textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Minor adjustments with respect to snowfall timing and accumulations tonight into Saturday morning.

Extreme Cold Watches converted to Extreme Cold Warnings for all zones Saturday through Sunday morning.

High Wind Watches for the southern coast of New Jersey and Delmarva converted to High Wind Warnings and Wind Advisories issued for all other areas.

Storm, Gale, and Heavy Freezing Spray Watches converted to Warnings.

KEY MESSAGES

1. A clipper system will bring a period of light snow to the region tonight, possibly ending with an isolated snow squall or two early Saturday morning.

2. Brutally cold and windy conditions develop on Saturday and continue into Sunday morning as Arctic high pressure builds into the region.

DISCUSSION

KEY MESSAGE 1...A clipper system will bring a period of light snow to the region tonight, possibly ending with an isolated snow squall or two early Saturday morning.

A robust clipper system will track through the Great Lakes today and quickly move across interior New York tonight. As a result, periods of light snow will begin to overspread western areas of eastern PA around 4-6pm, near the I-95 corridor around 6-8pm, and near coastal areas around/after 8pm. QPF amounts have remained generally the same, except in the higher elevations of Carbon and Monroe Counties where QPF has increased a bit. As a result, expecting snowfall totals around a coating to an inch for most locales. Isolated amounts up to 2 inches are possible in some spots, but would mainly be found across the higher elevations of northeast PA and northern NJ. No winter headlines are warranted as snowfall totals remain below advisory criteria.

Accompanied with the clipper is a strong arctic cold front which is progged to cross through the area during the pre-dawn hours Saturday morning. Basis available guidance, there remains a threat for snow squalls, but overall the threat is not overly impressive. While both NAM/RAP guidance indicate there will be sufficient low level (0-2km) RH and strong forcing with the arctic front, lapse rates are quite poor. Hence, the threat for snow squalls shown in their respective SNSQ parameter output is low/non-existent. However, cannot completely rule out snow squalls as many of the high resolution window (HRW) models highlight the potential for squalls, directly along the immediate front itself.

KEY MESSAGE 2...Brutally cold and windy conditions develop on Saturday and continue into Sunday morning as Arctic high pressure builds into the region.

Arctic front clears the region Saturday morning and strong cold air advection will be underway. 925mb-850mb temperatures drop to -17C to -20C throughout, and as a result, the max temperature for the day will likely occur just after midnight, and then temperatures crash into the single digits and teens by Saturday morning.

Meanwhile, as Arctic high pressure builds into the Great Lakes and Ohio Valley, low pressure over the western Atlantic will intensify and deepen. This results in a tight northwest pressure gradient over the Northeast and Mid-Atlantic. Winds increase to 20 to 30 mph with gusts up to 50 mph for most of the region and possibly as high as 60 mph across Delmarva and the southern coast of New Jersey. Will go ahead and convert High Wind Watches to Warnings for those areas, and will issue a Wind Advisory for all other areas.

Temperatures Saturday night will drop into the single digits for most of the region and as low as -5 in the southern Poconos and far northwest New Jersey. Wind chills will be as low as -20 in the southern Poconos on Saturday and as low as -30 Saturday night and from -10 to -20 elsewhere Saturday night. Will go ahead and convert Extreme Cold Watches to Extreme Cold Warnings. Cold headlines continue into Sunday morning, but will be in the single digits Sunday afternoon. Lows Sunday night will be in the single digits, and wind chills will be as low as -10 north and west of the Fall Line and as low as -15 in the southern Poconos.

A warmup occurs as the new week progresses.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

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

Today...Prevailing VFR with lowering and thickening clouds through the afternoon. Light snow possible at KRDG/KABE late in the afternoon. Light north winds in the morning, will settle out of the southwest around 3-6 kt this afternoon. Moderate-high confidence.

