textproduct: Philadelphia/Mt Holly

SYNOPSIS

Surface high pressure will slide offshore tonight. A weak clipper system will track through the area on Christmas Day. High pressure will build again into Friday morning. A low pressure system will bring widespread wintry precipitation to the region late Friday into early Saturday. Another system will impact the region late Sunday. Cold and dry high pressure will build and persist through Tuesday. Another weak clipper system is possible next Wednesday.

NEAR TERM /THROUGH THURSDAY NIGHT/

Partly cloudy tonight with seasonable temperatures expected. Lows in the 20s to low 30s. The winds will diminish through the afternoon, and become light/variable tonight.

For Christmas Day: not great weather, but not bad either for late December. A weak mid level shortwave trough will pass through during the morning hours, which could produce some brief sprinkles or flurries south of I-78. There's only a 20-30% chance of measurable precip. Skies will be mostly cloudy through the day, but temperatures will be near normal with highs in the 40s for most. Southwest winds 5-10 mph shifting northwest in the afternoon as a cold front passes through.

For Christmas night: The cold front will usher in a colder and drier airmass. Temperatures are expected to fall into the 10s near and north of I-78, and into the low to mid 20s south of I-78 by Friday morning. Skies will be partly cloudy. Winds relatively light, so wind chill won't be much of a factor.

SHORT TERM /FRIDAY THROUGH SATURDAY/

A winter storm is expected to impact the region on Friday and Friday Night, bringing impacts to the entire region in some form. Winter Storm Watches were issued from the Philadelphia metropolitan area on north. Details to follow...

Canadian high pressure will be situated off to the north to start the day, locking in some low-level cold air over most of the region. Low pressure comes out of the Great Lakes and dives southeast across the Applachians, moving offshore of the Mid-Atlantic by Saturday morning. As the surface low moves pass by to our west and eventual south, it will bring widespread precipitation to the area. Sub- freezing temperatures area wide will result in wintry precipitation across the region. The main uncertainty with this system remains where the mixing sets up and how far east the warm nose aloft gets. Another uncertainty point is where the strongest mesoscale banding sets up.

The thinking and trends at least for now point to this being an all snow event from I-195 on north, and a primarily snow event from I-78 on north. For these areas, a Winter Storm Watch was issued with the current forecast having 5-8" of snow in these locations. However, some area in this zone is likely going to see amounts potentially up to a foot depending on where the mesoscale banding sets up. The banding will produce snowfall rates over an inch per hour. Lift should be be quite strong in the northern half of our area which lies within the left exit region of the polar jet, coinciding with strong 700 mb frontogenesis nearby. Thermal profiles continue to show freezing temperatures through the atmosphere, and confidence is highest in these areas for warning-level snow. Probability of hitting warning level snow in these areas ( > 6 inches) was around 70-80%.

Confidence dwindles in the forecast the further south you go towards Philadelphia. For areas in PA south of I-78, a warm nose aloft will move in somewhere between 700 and 850 mb. A thicker warm nose would result more in freezing rain as the surface stays below freezing with a modestly strong cold air damming setup while a thinner warm nose and higher up towards 700-750 mb would result in more sleet. Regardless, this warm nose likely will cut into totals, but given a lower warning criteria in most of those counties south of I-78, there still is around 40-60% of warning-level snow based on the NBM and WPC Probabilistic Guidance. Thus, a Winter Storm Watch was issued for the remainder of our PA counties except Delaware and Chester County, where mixed precipitation totals will cut snow even further. Within this portion of the Watch, 4-6" of snow is expected, with the potential for ice accumulation up to two tenths of an inch. Some of this Watch may be converted to an Advisory depending how things trend in subsequent forecast cycles.

East of Philadelphia, but south of I-195, more of a marine influence will possibly cut into snow totals with some mixing, with a rain/snow/sleet mix. However, similar to adjacent areas in PA, probabilistic guidance is quite bullish on getting to warning level snow - around 50-60% or so. As a result, issued a Winter Storm Watch for Ocean, Burlington, and Camden County in this part of NJ. Important to note that the freezing rain potential in these areas is much lower compared to SE PA, but the possibility of a rain/snow/sleet mix cutting into totals is on the table. Looking at 4-6" currently within the Watch in this part of NJ.

