textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated rainfall forecast in Key Message 1.

KEY MESSAGES

1. A strengthening storm system organizes off the northern Mid- Atlantic coast Thursday into Friday.

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

DISCUSSION

KEY MESSAGE 1...A strengthening storm system organizes off the northern Mid-Atlantic coast Thursday into Friday.

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 portions of the Mid-Atlantic and then especially just offshore of the Northeast. It appears that low pressure will tend to consolidate off the New Jersey coast on Thursday, then deepen as it tracks north-northeast toward the Canadian Maritimes Friday into Saturday. While there are still some differences with the handling of the embedded short waves, there is better model clustering with the surface low placement/track. Overall, it will be unsettled across our region as we go through Thursday and Friday.

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). As the surface low consolidates off the northern Mid- Atlantic coast, the winds will turn more out of the northwest and this will increase the cold air advection. This will result in some colder air seeping southeastward later Thursday night but especially Friday into Friday night. The air mass may become sufficiently cold enough mostly across our far northwestern zones for a rain/snow mix or even some snow across the higher elevations, confined particularly to the Pocono Plateau. Any snowfall accumulation is expected to be less than an inch. Some elevated locations across the Lehigh Valley and Northwestern NJ may see a few wet flakes mixing in, but no accumulations are expected. Otherwise, a rain event is forecast. The amount of precipitation (rain and/or snow) will depend on the ultimate track of the surface low and whether a deformation zones wraps back into our northern to central areas. At the moment, rainfall amounts north of the PA Turnpike to I-195 corridor range around the 0.75-1.25" range while further south across Philly and south, we are expecting less than 0.75" of rainfall, with less than 0.25" expected for the southern Delmarva.

The model consensus looks to be more of a northward positioning of the surface low, which if this ends up occurring would result in less rainfall with a southward extent across our area. The coastal impacts, including the potential for stronger onshore winds and also (minor) tidal flooding, are a little less certain because the surface low track right over our area would lessen the residence time of the (stronger) onshore flow. Prior to the low fully consolidating offshore, some elevated instability could get into especially portions of our coastal plain and therefore a rumble of thunder is possible later Thursday night and Friday morning. The strongest winds with this look to occur as the storm moves farther away and the cold air advection then increases from the northwest. The main uncertainty is on how much precipitation occurs with this event region-wide and the extent/amount of any higher elevation snow (mainly for the southern Poconos).

KEY MESSAGE 2...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 will result in a period of breezy to windy conditions over the weekend. The strongest winds look to occur on Saturday, with peak gusts to about 35 mph, then while still gusty on Sunday it appears the speeds come down a little. 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. Where the growing season is still active across Delmarva, parts of southern New Jersey and far southeastern Pennsylvania. There could even be some lake effect snow and/or rain showers into the southern Poconos on Saturday.

Temperatures start to moderate a bit Sunday through Tuesday. Despite that, high temperatures are only in the upper 40s to low 50s each day which is still below average for this time of year.

While we are likely to get our first freeze sometime during this stretch, the Frost/Freeze program area-wide ends on November 15th. Beyond that, no further frost/freeze headlines will be issued.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

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

Rest of Today...VFR. W-SW winds less than 10 kt, becoming S-SE towards sunset. High confidence.

Tonight...VFR. Increasing clouds. E-SE winds 5 kt or less. High confidence.

Thursday... Early VFR lowering to MVFR most sites as rains arrive mostly W to E across the region. Some improvement back to VFR expected after 18Z. Light E/SE winds. Medium confid.

Outlook...

Thursday night 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 and Sunday...Mainly VFR. Northwest wind gusts 20-30 knots.

Monday...VFR. No significant weather.

MARINE

Sub-Small Craft Advisory conditions tonight and Thursday. Winds becoming onshore E/SE late today and increasing to 10 to 15 kts by dawn Thursday. Winds continue E/SE Thursday morning around 15 kts then winds will veer S/SW during the afternoon. Gusts around 20 knots possible. Winds near the North NJ waters may remain variable as low pressure passes overhead Thu afternoon. Fair tonight and rains for Thursday.

Outlook...

Thursday night and Friday...Small Craft Advisory conditions probable Thursday night and Friday. Some gale force wind gusts possible Friday afternoon and Friday night.

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

Sunday...Small Craft Advisory conditions possible.

Monday...Marine conditions will be below advisory criteria.

PHI WATCHES/WARNINGS/ADVISORIES

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