textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Areas of light rain, drizzle and fog are expected again tonight, however confidence is not quite high enough for another dense fog advisory at this time.

KEY MESSAGES

1. Occasional light rain, drizzle, mist and fog will linger across the area through Thursday morning.

2. Periods of rain Thursday night and Friday. Some wintry mix/ice for mostly the I-80 corridor Thursday night/Friday morning.

3. The region will be on the northwestern edge of a potential storm forecast to be just offshore on Sunday and Monday.

DISCUSSION

KEY MESSAGE 1...Occasional light rain, drizzle, mist and fog will linger across the area through Thursday morning.

Stalled frontal boundary remains straddled across the Delmarva. Warm advection overhead has resulted in plenty of clouds, and lingering snow on the ground has helped keep it murky and misty as it cools and condenses out the low-level moisture most efficiently. Passing disturbance tonight may bring a bit of increase in coverage of actual rainfall tonight, with POPs peaking this evening before dwindling toward dawn. As that impulse passes, reinforcing southwesterly push behind it looks to drive slightly drier air south and west across the region late tonight into Thursday morning, potentially reducing low clouds and mist/fog, especially across northeastern parts of the forecast area closer to NYC. As that push occurs, there may be a pulse of dense fog redevelopment expanding south and west ahead of it for a brief time before starting to clear back out, but confidence on longevity remains low so have not yet issued a new dense fog advisory for tonight. That said, the low clouds/fog/mist may linger most across areas west of Philly late tonight and early Thursday morning. Will also need to keep an eye on the far northern areas, where temps might touch freezing before precip is totally out of there, resulting in a chance of light freezing rain. Overall though, rainfall should generally be a quarter inch or less, most of it being up in the northern portion of the forecast area, with totals for the bulk of the region under one tenth inch. Temps currently mostly in the 40s will drop back into the 30s before rebounding back into the 40s on Thursday.

KEY MESSAGE 2...Periods of rain Thursday night and Friday. Some wintry mix/ice for mostly the I-80 corridor Thursday night/Friday morning.

A low pressure system is forecast to cross our area Thursday night into Friday. One low tracks across the lower Great Lakes and deepens while another one develops along a baroclinic zone across the Mid- Atlantic. A zone of warm air advection ahead of this system and associated front will result in some overrunning precipitation. It looks like a quick moving system and it is possible that the energy with this becomes elongated with time. Periods of rain will occur with the bulk of it falling Thursday night through Friday morning. Some wintry precipitation is anticipated, mainly for the I-80 corridor. Model forecast soundings show a warm layer aloft resulting in a freezing rain thermal profile. A period of freezing rain is anticipated Thursday night into Friday morning, mostly for the I-80 corridor, although some snow/sleet could mix in for a time. As of now, ice accretion from a light glaze to as much as 0.20 inches is forecast for the southern Poconos and far northwest New Jersey. If this continues to be forecast, then a Winter Weather Advisory will be needed for the I-80 corridor. Rainfall amounts, at this time, look to be around 0.50 inches across the region. Some fog will be possible as the milder and more moist air overruns some lingering chilly near-surface air, especially in areas that still have a snowpack along and north of the I-78 corridor. There looks to be a south to north temperature gradient across our area Friday due to the placement of a warm front. Milder air then arrives for Saturday as the low-level flow turns from northeasterly to westerly. Most places in the coastal plain Saturday should get into the 50s as cloud cover breaks some. A boundary may linger longer very close to our far northern zones Saturday with just a slight chance (20 percent) for some light precipitation.

KEY MESSAGE 3...Latest model guidance suggests the region will be on the northwestern edge of a potential storm forecast to be just offshore on Sunday and Monday.

Model guidance continues to show low pressure moving off the Southeast or Mid-atlantic coast Saturday night, and then deepening a few hundred miles off the Mid-atlantic coast Sunday into early Monday before moving eastward on Monday and out to sea. There is a large amount of uncertainty as to how impactful this system will be to the local region with some guidance suggesting light to moderate snowfall in the New Jersey- Philadelphia metro-Delaware regions, and other guidance showing no snow at all over the entire local area. That being said, as it stands currently, areas south and east of Philadelphia still have the greatest potential for impacts from any snow and wind Sunday afternoon into early Monday.

In the wake of this storm, a much colder air mass looks to settle into our region early next week. High temperatures probably do not get out of the 30s both Monday and Tuesday (colder in the Poconos), with lows mainly in the teens.

AVIATION /00Z THURSDAY THROUGH MONDAY/

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

Tonight... More LIFR/IFR conditions expected (or developing in the case of the southern terminals) with some patchy light rain and more mist/fog. Light North to Northeast winds. Moderate confidence.

Thursday...IFR/LIFR conditions in the morning with areas of patchy fog, potentially lifting to MVFR in the afternoon. Low confidence.

Outlook...

Thursday night through Friday night...Mostly IFR conditions along with periods of rain.

Saturday...Conditions may improve to VFR.

Sunday and Monday...Rain and snow likely (50-60%) that will result in restrictions. North/northeasterly winds up to 25 kts Sunday becoming Northwest on Monday.

MARINE

Main concern for marine waters will be intervals of dense fog through at least tonight. Dense fog advisory continues through tonight but may need extension into Thursday. Otherwise, winds look to stay below 25 kts and waves mostly below 5 feet, though a few may touch 5 feet well away from shore.

Regarding the river ice...the Delaware Bay continues to be mainly ice free outside of shallow near-shore areas. Ice coverage remains around 10-30% from the the Delaware Memorial Bridge up to the Commodore Barry Bridge. North of that up through Philadelphia, ice coverage is around 30-70%.

Ice coverage will continue to decline with warmer temperatures for the rest of the week.

Outlook...

Thursday night through Saturday night...The conditions are anticipated to be below Small Craft Advisory criteria.

Sunday and Monday...Small Craft Advisory conditions expected. A period of gale force wind gusts possible, depending on the track and strength of a coastal storm moving offshore.

EQUIPMENT

Some replacement parts for the KDIX radar arrived today, but whether they are sufficient to repair the system remains uncertain.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...Air Quality Alert until midnight EST tonight for NJZ001- 007>010-012-015-017>019. DE...None. MD...None. MARINE...Dense Fog Advisory until 6 AM EST Thursday for ANZ430-431- 450>455.