For areas outside the watch in far SE PA, Delmarva, and far South Jersey, snow is expected at onset, but a quick changeover to sleet, freezing rain, or all rain is anticipated. This will limit snow amounts to around a coating over southern Delmarva, and around 2 to 4 inches in far southern NJ, northern DE, and southeastern PA. The cutoff in snow amounts likely will be tighter than what the current forecast has, but did not have the confidence to nail down exactly where that would be. Would not be a surprise to see Advisories posted for parts of South Jersey, northern DE, and SE PA as snow could get to Advisory level. There also likely (60-80% chance) will be measurable ice over northern DE and SE PA.

Long story short, disruptions are to be expected Friday afternoon and Friday Night as this system moves through. Periods of snow and ice will make travel dangerous and anyone with travel plans will see disruptions. Stay tuned to the updated forecast as changes are likely.

The heaviest snow tapers off Saturday morning, but some lingering snow showers are still possible, mainly for the northern half of the area. Temperatures on Saturday will be in the upper 20s/low 30s for the northern half of the area, with mid to upper 30s and low 40s for the rest. There will not be any wind concerns with this event as the pressure gradient is weak.

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

This period starts out on the dry side in between systems. Saturday night lows look to be normal or slightly below under mostly cloudy skies. Clouds are expected to remain on Sunday and rain chances will increase as we move through the day. A wintry mix is possible across the southern Poconos and NW New Jersey. Highs on Sunday will be about normal for this time of year, maybe slightly below.

For Sunday night into Monday, it appears we're looking at an all rain event as temperatures rise a bit through the night. There's not a zero chance that some mixing will persist across the far NW. Temperatures will likely settle around normal. But if the cold front comes through sooner than currently forecast, Monday could be colder and windy.

It looks cold and blustery on Tuesday with highs near to below freezing for most. Lows in the 10s to low 20s.

Temperatures should rebound a bit on Wednesday. A weak clipper is possible.

AVIATION /20Z WEDNESDAY THROUGH MONDAY/

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

Thru 0Z this evening...VFR. Northwest wind 10-15 kts with gusts near 25 kts will gradually decrease, with gusts ending by 23Z. High confidence.

Tonight...VFR. Winds light/variable, then settling out of the south around 5 kts or less late. High confidence.

Thursday...Some brief MVFR conditions possible in low clouds, mainly from KPHL southwards. SW winds 5-10 kts shifting NW and increasing towards sunset. Low confidence.

Outlook...

Friday morning...VFR. No significant weather.

Friday afternoon through Saturday morning...An impactful winter storm will move through the region. KTTN likely will be all snow, with KRDG/KABE/KPNE/KPHL/KILG all seeing multiple precipitation types over this period. Precipitation will mainly be snow, but a changeover to sleet and/or freezing rain is expected at some point. KMIV/KACY should start out as snow with relatively quick change to a mix or all rain. IFR conditions likely through this time frame, but periods of LIFR cannot be ruled out.

Saturday Afternoon...Sub-VFR conditions expected with low clouds and scattered snow showers.

Saturday night...VFR conditions expected.

Sunday...Sub-VFR conditions likely in low clouds and rain, especially later in the day.

Sunday night...Sub-VFR conditions likely in low clouds and rain.

Monday...A return to VFR is likely.

MARINE

Small Craft Advisory conditions continue through this afternoon in the wake of a cold front on the northern ocean waters. Northwest wind gusts 25-30 kts and seas 3-5 feet. Winds and seas diminish below SCA levels all zones tonight. Small Craft Advisory conditions may redevelop Thursday into Thursday night as another cold front passes, but will wait until current headline expires before issuing next.

Outlook...

Friday through Friday Night...No marine headlines expected. Rain and snow anticipated over the waters on Friday Night, likely reducing visibility.

Saturday...SCA conditions possible (40-60%) as seas near 5 feet.

Saturday night...SCA conditions possible. Winds should relax, but seas are expected to remain near/around 5 feet.

Sunday...SCA conditions possible in the morning as 5 foot seas hang on. Sub-SCA expected in the afternoon.

Sunday night and Monday...SCA conditions expected once again. Gale force gusts possible.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Storm Watch from Friday afternoon through Saturday morning for PAZ054-055-060>062-071-103>106. NJ...Winter Storm Watch from Friday afternoon through Saturday morning for NJZ001-007>010-012>015-018>020-026-027. DE...None. MD...None. MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ450- 451.