textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Minor adjustment upward on snowfall accumulations Friday night.

Gale Watch upgraded to Storm Watch for coastal waters up to Ocean County NJ. High Wind Watch issued for adjacent land zones.

Extreme Cold Watch issued for all forecast zones for late Saturday through early Sunday.

KEY MESSAGES

1. A clipper system will bring light snow Friday night, possibly ending as a few snow squalls early Saturday morning.

2. Brutally cold and windy conditions set up beginning Saturday morning and persisting into Sunday morning in the wake of the arctic front as a strong storm develops off the coast.

DISCUSSION

KEY MESSAGE 1...A clipper system will bring light snow Friday night, possibly ending as a few snow squalls early Saturday morning.

There have been no big changes with the clipper system forecast, although timing may have nudged slightly later once again. QPF looks slightly more robust, so have around an inch across much of our northern/eastern zones, but still less than an inch southern/western zones. Still possible dry air could end up eating away at most of this warm advection precip Friday night, so kept POPs mostly in the low-end likely range versus categorical, though highest POPS definitely far northern and northeastern zones and lowest south and southwest. Snow squall potential still exists with the front early Saturday, but confidence remains pretty low. Either way, at least until the front roars through early Saturday, the weather will be comparatively benign.

KEY MESSAGE 2...Brutally cold and windy conditions set up beginning Saturday morning and persisting into Sunday morning in the wake of the arctic front as a strong storm develops off the coast.

The passage of the arctic front will usher in brutal weather for the bulk of our weekend. As the system passes off the coast, low pressure looks to rapidly strengthen, which will rapidly tighten the pressure gradient over our region. The end result will be rapidly increasing wind speeds, with gusts across much of the area potentially reaching 50 mph later Saturday into Saturday evening, with 60 mph possible near the coastal areas and across the Delmarva. For this reason, have issued a high wind watch across these areas, but will hold off on wind advisories until we determine whether to upgrade the watches or not, which will be another cycle or two. Winds likely peak late afternoon/early evening and then slowly dwindle from there.

This wind is unfortunately also being accompanied by a brutally cold arctic air mass, with temps probably falling a good portion of the day Saturday, spending much of the day in the teens before falling into the single digits at night, and below zero in the colder locales. With winds only slowly coming down, potential has increased for widespread wind chills of -10 to as low as -25, so have hoisted an extreme cold watch for the entire forecast area for late Saturday afternoon through Sunday morning. With winds relaxing during the day Sunday, wind chills should improve considerably despite temps not getting all that high.

Temperatures gradually warm as the new work week progresses, with only minor chances of precip through the period, at least until near the end of the week.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

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

Tonight... VFR. Increasing high clouds. North winds 5 knots or less. High confid.

Friday... VFR. Increasing high then mid clouds. Lower CIGS (035- 050) after (about) 21Z for KPHL. Light variable winds then South to Southwest 5 to 10 knots for the afternoon. High confid.

Outlook...

Friday Night...MVFR/IFR conditions possible with light snow.

Saturday and Saturday night...Brief sub-VFR possible with any snow showers early Saturday, then VFR. Strong winds with gusts to 45 kt.

Sunday...VFR with diminishing winds.

Monday and Tuesday...VFR.

MARINE

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

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 until at least next week. 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... Visibility restrictions possible Friday night in light snow, but generally sub-SCA.

Snow showers may linger early Saturday with visibility restrictions, but overall generally drying out. Main concern will be high winds developing behind arctic front as low pressure rapidly develops off shore. Winds may reach 50 kts over the waters, especially from Ocean County southward, so have hoisted storm watches for these zones. Maintained gale watch further north. Freezing spray watches also remain in place given arctic air and high winds.

Conditions will gradually subside late Saturday night through Sunday with sub-SCA conditions likely later Monday-Tuesday.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Extreme Cold Watch from Saturday afternoon through Sunday morning for PAZ060>062-070-071-101>106. Extreme Cold Watch from Saturday morning through Sunday morning for PAZ054-055. NJ...High Wind Watch from Saturday morning through Saturday evening for NJZ016-020>027. Extreme Cold Watch from Saturday afternoon through Sunday morning for NJZ001-007>010-012>027. DE...High Wind Watch from Saturday morning through Saturday evening for DEZ001>004. Extreme Cold Watch from Saturday afternoon through Sunday morning for DEZ001>004. MD...High Wind Watch from Saturday morning through Saturday evening for MDZ012-015-019-020. Extreme Cold Watch from Saturday afternoon through Sunday morning for MDZ012-015-019-020. MARINE...Storm Watch from Saturday morning through late Saturday night for ANZ430-431-451>455. Heavy Freezing Spray Watch from Saturday morning through late Saturday night for ANZ430-431-450>455. Gale Watch from Saturday morning through late Saturday night for ANZ450.