textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A dry trend continues in the forecast for Thursday and Thursday night. Light wintry precipitation remains possible over the weekend, however the chance of any significant amounts appears low.

KEY MESSAGES

1) Seasonably warm and tranquil through tonight.

2) A storm system will bring some light rain and snow Wednesday afternoon through early Thursday morning, followed by much colder and blustery conditions through Friday.

3) A couple systems could bring some light wintry precipitation over the weekend.

DISCUSSION

KEY MESSAGE 1...Seasonably warm and tranquil through tonight.

A warm front will lift north of the area later this afternoon as low pressure tracks across southern Canada. Otherwise, broad and elongated high pressure over the Deep South extending into the western Atlantic remains in control through tonight. While mostly sunny skies are expected for the first half of the day, clouds will be increasing late in the day and into tonight with the warm front passing through. Under a warm air advection regime with light southerly flow, anticipate highs to top out in the mid 40s to low 50s. Lows tonight will remain above freezing for most locales, mainly in the mid to upper 30s. Aside for a stray shower north and west of the I-95 corridor tonight, dry weather is expected.

KEY MESSAGE 2...A storm system will bring some light rain and snow Wednesday afternoon through early Thursday morning, followed by much colder and blustery conditions through Friday.

A strong upper-level trough is forecast to pivot into the East late Wednesday through late Thursday before lifting out into Friday. The strength and orientation of this trough will be a key driver regarding surface low pressure strength and track. A positively tilted upper-level trough lasting longer will keep low pressure weaker and more out to sea, while a neutral to especially negatively tilted trough will result in a stronger and westward storm track. The model consensus continues to support a low impact system due to the trough closing off farther north and the trough axis remaining more positive to neutral longer.

An initial system tracks to our north with its cold front arriving during Wednesday. As the trough continues to amplify eastward, a new surface low develops along or near an existing baroclinic zone mostly off the Carolina coast, however it looks to mainly miss our area. The overall synoptic lift with the initial system and cold front should result in the development of a precipitation shield Wednesday and Wednesday night. This could be more focused initially across eastern Pennsylvania before pivoting eastward Wednesday night into early Thursday morning as colder air works in. If enough moisture remains as the colder air deepens, then rain would change to some snow before ending for parts of the area. Any snowfall accumulations would be very light, if any. A dusting is possible northwest of the fall line, and perhaps as much as 1" in the southern Poconos. There will probably be another area of precipitation developing well offshore associated with the secondary low, but this now looks to remain well offshore.

One of the forecast challenges has been whether this system can consolidate into one main storm for our area, and the overall trend remains much less conducive for this. As a result, chances for precipitation Thursday remain low (10-20%), which is mainly due to the potential for lingering light precip early in the day. The forecast for Thursday night is now completely dry.

It will be mild ahead of this upper-level trough Wednesday (highs in the mid 40s to low 50s), which will support initially rain everywhere, then strong cold advection takes place Wednesday night through Thursday night. Much colder air follows for Thursday through Friday. Temperatures should be in the mid 20s to near 30 degrees in most areas by dawn Thursday, but a breezy WNW wind will produce wind chills in the teens to low 20s. These wind chills will continue much of the day Thursday as WNW winds 15-25 mph persist with gusts around 30-35 mph possible at times. Highs in the upper 20s to mid 30s.

As strong cold advection continues into Thursday night, temperatures are forecast to fall into the teens to low 20s by dawn Friday. A persistent westerly wind will produce wind chills in the single digits area wide, and below zero in the southern Poconos, some of the coldest so far this winter season. Remaining cold Friday with highs in the upper 20s to mid 30s and maximum wind chills only in the low to mid 20s.

KEY MESSAGE 3...A couple systems could bring some light wintry precipitation over the weekend.

Following an upper-level trough in the previous key message, the upper-level trough pattern looks to remain in the East for the weekend and early next week. The depth of the trough looks to be strong on much of the guidance, which will support surface low pressure in the East. It will also keep the cold air in place. The latest trend in guidance for this period is toward a less impactful, lower QPF event that could come from 2 separate rounds (one Saturday, one Sunday), however uncertainty remains high during this period. Details regarding the evolution of the mid level dynamics remain unclear at the moment, which will ultimately determine timing and type of any precip that occurs. At the moment, combined QPF appears quite low, and it is possible some areas end up seeing just some sprinkles or flurries at most. We'll need to keep an eye on the trends for the weekend period given the dynamics at play. PoPs were lowered to 20% with this update. Following a brief return to normal temperatures Saturday, below normal temperatures look to return Sunday into early next week.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

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

Today...VFR. Mostly clear skies with increasing clouds in the afternoon. South-southwest winds around 7-12 kt with a few gusts up to 15-20 kt possible in the afternoon. High confidence.

Tonight...VFR with mostly cloudy to overcast skies. A few showers possible after 06Z, mainly northwest of I-95 corridor, but confidence is not high enough to include in TAFs at this time. South-southwest winds around 5-10 kt. Low-level wind shear likely between 00-09Z. Moderate-high confidence.

Outlook...

Wednesday through Wednesday night...Restrictions likely developing Wednesday afternoon through Wednesday night in low clouds and periods of light rain/snow. Gusty WNW winds developing late Wednesday night.

Thursday...VFR. Gusty WNW winds.

Friday and Saturday...VFR. Gusty SW winds possible Friday. LLWS possible Friday night.

MARINE

A Small Craft Advisory has been issued for all Atlantic Coastal Waters beginning 6 PM today through tonight. No marine headlines are in effect for the Delaware Bay.

West-southwest winds around 10-15 kt this morning will settle out of the south-southwest this afternoon. Winds will increase to around 15- 20 kt late in the day with gusts up to 25-30 kt through tonight. Seas around 2-3 feet today building to 3-6 feet tonight. Fair weather expected outside of SCA conditions.

Outlook...

Wednesday...No marine hazards expected.

Wednesday night through Thursday...Small Craft Advisory conditions expected with WNW winds 20-30 kts, and a brief period of gale force wind gusts possible.

Thursday night...Freezing spray possible. Seas may linger near 5 feet across the Atlantic waters.

Friday and Saturday...No marine hazards anticipated, however wind gusts may briefly increase to around 25 kts Friday evening.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 6 PM this evening to 10 AM EST Wednesday for ANZ450>455.