Tonight...MVFR/IFR conditions likely with periods of light snow at all terminals through 06-09Z. After 09Z, anticipate ceilings and visibilities to improve as a cold front tracks through the area. Southwest winds around 5 kt will sharply become northwest winds around 10-15 kt with gusts up to 25-30 kt by day break on Saturday. Low-moderate confidence overall.

Outlook...

Saturday through Saturday night...Lingering sub-VFR conds in SHSN at KACY Saturday morning, otherwise, VFR. NW winds 20 to 25 kt with 40 to 50 kt gusts Saturday, diminishing Saturday night.

Sunday...VFR with continued diminishing winds.

Monday through Tuesday...VFR/NSW.

MARINE

No marine headlines are currently in effect through tonight. North winds around 10-15 kt this morning will settle out of the west-southwest around 5-10 kt this afternoon. Winds gradually increase this evening to around 10-15 kt before sharply veering to west- northwest around 15-25 kt with gusts up to 30 kt by early Saturday morning in wake of strong arctic cold front. Seas around 2-4 feet. Periods of light snow likely tonight with areas of freezing spray developing late.

Regarding the river ice threat...Ice has formed across area bay waters and inland estuaries. This includes, but is not limited to the Delaware estuary. On rivers, the ice has grown in thickness enough to start causing restrictions in flow. An example of this can be seen at the Trenton River gauge. Per satellite imagery, the Delaware River is pretty much ice covered from just south of Trenton upstream to about Washington Crossing.

The most common ways river ice can break up is either through a thermal or mechanical means. We would like to see temperatures rise into the 40s for a few days, but that's not expected within the next 7 days. Rainfall, producing river rises, can also break up ice. But again, that's not in the forecast either this coming week.

As a result, we expect to continue to see ice expand before it contracts, especially with another deep freeze expected this weekend. For next week, there may be a pause in the growth during the daylight hours as temperatures warm above freezing, however temperatures at night will continue to support expansion.

Outlook...

Strong cold front moves across the waters Saturday morning and winds ramp up quickly. A Storm Warning is now in effect for Delaware Bay and most ocean waters except for ANZ450, which now has a Gale Warning as wind gusts will range from 45 to 50 kt. In addition, a Heavy Freezing Spray Warning is now in effect for Saturday through Saturday night.

Gale force winds may continue into Sunday, otherwise, SCA will be needed through Sunday night.

Lingering VSBY restrictions possible in snow showers Saturday morning.

Sub-SCA conditions Monday through Tuesday.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Extreme Cold Warning from 3 PM Saturday to 10 AM EST Sunday for PAZ060>062-070-071-101>106. Wind Advisory from 8 AM to 9 PM EST Saturday for PAZ060>062- 070-071-101>106. Extreme Cold Warning from 5 AM Saturday to 11 AM EST Sunday for PAZ054-055. Wind Advisory from 6 AM Saturday to midnight EST Saturday night for PAZ054-055. NJ...High Wind Warning from 8 AM to 9 PM EST Saturday for NJZ016- 020>027. Extreme Cold Warning from 3 PM Saturday to 10 AM EST Sunday for NJZ001-007>010-012>027. Wind Advisory from 8 AM to 9 PM EST Saturday for NJZ001- 007>010-012>015-017>019. DE...High Wind Warning from 8 AM to 9 PM EST Saturday for DEZ001>004. Extreme Cold Warning from 3 PM Saturday to 10 AM EST Sunday for DEZ001>004. MD...High Wind Warning from 8 AM to 9 PM EST Saturday for MDZ012- 015-019-020. Extreme Cold Warning from 3 PM Saturday to 10 AM EST Sunday for MDZ012-015-019-020. MARINE...Storm Warning from 6 AM Saturday to 6 AM EST Sunday for ANZ430- 431-451>455. Heavy Freezing Spray Warning from 6 AM Saturday to 6 AM EST Sunday for ANZ430-431-450>455. Gale Warning from 6 AM Saturday to 6 AM EST Sunday for ANZ450.