textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated Key Message 1 and the Aviation Section for the 18Z TAFs.

KEY MESSAGES

1. Surge of warmth and humidity through Wednesday ahead of a cold front.

2. A strong cold front Friday brings an end to the warmth as much colder air arrives for the weekend.

DISCUSSION

KEY MESSAGE 1...Surge of warmth and humidity through Wednesday ahead of a cold front.

A frontal zone draped to our north today will result in surface low pressure riding along it through Wednesday, pulling a cold front across our area by later Wednesday. Ahead of this feature, southerly flow will boost the low-level warm air advection regime, resulting in much warmer air surging into our area through Wednesday. High temperatures will be in the upper 70s to around 80 degrees for many areas today. Wednesday's highs are forecast to mainly range in the low to mid 80s, with even some upper 80s to near 90 degrees possible over southern Delaware.

In addition, dew points will increase back through the 60s into Wednesday. This will result in a much more humid feel over the next couple days, with it being the most humid on Wednesday (heat index values should approach 90F for areas from about Philadelphia south and east; away from the coast). There does not appear to be much in the way of forcing, therefore just a slight chance (less than 25%) for showers is expected. After the front moves through, cooler air will briefly return on Thursday.

KEY MESSAGE 2...A strong cold front Friday brings an end to the warmth as much colder air arrives for the weekend.

A potent upper-level trough is forecast to arrive across the East during Friday, then gradually lift out over the weekend. This will drive surface low pressure just north of our area Friday with an associated strong cold front sweeping through.

A mild and increasingly moist air mass ahead of this feature will provide some instability as the strong cold front arrives on Friday. Large scale forcing for ascent will increase with the arrival of the robust upper-level trough and cold front, therefore showers will increase for a time (peaking on Friday). Some showers though look to develop Thursday night ahead of the main cold front. Some thunderstorms cannot be ruled out given the strength of the system, however this will ultimately depend on the magnitude of the instability and timing of the cold front. In the wake of the cold front, strong cold air advection occurs and this will result in much colder temperatures for the weekend. As of now, high temperatures both Saturday and Sunday look to struggle to get out of the 50s (colder in the higher terrain). A gusty wind Saturday due to cold air advection and a tightened pressure gradient will add to the chill. If the wind drops off enough Saturday night, some frost or freezing temperatures will become a concern for portions of our area. The air mass then starts to modify early next week.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

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

Rest of Today... VFR with mainly clear skies. Southwest winds around 10 kt. High confidence.

Tonight...VFR with mostly clear skies. A period of LLWS is possible mainly between 00Z-08Z. West-southwest winds 4-8 kt. High confidence.

Wednesday...VFR with increasing clouds after daybreak. West winds around 5-10 kt early, increasing to 10-15 kt in the afternoon. A period of showers is possible from west to east, but confidence is not high enough to include in TAFs at this time. Moderate-high confidence overall.

Outlook...

Wednesday night...VFR. Slight chance for brief period of sub- VFR conditions Wednesday night from showers.

Thursday and Friday...A period of sub-VFR conditions possible with showers. Gusty southwest to west winds on Friday.

Saturday...VFR. Gusty northwest winds.

MARINE

No marine headlines are in effect through Wednesday. South- southwest winds are expected to increase to around 15-20 kt later this afternoon through tonight with gusts up to 22-23 kt possible. Winds will become west around 10-15 kt on Wednesday following a cold frontal passage. Seas around 2-4 feet. Fair weather expected outside of gusty winds.

Outlook...

Wednesday night through Thursday...No marine hazards are anticipated.

Friday and Saturday...Small Craft Advisory conditions probable, with a period of gale-force wind gusts possible.

PHI WATCHES/WARNINGS/ADVISORIES

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