textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes to the forecast.

KEY MESSAGES

1. Weak low pressure moves through the region this evening, followed by high pressure through Thursday.

2. A wave of low pressure will bring widespread rainfall to the region this weekend with seasonably cool temperatures.

DISCUSSION

KEY MESSAGE 1...Weak low pressure moves through the region this evening, followed by high pressure through Thursday.

Weak low pressure passes through the region early this evening. Any showers out there will taper off late this afternoon/early this evening, then there may be another brief shot of showers after sunset as a shortwave moves through the region. Any QPF will be minimal.

High pressure then builds both over Hudson Bay in Canada and over the Mid-Atlantic and Southeast. A light onshore flow develops tonight, and with increasing low level moisture, some patchy fog will develop.

Winds turn to the west-northwest on Thursday and will increase to 10 to 15 mph with gusts up to 20 mph in the afternoon. Skies will be sunny, and it will be quite warm with highs in the mid to upper 70s, though some spots along the I-95 corridor and interior portions of Delmarva may top off around 80.

Surface dew points will start out in the upper 40s, then will mix down to the low 40s with efficient mixing. This results in MinRH values from 30 to 35 percent away from the coasts.

KEY MESSAGE 2...A wave of low pressure will bring widespread rainfall to the region this weekend with seasonably cool temperatures.

A back door cold front will sink south through the region on Friday as high pressure settles near New England. A few showers may develop in the afternoon as some energy embedded in the northwest flow aloft clips the region, but overall conditions look to be mainly dry and cool. Forecast highs on Friday will mainly be in the 60s to around 70, perhaps pushing into the mid 70s near the Eastern Shore of MD. It should be noted that forecast guidance notoriously struggles with back door fronts and it is entirely possible that the front sinks through much faster than currently progged. This would result in much cooler temperatures than what is currently in the forecast.

Low pressure will then begin to organize over the Great Lakes region by Friday night and push toward the Northeast and Mid-Atlantic region over the weekend. As of now, there are still some timing differences on how quickly the low tracks through the Mid-Atlantic. Some guidance indicates a quick moving low that will be off the coast by Sunday, whereas some depict a slower moving low that won't move off the coast until Sunday night. For this reason, have carried at least a chance of PoPs through the entire weekend, with the most likely timing of precip being Saturday afternoon and evening. While not expecting a washout both days, showers will likely be around at times. Current rainfall amounts look to total around 0.5-1.0 inches to the north of Philadelphia and around 0.1-0.5 inches to the south of Philadelphia.

Saturday will be dreary, damp, and raw with highs only in the upper 40s to upper 50s. Some improvement should be expected by Sunday (though is dependent on low timing mentioned above), with temps increasing into the mid 50s to mid 60s. By early next week, dry and warmer conditions return with highs returning back to normal.

AVIATION /00Z THURSDAY THROUGH MONDAY/

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

Tonight...VFR initially, then at least MVFR conds in patchy fog. Some low stratus is possible, but with light winds favoring fog development, kept ceilings around a SCT020 in most spots. E winds less than 5 kt, though mostly LGT/VRB. Low confidence.

Thursday...Morning fog and stratus will lift and dissipate shortly after sunrise, then VFR. NW winds 5 to 10 kt, increase to 10 to 15 kt with gusts up to 20 kt in the afternoon. Moderate confidence.

Outlook...

Thursday night through Friday night...Primarily VFR expected with a chance of rain showers on Friday night.

Saturday through Sunday...Sub-VFR conditions likely. Rain showers likely, especially Saturday afternoon into Saturday night. Patchy fog possible.

Sunday night through Monday...VFR expected. A slight chance of rain showers.

MARINE

Sub-Small Craft Advisory conditions tonight through Thursday. South winds 10 to 15 kt with gusts up to 20 kt become NE 5 to 10 kt tonight, then NW 10 to 15 kt Thursday morning. Winds then become S in the afternoon behind afternoon sea breezes. Seas overall 2 to 4 feet.

VSBY restrictions possible in fog, mostly near the coasts, late tonight through Thursday morning.

Outlook...

Thursday night through Friday night...No marine headlines expected. Winds around 5-15 kt with seas of 2-3 feet.

Saturday through Sunday night...SCA conditions likely to develop by Saturday afternoon. Winds around 15-25 kt with gusts up to 30 kt possible. Seas of 4-7 feet. Rain showers likely.

Monday...Lingering SCA conditions possible. Winds generally around 15-20 kt with seas of 3-5 feet.

PHI WATCHES/WARNINGS/ADVISORIES

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