textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Widespread fog will develop tonight with patchy dense fog likely around daybreak Friday.

KEY MESSAGES

1.Stationary boundary south of the Delmarva Peninsula will lift north as a warm front late tonight through Friday morning. Conditions warm up Friday afternoon.

2.A warm front lifts north of the region Saturday before a cold front crosses the region Sunday, bringing a few rounds of showers and thunderstorms before ushering in more seasonable temperatures.

DISCUSSION

KEY MESSAGE 1...Stationary boundary south of the Delmarva Peninsula will lift north as a warm front late tonight through Friday morning. Conditions warm up Friday afternoon.

High pressure remains over eastern Canada, and onshore flow keeps a relatively cold for this time of the year airmass over the area for the rest of the day and through tonight. This high will move over the Canadian Maritimes this evening before departing Friday morning. Meanwhile, low pressure over the Midwest will lift to the north and east tonight into south- central Canada. This low will track across southern Canada on Friday. A stationary boundary extend out from this low and lies across the Mid-Atlantic, just south of the Delmarva Peninsula. As low pressure lifts to the north and east, then eventually to the east, that stationary boundary will begin to lift north as a warm front late tonight. The warm front does not look to clear the northern zones until late Friday afternoon.

In terms of sensible weather, conditions remain cloudy, cool, and foggy for the rest of the day and into tonight. Another weak boundary out ahead of that low will lift north through the northern half of the forecast area, and this may touch off some showers with minimal QPF. With increasing low level moisture and a strong inversion up to around 950 mb, fog and stratus is likely to develop. The true surge in low level moisture arrives with the passage of the main warm front Friday morning. However, it does not look as if the inversion will break until sometime Friday afternoon. As a result, conditions will remain cloudy and foggy through much of Friday morning, and it will take some time for temperatures to warm up. Look for temperatures to remain in the 40s and 50s for most of the morning, not really warming up until skies clear out and the inversion breaks late Friday afternoon. There could even be some patchy dense fog just after sunrise as well, and a Dense Fog Advisory may be needed for a few hours Friday morning. Strong warm air advection will be underway late Friday afternoon. and highs will be well in the 70s.

KEY MESSAGE 2...A warm front lifts north of the region Saturday before a cold front crosses the region Sunday, bringing a few rounds of showers and thunderstorms before ushering in more seasonable temperatures.

A warm front is expected to lift north through the region on Saturday as low pressure begins to shift northeastward out of the Great Lakes region. Scattered showers will be possible in the vicinity of the front through the day Saturday, with highest chances across northern parts of the area. There may be enough elevated instability to allow for a few rumbles of thunder. Otherwise, much warmer temperatures will surge into the region, highs on Saturday rising into the mid 70s to low 80s outside of the Poconos and near the coast, where mid 60s are expected. Lows Saturday night look to generally be in the 40s and 50s, but may remain above 60 in parts of Delmarva.

There will likely be a relative lull in shower chances early Sunday with the region in the warm sector ahead of a cold front that will sweep across the region by Sunday afternoon. High temperatures on Sunday will be highly dependent on the timing of the frontal passage which remains somewhat in question, but there should be enough time for most of NJ southward into Delmarva to rise into the low-mid 70s. Widespread showers and scattered thunderstorms are expected along and just ahead of the cold front. As of right now, any severe weather threat looks quite limited with the primary limiting factor being the degree of destabilization, with meager instability at best depicted in model guidance. Locally heavy rainfall will be possible given rich moisture in place, and PWATs around 1.5", which is well above the 90th percentile for this time of year. However, given that this is expected to be a fast moving system, flash flooding is not currently expected to be a concern. Total rainfall amounts look to be in the neighborhood of 0.5", though locally higher amounts could occur in heavier showers or storms. Breezy conditions are likely on Sunday even outside of any showers and storms with southwesterly winds gusting 25-35 mph ahead of the front, with similar westerly winds behind it.

In the wake of the cold front, temperatures are expected to return to near or even slightly below normal, with highs mostly in the 50s and lows in the 30s and 40s through mid-week. A slow warm up may begin Thursday and beyond.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

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

Rest of today...Mostly IFR CIGs in low stratus with patchy MVFR VSBYs in BR. CIGs may lift to MVFR (1000 to 1500 ft) from time to time prior to 00Z, mainly at KABE-KTTN-KPNE-KPHL. E winds 10 to 15 kt with 20 to 25 kt gusts. Moderate confidence.

Tonight...CIGs lower to LIFR prior to 06Z. MVFR to IFR VSBYs in BR for most of the night. VSBYS will drop to less than 1SM just before 12Z. E winds 5 to 10 kt, becoming LGT/VRB after 09Z. Moderate confidence.

Friday...Low CIGs and VSBYs for much of the morning, with VSBYS less than 1SM and CIGs less than 500 ft. Conds improve by 18Z, and should be VFR by 21Z. S winds 5 to 10 kt in the morning, increasing to 10 to 15 kt with gusts up to 20 kt after 18Z. Moderate confidence.

Outlook...

Friday night...Primarily VFR. A few showers during the evening could lead to brief sub-VFR conditions.

Saturday through Saturday night...Sub-VFR conditions are possible at times in showers and perhaps a few thunderstorms.

Sunday...Sub-VFR expected with a period of showers and storms. Gusty winds likely through the day Sunday.

Sunday night...Conditions improve to VFR after the evening hours.

Monday through Tuesday...Primarily VFR. There is a slight chance (around 20%) for a few showers Monday night and Tuesday.

MARINE

A Small Craft Advisory remains in effect for the ocean waters from Sandy Hook NJ to Great Egg Inlet, NJ, but has been extended to 6 am Friday. Though winds will diminish this evening, seas will remain elevated for much of tonight.

E winds 15 to 20 kt with 25 to 30 kt gusts with 6 to 8 ft seas where the SCA is in effect, with winds diminishing to around 10 kt tonight. Seas subside to 3 to 5 ft. Outside of the SCA, E winds range from 10 to 15 kt with gusts up to 20 kt with 2 to 4 ft seas. Winds diminish to 5 to 10 kt tonight.

On Friday, winds turn S 5 to 10 kt in the morning, increasing to 15 to 20 kt. Gusts up to 25 kt possible late. Seas 2 to 4 ft, possibly building to 3 to 5 ft late. A new SCA may be needed for late Friday.

Fog will be developing on the waters tonight, with VSBYs from 1 to 3 NM, and VSBYs may even be less than 1 NM Friday morning. A Marine Dense Fog Advisory may be needed in the morning. VSBYs should improve Friday afternoon.

Outlook...

Friday night...SCA conditions likely in the evening hours. Southwest winds 10-15 kt with gusts up to 25 kt, decreasing to around 10 kt with gusts up to 20 kt late. Seas 3-6 feet decreasing to 2-4 feet.

Saturday...No marine headlines currently anticipated with winds under 25 kt and seas under 5 feet.

Saturday night through Sunday...SCA conditions likely with gales possible. Southerly winds Saturday night will increase to 15-25 kt with gusts to 30-35 kt. Winds will become southwesterly on Sunday. Seas 4-8 feet.

Sunday night through Monday...Lingering SCA conditions likely. Winds quickly subside to below 25 kt. Seas remain elevated near 5 feet through Monday morning before decreasing below 5 feet by Monday afternoon.

Monday night through Tuesday...No marine headlines currently anticipated with winds under 25 kt and seas under 5 feet.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Friday for ANZ450>452.