textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes through tonight.

Minor timing tweaks to the timing of the snow Friday night followed by the timing of the Arctic air over the area.

Gale Watch remains in effect for all marine waters for Saturday and Saturday night, and a Heavy Freezing Spray Watch is now in effect for Saturday and Saturday night.

KEY MESSAGES

1. A clipper system will be followed by an Artic front Friday night.

2. Brutally cold and windy conditions set up this weekend as an Arctic front moves into the region.

DISCUSSION

KEY MESSAGE 1...A clipper system will be followed by an Artic front Friday night.

The timing of the passage of the clipper system has trended a bit slower, and it looks like any snow will not make it to far western portions of the forecast area until Friday evening. Any snow will be light with minimal, if any, snow accumulation.

The Arctic front then passes through the region after midnight Friday night and will clear the area by Saturday morning. This front will be accompanied by a deep H5 trough with strong shortwave energy. As a result, snow showers become likely across most of the area. Stronger snow showers may become squalls with brief heavy snow showers reducing visibility to under 1 mile, wind gusts up to 40 mph, and a quick accumulation of up to and just over 1 inch of snow. This will create slick and hazardous driving conditions.

KEY MESSAGE 2...Brutally cold and windy conditions set up this weekend as an Arctic front moves into the region.

Arctic high pressure then builds east into the Great Lakes and Ohio Valley on Saturday. Strong cold air advection will be underway, and a tight pressure gradient forms between that high and low pressure over the Atlantic ocean. The high temperature for Saturday will most likely be early Saturday morning, then temperatures by late Saturday morning will crash into the teens, and northwest winds will increase to 20 to 30 mph with 40 to 50 mph gusts. Wind Advisories will likely be needed. Wind chills on Saturday will mostly be in the single digits, with wind chills as low as -10 north and west of the Fall Line, and possibly as low as -20 in the southern Poconos. Lows Saturday night will be in the single digits to as low as -5 in the southern Poconos and far northwest New Jersey. Winds diminish a bit, down to 15 to 20 mph with 25 to 35 mph gusts. This results in wind chills as low as -10 to -15 for most of southern New Jersey, the Delaware Valley, and Delmarva, and as low as -15 to -20 for the Lehigh Valley and northern New Jersey, and as low as -30 in the southern Poconos.

Artic air remains in place on Sunday with highs in the teens to low 20s, but winds will diminish, resulting in single digits wind chills during the day Sunday.

Temperatures gradually warm as the new work week progresses.

AVIATION /12Z THURSDAY THROUGH MONDAY/

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

Today...VFR. Mainly clear skies through the day. North- northwest winds around 7-12 kt. High confidence.

Tonight...VFR. Increasing and thickening clouds. North- northwest winds around 5 kt or less, becoming calm at times. High confidence.

Outlook...

Friday...VFR.

Friday night...Overall, VFR, but brief sub-VFR conds, possibly as low as IFR, in SHSN.

Saturday through Saturday night...VFR. NW winds 15 to 25 kt with 30 to 40 kt gusts. Winds diminish Saturday night.

Sunday through Monday...VFR.

MARINE

No marine headlines are in effect through tonight. North winds around 10-15 kt with seas around 2 feet. 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 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...

Sub-SCA conditions Friday and most of Friday night. Arctic front passes through the waters late Friday night through Saturday morning, then conditions quickly ramp up from SCA to Gales. A brief period of Storm Force winds is also possible during the day Saturday. A Gale Watch is in effect for Saturday and Saturday night. Heavy freezing spray is also likely to develop, so a Heavy Freezing Spray Watch is now in effect for Saturday and Saturday night as well. Winds diminish to SCA levels late Saturday night, and then SCA conditions continue through Sunday. Sub-SCA conditions Sunday night and Monday.

Brief visibility restrictions likely in snow showers and possible snow squalls late Friday night through Saturday morning.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Saturday morning through late Saturday night for ANZ430-431-450>455. Heavy Freezing Spray Watch from Saturday morning through late Saturday night for ANZ430-431-450>455.