textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated Key Messages for newly named Tropical Storm Isaias and Aviation section for 12Z TAFs.

KEY MESSAGES

1. A cool air mass in place is bringing frost to parts of the region early this morning, particularly across the Poconos and northwestern New Jersey.

2. Tranquil weather will prevail this week continuing into the weekend with no significant weather hazards anticipated.

3. The remnants of what is now Tropical Storm Isaias will approach the area later this weekend bringing chances for rain as early as Sunday.

DISCUSSION

KEY MESSAGE 1...A cool air mass in place is bringing frost to parts of the region early this morning, particularly across the Poconos and northwestern New Jersey.

Canadian high pressure has brought a cool airmass to the area. The high is gradually beginning to slide offshore this morning. This setup has lead to very good radiational cooling conditions early this morning due to the mainly clear sky and near calm wind under the center of the high. For these reasons our forecast continues to undercut NBM temps for most areas with the mention of patchy frost in the normally cooler areas including interior southern NJ. Farther north, frost is more extensive so we continue with the Frost Advisory for early this morning for our Pocono zones and Sussex County NJ. These area have the best chances for temperatures radiating to freezing, and in fact KMPO reached freezing already, marking the end of the growing season at least in the coldest portions of our region. Whether that causes us to declare any entire forecast zones as having ended the growing season remains to be seen, as it generally needs to be at least half the zone and that part is not yet clear. Some colder locations farther south, particularly across the NJ Pine Barrens, are also making a run at 32 early this morning, but we haven't seen any other major locations reach there just yet.

KEY MESSAGE 2...Tranquil weather will prevail this week continuing into the weekend with no significant weather hazards anticipated.

High pressure will be in control through most of the week resulting in mostly clear/sunny skies. Following this morning's chilly weather temperatures will moderate later today through Thursday as the flow turns southwesterly ahead of the next cold front. In fact highs by Thursday afternoon look to reach the middle to upper 70s for most of the area, perhaps even touching 80, though latest NBM has nudged the temps down a bit. The cold front moves through by late Thursday as a dry front and will be followed by high pressure re- establishing itself over the area for Friday through at least Saturday with seasonable temperatures.

KEY MESSAGE 3...The remnants of what is now Tropical Storm Isaias will approach the area later this weekend bringing chances for rain as early as Sunday.

High pressure shifts offshore by Sunday as the next low pressure system (what is now Tropical Storm Isaias) approaches from the south and west. There are some model differences regarding this system and its arrival time in the area. The latest ECMWF slowed down a bit compared to its earlier run, while the GFS has sped up. In any event, the system should be fairly progressive but will likely have significant moisture with it (PWATs potentially reaching a rather tropical 2 inches) so the rain could be heavy at times with the potential for some areas to see 1-2 inches.

AVIATION /09Z WEDNESDAY THROUGH SUNDAY/

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

Today...VFR. South-southwest winds around 7-12 kt. High confidence.

Tonight...VFR. South-southwest winds around 5 kts may become light/variable late. High confidence.

Thursday...VFR. Southwest winds 5-10 kts. High confidence.

Outlook...

Thursday night through Saturday...VFR. No significant weather expected.

Sunday and Monday...Restrictions possible with the remnants of what is now Tropical Storm Isaias moving through, accompanied by low stratus and rain.

MARINE

Today into Thursday, winds will be southwest and increase to 15-20 kt by late day through early Thursday. Some gusts of 20 to 25 kt will be possible so it will be at least close to Small Craft Advisory levels. However given that it's marginal, we are holding off on issuing a Small Craft Advisory at least at this point.

Outlook...

Friday through Saturday...No marine hazards expected.

Sunday into Monday...SCA possible as the remnants of what is now Tropical Storm Isaias moves through the region.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Frost Advisory until 9 AM EDT this morning for PAZ054-055. NJ...Frost Advisory until 9 AM EDT this morning for NJZ001. DE...None. MD...None. MARINE...None.