textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Timing of widespread light snow on Friday evening and potential snow squalls late Friday night has been tweaked. Gale Watch has been raised for all marine waters late Friday night through Sunday morning, with potential for Storm conditions.

KEY MESSAGES

1. An arctic cold front will bring a period of light snow Friday evening followed by potential snow squalls late Friday night.

2. Strong wind gusts behind the arctic front may exceed 50 mph across much of the area on Saturday, with air temperatures plunging into the single digits or below zero and wind chills diving possibly to -10 to -20 degrees by late Saturday night.

DISCUSSION

KEY MESSAGE 1...An arctic cold front will bring a period of light snow Friday evening followed by potential snow squalls late Friday night.

Relatively tranquil, if cold, conditions will prevail through Friday morning, with relatively light winds and a mix of clouds and clear skies as weak high pressure dominates. This all changes Friday afternoon and evening as a clipper system passes by to the north. At first, the system will be *relatively* benign, with clouds increasing, followed by a period relatively light warm advection-induced snow across much of the region. Still some uncertainty about just how much falls, but overall guidance is pretty light here... generally a coating to upwards of an inch or so in most of the region, with best chances of an inch or more being across our northern tier, particularly in the Poconos. Winds should be fairly light during this snowfall, but with cold temperatures preceding it, any untreated roads could quickly get slippery.

Things get much more interesting late at night, as an arctic cold front barrels southeastward across the region. Right now, guidance has honed in on a timing late Friday night, after midnight but before dawn. With the front itself, scattered snow showers, perhaps even some heavier snow squalls, are possible. This might not be especially dissimilar from the event on New Years Day, since its a late night before a non-weekday, but impacts may yet be felt. Roads after that event were quite icy during the morning, and this may occur yet again. As with the New Years Day event, actual snowfall with the squalls will likely be very light, less than an inch, but still potential for some rapid drops in visibility and roads quickly becoming icy.

KEY MESSAGE 2...Strong wind gusts behind the arctic front may exceed 50 mph across much of the area on Saturday, with air temperatures plunging into the single digits or below zero and wind chills diving possibly to -10 to -20 degrees by late Saturday night.

Arctic high pressure will build southeastward into the region slowly but surely on Saturday in the wake of the arctic front. As low pressure rapidly strengthens off the coast while the strong high slowly builds in, winds will be quite strong and gusty, with gusts potentially reaching 50 mph or greater. Wind advisories may be required at minimum.

This wind will also be transporting another round of bitter arctic air into the region, with air temperatures plunging into the single digits region wide and below zero in the colder northwestern zones. Combined with the strong winds, we'll likely see wind chills below zero region wide, possibly reaching -10 in the lower elevations and -20 in the Poconos. Widespread cold weather advisories look likely, with some risk of extreme cold weather warning criteria.

Conditions should gradually wane later Sunday into early next week as the high moves overhead, winds relax and the air mass starts to moderate.

AVIATION /00Z THURSDAY THROUGH MONDAY/

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

Tonight... VFR with just some periods of high cloudiness moving through. North to NW winds generally around 5 knots or less. High confidence.

Thursday... VFR expected. Mainly clear skies through the day. Winds will be mainly North to Northwest around 10 knots after 14Z/15Z. High confid.

Outlook...

Thursday night...VFR.

Friday into Friday night...Sub-VFR likely by late Friday into Friday evening, possibly briefly IFR, with a period of light snow reducing cigs and vsby. Arctic front late Friday night may bring snow squalls.

Saturday...Returning to VFR with the snow squall threat ending early, but with strong northwest winds gusting possibly as high as 40 kts during the day and possibly lingering into Saturday night.

Sunday...VFR with winds diminishing.

Monday...VFR.

MARINE

Sub-SCA conditions tonight and Thursday. Winds will be North thru the period 5 to 10 knots tonight and 10 to 15 knots Thursday. Fair weather expected.

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 this week into the weekend. 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 the ice we're seeing to expand before it contracts. Even though there may be a pause in the growth during the daylight hours this coming week, temperatures at night will support expansion.

Outlook...

Sub-SCA conditions Thursday night through Friday evening, though light snow Friday evening may reduce visibility.

Strong winds likely late Friday night through Sunday morning as arctic front moves through late Friday night, possibly accompanied by snow squalls. Winds look likely to peak later Saturday into Saturday evening, possibly reaching storm force, but for now have held to high-end gale watch until certainty in storm-force winds grows. Winds should gradually diminish as we head through Sunday. In addition to the wind risk, with much colder air moving back in, high risk of at least freezing spray advisory conditions, possibly reaching warning levels. For now held off on any freezing spray headlines, but that will be revisited in future shifts.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from late Friday night through Sunday morning for ANZ430-431-450>455.