textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Rainfall chances and confidence continues to increase on Friday, otherwise no significant changes to the forecast.

KEY MESSAGES

1. Patchy fog possible late tonight/early tomorrow morning. No significant weather otherwise through Wednesday.

2. A slow moving cold front approaches the area Thursday night into Friday bringing the potential for heavy rain to portions of the area.

DISCUSSION

KEY MESSAGE 1...Patchy fog possible late tonight/early tomorrow morning. No significant weather otherwise through Wednesday.

An expansive area of high pressure offshore is in control, resulting in warm south/southwesterly flow and tranquil weather. A stretch of nice mid-Fall days will continue through the middle of the week. With some weak moisture advection, mostly clear skies, and light winds tonight, some fog may develop late tonight/early tomorrow morning. High-res guidance focuses fog development over the Lehigh Valley and New Jersey coastal plain. Not expecting widespread dense fog, but some areas could see some lower visibility for the morning commute. Any fog dissipates by 9-10 AM and it should be a nice Wednesday with warm temperatures in the 70s yet again.

KEY MESSAGE 2...A slow moving cold front approaches the area Thursday night into Friday bringing the potential for heavy rain to portions of the area.

Upper level ridging will finally begin to break down by Thursday as a potent upper trough over the central CONUS cuts off and lifts northeast into the Great Lakes region by Friday. This upper level low is then forecast to slowly move east toward New England over the weekend. At the surface, low pressure over the Central Plains will lift north into the northern Great Lakes by Wednesday and up into Quebec/Ontario on Thursday. Its associated cold front will trail back to the southwest but will begin to slow down in forward progression as it tracks toward the region before eventually stalling somewhere over/offshore of the Mid-Atlantic late Thursday. The biggest question that remains at this time is where exactly and in what orientation does the front stall/wash out, as this will play a major factor on how things pan out later Thursday and Friday. Given the latest RRFS and ensemble guidance it is becoming increasingly likely that the front will stall out within close vicinity to our area. Guidance is then keen on developing a surface low along the boundary late Thursday night and into Friday while tracking northeast along the stalled boundary before lifting up into New England on Saturday and pulling the front offshore.

All in all, this will yield the potential for some showers and light rain to begin on Thursday for western areas as the front approaches. But as the front weakens, the forcing will too, meaning that many areas will stay mainly dry on Thursday as PoPs are generally less than 30% along and southeast of the I-95 corridor. This changes by Thursday night and into Friday, as a higher plume of PWATs will surge northward upwards of 1.5-1.7 inches, which would push close to the climatological max for early November. While not currently outlooked by WPC in the Excessive Rainfall Outlook, it is in the realm of possibility that areas of heavy rain may occur from time to time late Thursday night into Friday. The greatest potential of this threat and where there may be a localized maxima of higher QPF is targeted to be south and east of the I-95 corridor at this point in time. However, it should be noted that this is highly uncertain and dependent on the placement of the stalled front. To summarize, a slower front will yield more rainfall across the region, in comparison to a faster front which will yield less rainfall across the region.

Rain chances should then decrease heading into the weekend as surface high pressure attempts to build in from the west. However, as the upper low meanders across the Northeast, this is highly dependent on how lobes of shortwave energy evolve. Thus, while it has trended drier over the weekend, there is quite a bit of uncertainty. The takeaway message here is that the weekend may not be fully dry and could remain somewhat unsettled into early next week with some diurnal opportunities for showers.

AVIATION /19Z TUESDAY THROUGH SUNDAY/

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

Rest of Today...VFR. South/southwest winds around 5 to 10 kt.

Tonight...Primarily VFR through most of tonight but areas of fog likely develop. High resolution guidance has fog mainly focused around the coastal plain of New Jersey and the Lehigh Valley. This could result in MVFR/IFR restrictions after 09z for KRDG/KABE/KACY/KMIV. Given the more rural micro-climate of KTTN, a group of MVFR restrictions was added. Confidence remains low in where exactly fog develops and restrictions could extend all the way down into Philadelphia and Wilmington. Winds generally light and variable, though a southerly direction may be favored at times.

Wednesday...Fog mixes out by 13z-14z with VFR conditions expected after. South/southwest winds 5 to 10 kt. High confidence.

Outlook...

Wednesday night...VFR. A slight chance of showers near KRDG, otherwise no significant weather expected.

Thursday through Saturday...Periods of sub-VFR conditions likely, especially Thursday night into Friday. Periods of rain expected with a slight chance of a thunderstorm.

Saturday night through Sunday...VFR. No significant weather expected.

MARINE

Tranquil conditions expected on the waters through tomorrow with southerly winds around 10 kt and seas 1 to 2 feet.

Outlook...

Wednesday night through Thursday...No marine headlines expected. Winds will generally remain less than 20 kt with seas of 2-4 feet. Fair weather.

Thursday night through Sunday...Periods of SCA conditions possible. While winds will largely remain less than 25 kt, seas are expected to range between 3-5 feet. Periods of rain likely through Saturday.

PHI WATCHES/WARNINGS/ADVISORIES

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