textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Small Craft Advisory has expired for all marine zones.

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.

Following the frontal passage earlier today and associated blustery conditions, mainly tranquil weather is expected through mid-week. The pressure gradient will lessen this evening as high pressure nudges in from the southwest and remains within vicinity of the Mid- Atlantic through Wednesday. As a result, dry conditions are expected with partly to mostly clear skies.

High temps through Wednesday will generally be in the low to mid 60s (upper 50s in the higher terrain). Low temps the next couple of nights will fall into the mid to upper 30s for most of the region with low to mid 40s along the urban corridor and along the coast. Suppose some patchy frost is possible where the growing season remains active across portions of southern NJ and into the Delmarva. However, forecast confidence in occurrence is low as there is indication of a mid-level cloud deck developing after midnight tonight, which would preclude the development of frost. For this reason, have not issued any frost advisories at this time.

KEY MESSAGE 2...A strengthening storm system will impact the region Thursday through Friday night, with much colder air on Friday and over the weekend.

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 /03Z TUESDAY THROUGH SATURDAY/

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

Tonight...VFR. Winds diminish quickly after 00z, with winds generally out of the west around 5 to 8 kt. High confidence.

Tuesday...VFR. Winds out of the west/southwest around 10-15 kt with a period of gusts from 16z-22z around 20-25 kt. High confidence.

Tuesday Night...VFR. West/southwest winds around 5 to 8 kt. 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

Sub-SCA conditions tonight with west/northwest winds around 10-20 kt and seas 2 to 4 feet.

For Tuesday, winds will settle out of the west-southwest around 15 kt with gusts around 20-25 kt possible, especially along the northern New Jersey coast. Have opted to hold off on further SCA's at this time due to overall localization of 25 kt gusts. Seas around 2-4 feet. Fair weather.

Outlook...

Tuesday night and 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.