textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes to the overall forecast.

Aviation section updated for the 12Z TAFs.

Added a key message highlighting a period of below normal temperatures expected to begin in the wake of the late week system.

KEY MESSAGES

1. Dry and seasonable through mid-week. Some patchy frost possible tonight.

2. A strengthening storm system will impact the region Thursday through Friday night.

3. A stretch of below normal temperatures begins on Friday, continuing through the weekend as a much colder airmass moves overhead.

DISCUSSION

KEY MESSAGE 1...Dry and seasonable through mid-week. Some patchy frost possible tonight.

High pressure over the Gulf Coast states will build east through the middle of the week before moving offshore on Wednesday. Tranquil conditions expected during this period.

The pressure gradient tightens once again today, and west to northwest winds will increase to around 15 mph with gusts to around 20 mph this afternoon. Winds diminish tonight. Highs today will be a bit warmer than they were on Monday, topping off in the low to mid 60s.

Under clear skies and with light to nearly calm winds, lows tonight will drop into the 30s to low 40s. Suppose some patchy frost is possible where the growing season remains active across portions of southern NJ and into the Delmarva.

KEY MESSAGE 2...A strengthening storm system will impact the region Thursday through Friday night.

A vigorous upper-level trough continues to be forecast to evolve across the eastern U.S. later this week and into the weekend. This supports a surface low that strengthens as it tracks across the Northeast and Mid-Atlantic region. The main low main tracks into the Ohio Valley while a secondary low develops in the vicinity of the northern Mid-Atlantic. The model guidance, including ensemble data, continues to be in good agreement that a deep upper-level trough evolves in the East later this week. There are however some differences remaining with the handling of the embedded shortwaves and thus the details of the surface low. This will determine whether the strengthening storm tracks over our region or slides just south and then east/offshore. Despite the continued uncertainty on the finer details, our region will still be impacted by a storm.

The strength of the parent upper-level trough results in a colder air mass moving in with time, however the initial air mass is not all that cold (the lack of a cold antecedent air mass). The air mass may become sufficiently cold enough on the west to northwestern edge of the precipitation shield for a rain/snow mix or even some snow across the higher elevations of our far western to northwest zones, confined particularly to the southern Poconos. Otherwise, a rain event is forecast. The amount of precipitation (rain and/or snow) will depend on the ultimate development and track of the main surface low. The GEFS and ECMWF members suggest a spread still with the placement/track of the surface low, however overall this looks to be mostly a rain event for the majority of our region. The coastal impacts, including the potential for stronger winds and also tidal flooding, are less certain because if the surface low tracks right over our area that would lessen the residence time of the (stronger) onshore flow. If an inland track to the surface low occurs, then portions of our area could get briefly into some milder air and therefore some thunder would be possible. The strongest winds with this may end up occurring as the storm exits and the cold air advection then increases from the northwest. The main uncertainty is on how much precipitation occurs with this event and the extent of any higher elevation snowfall (southern Poconos).

KEY MESSAGE 3...A stretch of below normal temperatures begins on Friday, continuing through the weekend as a much colder airmass moves overhead.

In the wake of the storm, as it deepens and tracks toward the Canadian Maritimes over the weekend, a colder air mass will be in place across our area. In addition, a tightened pressure gradient coupled with deeper mixing should result in a period of breezy to windy conditions over the weekend. Daytime temperatures on Friday and Saturday should not get out of the 40s for many areas, with temperatures at night dropping into the 20s to low 30s. Some areas likely will see their first freeze of the season as the growing season is still active across Delmarva, parts of South Jersey, and far southeastern Pennsylvania. There could even be some lake effect snow and/or rain showers into the southern Poconos on Saturday but things like dry for the most part elsewhere.

Temperatures should moderate some on Sunday and Monday, but the current forecast still has temperatures in the upper 40s/low 50s each day, which would still be below normal for mid- November.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

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

Today...VFR. W-SW winds 10 to 15 kt with 18 to 23 kt gusts. High confidence.

Tonight...VFR. W-NW winds 5 kt or less. High confidence.

Outlook...

Wednesday...VFR. No significant weather.

Thursday and Friday...Sub-VFR conditions with periods of rain (rain/snow possible later Friday, mainly north of KABE/KRDG). North to northwest wind gusts 20-25 knots Friday.

Saturday...Mainly VFR. Northwest wind gusts up to 30 knots possible during the day.

MARINE

West to southwest winds around 10 kt will increase to around 15 kt with gusts up to 20 kt this afternoon. A few gusts to 25 kt are possible off the New Jersey coast this afternoon, but do not think those gusts will be frequent enough nor widespread enough to warrant a Small Craft Advisory at this time. Seas will average 2 to 4 feet.

West to northwest winds around 10 kt tonight with 2 to 4 ft seas.

Outlook...

Wednesday...The conditions are expected to be below Small Craft Advisory criteria.

Thursday and Friday...Small Craft Advisory conditions probable Thursday night and Friday. Gale force wind gusts possible on Friday.

Saturday...Small Craft Advisory conditions probable. Near gale force wind gusts possible.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...None.