textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated key messages.

KEY MESSAGES

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

2. Minor tidal flooding expected with this morning's high tide.

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

A vigorous upper-level trough continues to be forecast to evolve across the eastern U.S. from today into the weekend. This supports a surface low tracking over the Mid-Atlantic that will then transition its energy to a developing area of low pressure offshore which will tend to consolidate off the New Jersey coast later today, and deepen as it tracks north- northeast toward the Canadian Maritimes on Friday into Saturday. While there are still some differences with the handling of the embedded shortwaves, there is better model clustering with the surface low placement/track. Overall, it will be unsettled across our region as we go through today and Friday.

While showers have developed over northwestern portions of the area late tonight as the initial low tracks towards the area, the bulk of precipitation looks to hold off until the afternoon hours today. Any showers may be moderate to occasionally heavy at times, where even a few embedded thunderstorms cannot be ruled out. Guidance continues to indicate an uptick in the instability department, albeit is very meager at best around 250-500 J/kg. Guidance has also hinted at a localized convergence zone as the low transitions some energy offshore. While strongest flow aloft is displaced to our south and west, bulk shear values will be around 40 kt. As a result, SPC has introduced a Marginal Risk for severe weather today (primarily in the afternoon hours). Primary threat would be localized damaging winds, but also cannot rule out a few instances of hail with cold temperatures aloft. Overall, this set-up is low CAPE, modest shear, so confidence in occurrence is less than usual.

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 tonight but especially Friday into Friday night. The air mass may become sufficiently cold enough mostly across our far northwestern zones for some snow across the higher elevations, confined mainly 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 0.50-1.25" range while further south across Philly and south, we are expecting less than 0.50" of rainfall, with less than 0.10" 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. Consequently, the forecast rainfall have diminished a bit. 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 tonight 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...Minor tidal flooding expected with this morning's high tide.

A brief period of onshore flow will result in some elevated tides. For the most part, tidal flooding will be relatively spotty for communities along the Atlantic Ocean and Delaware Bay. More widespread tidal flooding though is currently forecast for areas around Barnegat Bay, which could result in some inundation of low lying areas and roadways this morning. As a result, a Coastal Flood Advisory was issued for Ocean County.

No other headlines are in place. No tidal flooding is expected along the tidal Delaware River and Chesapeake Bay.

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 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 30 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, frost/freeze headlines may be necessary. 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 Sunday November 15th. Beyond that, no further frost/freeze headlines will be issued.

AVIATION /07Z THURSDAY THROUGH MONDAY/

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

Thru 12Z this morning...VFR. Clouds increase overnight, although may scatter out at times. Some showers likely at KRDG and KABE between 04-09Z, with perhaps a few showers reaching as far southeast other terminals. Winds will settle out of the east around 5 kt. Moderate-high confidence overall.

Today...VFR expected through at least mid-morning, before conditions lower to MVFR/IFR from west to east as showers slowly creep into the area from northwest to southeast. Isolated heavy rain and thunderstorms possible. Most likely locations for thunder will be for I-95 terminals southeast, but confidence low on thunder. Locally gusty and erratic winds will be possible in/near any thunderstorms. Overall, winds will be variable but favoring an easterly component through midday before varying between west and northeast thereafter. Moderate-low confidence.

Tonight...Cigs gradually slip to IFR with showers. Winds north to northwest 5-10 kts. Moderate confidence.

Outlook...

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

No marine headlines are in effect through tonight. Winds E-SE increasing to 10 to 15 kt this morning. Winds will veer S-SW during the afternoon. Gusts around 20 kt possible. Seas of 2 to 3 feet.

Rain and some thunderstorms possible today. Locally gusty and erratic winds possible near any thunderstorm -- special marine warnings may be warranted if winds are greater than 34 kt for a short duration.

NW winds increase Thursday night with Small Craft Advisory conditions probable Friday. Some gale force wind gusts possible Friday afternoon and Friday night. Outlook...

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...Coastal Flood Advisory from 8 AM this morning to 2 PM EST this afternoon for NJZ020-026. DE...None. MD...None. MARINE...Small Craft Advisory from 6 AM to 6 PM EST Friday for ANZ430- 431-450>455.