textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Freeze Watch upgraded to Freeze Warning in Poconos with Frost Advisory added for adjacent areas.

KEY MESSAGES

1. Freeze likely in the southern Poconos tonight with areas of frost across the far northwestern suburbs of Philly and NYC along with the Lehigh Valley.

2. Marginal fire weather conditions are possible 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 and NYC along with the Lehigh Valley.

Low pressure departs 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 now in effect for Carbon and Monroe counties tonight, with frost advisories added across adjacent areas including the far northwestern suburbs of Philly and NYC. Its a little uncertain in the frost advisory as clouds may linger as the low pressure remains nearby this evening and then winds may pick up late tonight after clouds clear, so confidence on frost is not great. Feel more confident about the freeze warning in the Poconos where advection should be more effective at getting the plateau down to freezing or below. Valleys there are more questionable.

KEY MESSAGE 2...Marginal fire weather conditions are possible on Sunday.

Departing low pressure and incoming high pressure will result in a tight pressure gradient setting up for the back half of the weekend. This will translate to a breezy Sunday, with downsloping northwest flow and wind gusts around 20-25 MPH through the late morning and the afternoon.

With breezy and dry northwest winds, combined with deep mixing setting up (tapping into very dry air aloft), there is a marginal risk for increased fire spread as minimum RH values will fall into the 25-35% range.

Based on collaboration with 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 /18Z SATURDAY THROUGH THURSDAY/

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

Rest of Today...VFR. NW winds around 10 kt. Moderate confidence.

Tonight...VFR. NW winds 5-10 kt. High confidence.

Sunday...VFR. NW winds around 10 kts with a few gusts of 20 kts. High confidence.

Outlook...

Sunday Night through Monday Night...VFR. No significant weather.

Tuesday through Tuesday Night...VFR. South/southwesterly wind gusts 20-25 kt.

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

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

MARINE

Sub-Small Craft Advisory conditions today and tonight. Winds northwest becoming variable late this afternoon, then resuming a northwest direction and increasing to 10-15 kt with gusts up to 20 kt tonight. Seas 2 to 4 ft.

VSBY restrictions in rain possible from time to time this afternoon over the open ocean waters.

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.