textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Aviation section updated for 18Z TAFs.

Frost Advisory now in effect for Carbon and Monroe counties for late tonight through Tuesday morning.

KEY MESSAGES

1. A drier airmass spreads into the region. A Frost Advisory is now in effect for the southern Poconos late tonight through Tuesday morning.

2. Marginal fire weather conditions may develop Tuesday afternoon.

3. Widespread rain with potentially a few thunderstorms moves in for the PM hours on Wednesday, continuing into Thursday morning.

4. Above normal temperatures take over for the weekend and potentially continue into next week.

DISCUSSION

KEY MESSAGE 1...A drier airmass spreads into the region. A Frost Advisory is now in effect for the southern Poconos late tonight through Tuesday morning.

Low pressure south and east of the Delmarva Peninsula will continue to drift out to sea as the afternoon progresses, and any light rain over Delmarva and southeast New Jersey will taper off by early this evening. While skies will begin to clear out, some shortwave energy passing north of the area may keep some mid and high clouds in northern portions of the region early tonight.

High pressure over the Great Lakes will build south and will be centered over central Pennsylvania by Tuesday morning, and shifts off the Mid-Atlantic coast by early Tuesday evening.

A light northerly flow develops tonight, and weak cold air advection will be underway. Lows tonight will drop into the mid and upper 30s in the southern Poconos and far northwest New Jersey and in the low to mid 40s elsewhere. With frost development becoming likely for Carbon and Monroe counties, will go ahead and hoist a Frost Advisory for those counties late tonight through Tuesday morning. Some patchy frost may develop in portions of far northwest New Jersey as well, but the airmass may be too dry for frost to be widespread.

KEY MESSAGE 2...Marginal fire weather conditions may develop Tuesday afternoon.

Sunny and mild conditions on tap for Tuesday. Highs will mostly be in the mid and upper 60s to around 70, thought cooler in the southern Poconos and along the coasts. With decent mixing up to around 850 mb, dew points should mix down to the upper 20s and low 30s in the afternoon during peak heating of the day. MinRH values will generally be 25 to 30 percent, though as low as 20 to 25 percent over the Pine Barrens of southern New Jersey. However, with the center of the high nearby, surface winds will turn from the north at less than 10 mph in the morning to the west in the afternoon. Though dry, recent rainfall and relatively light winds are limiting factors for fire weather conditions despite the low RH values.

KEY MESSAGE 3...Widespread rain with potentially a few thunderstorms moves in for the afternoon hours on Wednesday, continuing into Thursday morning.

The upper-level pattern is forecast to become rather amplified through this week. A strong upper-level trough axis shifts to our east Tuesday as high pressure will also be shifting offshore. Another upper-level trough then moves over the Eastern US Wednesday into Thursday before lifting out Friday into Saturday. Guidance seems to be trending toward the earlier ECMWF's idea that the incoming trough will close off either over our area or just off to the east. The timing and location of where the cutoff occurs will determine when things dry out. If the trough cuts off over our area, showers could linger into Thursday. If it cuts off just to the east, showers should move out by Thursday, giving way to a nice weekend (more on that in Key Message 3). Regardless of how this transpires, a round of widespread rain is expected with as the incoming trough and cold front arrive.

As the aforementioned upper-level trough moves into the region Wednesday, low-level warm air advection will increase ahead of it as the low to mid level flow also increases. This will result in an increase in forcing for ascent, and some instability should develop ahead of the main forcing. A warm front should also lift across our area Wednesday with some ascent associated with it. As a result, showers should increase Wednesday from west to east, especially in the afternoon with forcing peaking Wednesday night as the cold front arrives. The amount of destabilization is more uncertain, however guidance does show some instability which should be enough to support at least a few thunderstorms. The intensity of any thunderstorms however will depend on the magnitude of the instability and shear. However, guidance this morning does not show much in terms of instability, with shear looking rather meager as well. This would result in a low to zero severe threat, with better kinematics well off to our west.

The main forcing should shift to our east and northeast Thursday with the bulk of the showers ending. However, if the closed low scenario ends up happening, then the core of the cold air aloft with this slides overhead and this could result in additional shower development on Thursday before the system departs our region. Rainfall amounts are trending to be right around a half inch or so.

KEY MESSAGE 4...Above normal temperatures take over for the weekend and potentially continue into next week.

Medium to long range guidance continues to hint at a ridge setting up over the Eastern US with an associated area of high pressure over the southeast US and slowly pushing offshore. This would result in a warm southerly return flow setting up and temperatures likely getting into the 80s. Still several days out though and things could change, but consensus among the deterministic and ensemble guidance has a stretch of above normal temperatures beginning this weekend. The current forecast does not have much in terms of precipitation chances for this stretch either.

AVIATION /18Z MONDAY THROUGH SATURDAY/

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

Rest of today...VFR. Lingering SHRA at KMIV/KACY without restrictions. NW winds 5 to 10 kt, though the direction may be more variable at KMIV/KACY. Moderate confidence.

Tonight...VFR. N winds 5 kt or less. High confidence.

Tuesday...VFR. N winds around 5 kt in the morning, then winds become W to SW 5 to 10 kt in the afternoon with sea breezes likely at KACY with S to SE winds around 10 kt.

Outlook...

Tuesday Night through Wednesday Morning...VFR. No significant weather.

Wednesday Afternoon through Thursday Morning...IFR conditions expected at times with periods of rain and a slight chance (15%) of thunderstorms. Southerly wind gusts around 20-25 kt expected on Wednesday Afternoon.

Thursday Afternoon through Friday...Primarily VFR. Cannot rule out some lingering showers which could result in some MVFR conditions.

MARINE

Sub-Small Craft Advisory conditions for the rest of today, tonight, and Tuesday. Northeast winds around 10 kt will become variable this evening as low pressure passes over the ocean waters. Winds then shift to the west, then northwest at 10 to 15 kt with gusts up to 20 kt by midnight or so tonight. Winds diminish Tuesday morning to around 10 kt, then winds take on an east to southeast flow at 10 to 15 kt with gusts up to 20 kt in the afternoon. Seas will generally average 2 to 4 ft.

Outlook...

Tuesday Night...No marine headlines expected.

Wednesday through Wednesday Night...SCA conditions likely (70-80%) with southerly winds around 20-30 kt and seas 4 to 7 feet.

Thursday...Cannot rule out some lingering 5 foot seas, but primarily sub-SCA conditions expected.

Thursday Night through Friday...No marine headlines expected.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Frost Advisory from 3 AM to 8 AM EDT Tuesday for PAZ054-055. NJ...None. DE...None. MD...None. MARINE...None.