textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Aviation section updated. Some adjustments made to key messages one and two.

KEY MESSAGES

1. Freeze likely in the southern Poconos tonight with areas of frost across the far northwestern suburbs of Philly, the Lehigh Valley to northwest New Jersey.

2. Limited fire weather conditions on Sunday.

3. A series of systems on Wednesday and Thursday likely brings the next widespread rainfall.

DISCUSSION

KEY MESSAGE 1...Freeze likely in the southern Poconos tonight with areas of frost across the far northwestern suburbs of Philly, the Lehigh Valley to northwest New Jersey.

Low pressure will continue to move farther away across the western Atlantic this evening, and surface high pressure builds over the Tennessee Valley and Gulf Coast states. Meanwhile, an upper trough remains over the Northeast. This allows for a northwest flow over the area with cold air advection. Lows will drop into the 30s and low 40s with temperatures as low as the upper 20s in the southern Poconos. A Freeze Warning is in effect for Carbon and Monroe counties tonight, with a Frost Advisory across adjacent areas including the far northwestern suburbs of Philly to northwest New Jersey. It is a little uncertain in the Frost Advisory as clouds may linger given mostly the incoming upper trough and then the winds may remain up enough through the night. More confident about the Freeze Warning in the Poconos where cold air advection should be more effective at getting the plateau down to freezing or below. Valleys there are more questionable.

KEY MESSAGE 2...Limited fire weather conditions on Sunday.

Low pressure well offshore and incoming high pressure will result in a tight pressure gradient setting up for Sunday. Northwest winds will increase to 15-20 mph with gusts 20-30 mph from about late morning and continuing during the afternoon.

With breezy and dry northwest winds, combined with deep mixing setting up (tapping into very dry air aloft), minimum RH values will fall into the 25-35 percent range Sunday afternoon.

Based on collaboration with state fire weather partners though, fuel moistures remain damp with the rainfall over the last few days along with the green up across most of the region. Therefore, no statements will be issued at this time.

KEY MESSAGE 3...A series of systems on Wednesday and Thursday likely brings the next widespread rainfall.

As high pressure shifts further away on Tuesday, a low passing well to the north may bring a few showers late Tuesday into Tuesday night for areas northwest of the I-95 corridor. A cold front then approaches from the west on Wednesday and will stall out over the area. A potentially more dynamic area of low pressure develops over the southeast and cuts inland to our west on Thursday. This will result in widespread rainfall over those two days.

The latest guidance has the bulk of the precipitation falling with that second system sliding through Thursday though the latest guidance has the system moving through faster. As a result the peak PoPS (70-90%) come a little bit earlier (late Wednesday night/early Thursday through Thursday evening).

Timing of the peak activity and the front will play key roles in the potential for any strong to severe thunderstorms. If the peak of the rainfall comes Thursday morning, that may inhibit the ability for the low-levels of the atmosphere to become unstable enough for stronger storms in the afternoon before the front arrives. However, it is still much too early to have any level of certainty on this timing so we will have to continue to watch this period over the next couple of days.

The NBM Probability of total rainfall from those two systems is about 40-70% for 1 inch or more from Philadelphia on north and about 20-40% south of Philadelphia. With how dry we have been, flooding concerns appear to be minimal, which means these systems should largely be beneficial rainfall for the area.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

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

Tonight...VFR ceilings should mostly become scattered to few overnight. Northwest winds around 10 knots, diminishing to 5 knots or less for a time. Moderate confidence.

Sunday...VFR cloud bases increase. Northwest winds 10-15 knots with gusts 20-25 knots. Moderate confidence.

Outlook...

Sunday Night through Monday Night...VFR.

Tuesday through Tuesday Night...VFR. Southwesterly wind gusts 20-25 knots.

Wednesday...Primarily VFR. Some showers move in for the afternoon and evening which could result in some restrictions. South/southwest wind gusts 20-25 knots.

Thursday...Sub-VFR conditions possible with showers. South/southwest wind gusts 20-25 knots.

MARINE

Sub-Small Craft Advisory conditions tonight into Sunday. Northwest wind gusts to around 20 knots by later tonight and on Sunday.

Outlook...

Sunday...No marine headlines expected. Cannot rule out a brief period of wind gusts around 20-25 knots but the potential for widespread SCA conditions is low (10% chance or less).

Sunday Night...No marine headlines expected.

Monday...SCA conditions possible (30-40% chance) as seas begin to build near 5 feet with wind gusts around 20-25 knots.

Tuesday through Thursday...SCA conditions expected (90-100% confidence). Wind gusts out of the south around 20-30 kt. Gale conditions possible (30-40% chance) on Tuesday Night. Seas of 4 to 7 feet anticipated.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Frost Advisory from 11 PM this evening to 9 AM EDT Sunday for PAZ060>062-101-103-105. Freeze Warning from 11 PM this evening to 9 AM EDT Sunday for PAZ054-055. NJ...Frost Advisory from 11 PM this evening to 9 AM EDT Sunday for NJZ001-007>009. DE...None. MD...None. MARINE...None.