ridge Mountains of western Virginia and eastern Kentucky while a secondary surface low develops across coastal North Carolina. This complex will continue to push northeastward across the East Coast this afternoon and evening, prolonging the winter weather impacts across the greater Delaware Valley. As previously stated, pure snowfall is mostly confined now along the I-80 corridor and points north while the rest of the region is seeing widespread sleet with some mixing. The current setup will be a tug of war between the two lows. The stacked parent low across the Blue Ridge Mountains continues to bring strong warm air advection aloft, resulting in a strong warm nose in the 700-850mb layer. At the same time, the developing surface low off of coastal North Carolina will team up with an arctic high pressure system to the north to bring in a cold northeast wind a the surface. The resulting atmospheric profiles will be highly favorable for persistent sleet this afternoon and evening with an increasing chance for freezing rain as well. While high res guidance has been insistent on pure rain penetrating inland north and west towards the I-95 corridor, surface observations over the last several hours have shown temperatures running several degrees below even the coldest forecast guidance. As a result, it is fair to conclude surface temperature will not cede ground as easily as guidance suggests. At the same time, it is evident that guidance has forecasted the warm nose and warm air advection aloft quite well. Hence the emphasis on our aforementioned ptype tug of war.

As of now, the forecast calls for much of the pure rain to be confined to southern Delaware and extreme southeast New Jersey and not reaching the I-95 corridor. Sleet is expected to be the predominant ptype the rest of the afternoon before guidance shows the warm nose wedging in more intensely later this evening and into tonight. This would give way to better and melting aloft, but should surface temperatures remain below freezing as currently forecast, we could very well see a more predominant period of freezing rain along the I-95 corridor and areas just south and east. All this said, this forecast between sleet vs freezing rain is rather low confidence. Regardless of which ptype we see, major impacts from wintry precip will continue into tonight.

In terms of totals, an additional 4-8 inches of snow and sleet are possible along the I-80 corridor and points north with mainly 1-4 inches of sleet falling across the I-95 corridor and north towards I-80. Flat ice accretion of 0.10-0.25 inches from freezing rain is forecast to fall across the I-95 corridor and nearby areas south and east.

Precipitation tapers off Sunday night with some drizzle or even freezing drizzle possible overnight. 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. Lows come Monday morning will be very cold, in the low teens to mid 20s across the region. This in mind, travel could remain hazardous as anything on the roads and sidewalks will remain frozen or refreeze tonight, resulting in a hard slush or black ice.

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 /21Z SUNDAY THROUGH FRIDAY/

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

Rest of this afternoon...Snow or sleet, except mostly rain at ACY, with IFR/MVFR conditions. Some freezing rain should develop late this afternoon from near KPHL to the south and east. East-northeast winds 10-15 knots with some gusts to around 25 knots. Low confidence.

Tonight...IFR ceilings overall. Areas of sleet or freezing rain/drizzle may end as some light snow during the evening with some visibility restrictions. North-northeast winds 10-15 knots becoming northwest 5-10 knots. Low confidence.

Monday...MVFR ceilings to start improve to VFR. West-northwest winds increasing to 15-20 knots with gusts to around 25 knots. Moderate confidence.

Outlook...

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

MARINE

Gales continue this afternoon before subsiding briefly Sunday night. Winds diminish to around 15kts with gusts around 20 kts and back from the NE to WNW overnight. WNW winds surge again Monday afternoon, bringing gusts back up to gale-force. Seas will be 7-10 feet Sunday night and only diminish to 5-7 feet Monday.

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.

Friday...SCA conditions probable. WNW gusting to 25 kts and seas 3-5 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- 022-025>027. DE...Winter Storm Warning until 1 PM EST Monday for DEZ001-002. MD...Winter Storm Warning until 1 PM EST Monday for MDZ012-015-019- 020. MARINE...Gale Warning until 6 PM EST this evening for ANZ430-431- 452>455. Gale Warning until midnight EST tonight for ANZ450-451.