textproduct: Philadelphia/Mt Holly

SYNOPSIS

A weak front moves through the region this evening, then high pressure builds over the area tonight before moving offshore on Thursday. A warm front lifts north through the region Thursday night, followed by a strong cold front on Friday. High pressure moves through the Mid-Atlantic on Saturday, followed by a cold front Sunday. High pressure briefly returns on Monday, and then another frontal boundary impacts the area Tuesday. High pressure returns on Wednesday.

NEAR TERM /THROUGH THURSDAY/

The area will be near the tail end of a cold front early in the night as low pressure moving through eastern Canada passes by far to our north. This will result in winds going W/NW for a time before becoming very light to even calm in spots overnight as high pressure builds in right over the area. This will create some challenges in the forecast.

Radiational cooling conditions will be good with this set up and so temperatures have the potential to fall close to dew points resulting in some fog or low stratus potentially developing. There will also be added low level moisture from the snow pack. Mitigating factors though are that slightly drier air will be trying to work in for a time this evening behind the cold front and also that the winds not too far off the ground should remain elevated overnight which could cause enough mixing to keep widespread fog at bay. For the time being, added in patchy freezing fog for most of the area. Expect lows tonight generally in the 20s except low 30s near the coast.

For Thursday, high pressure moves eastward off the coast as a large area of low pressure advances eastward into the western Great Lakes region. This will result in the flow turning more S/SE. It will once again be a tricky forecast as there's the potential for increasing low level moisture to remain trapped below an inversion which could keep the area under a deck of low stratus. Best chance of seeing any sun will be in the morning to early afternoon before clouds associated with the system to our west move in by later in the day.

In terms of temperatures, there will be a warm front beginning to move back into the area from the south so the warmest temperatures and areas most favored to see any sun will be our southern most zones over Delmarva into adjacent far southern NJ. These areas look to get into the 50s. Farther north, generally expect daytime highs in the 40s except upper 30s over the southern Poconos. One thing to note though is that these temperatures will continue to warm at night due to strengthening warm air advection. The daytime should be mainly free of any precipitation except by late day there could be some showers developing over portions of northern Delmarva into eastern PA and nearby locations in NJ near the PA border. POPs though are only around 20-30 percent. Some advection fog is possible during the daytime as well, due to the higher dewpoints moving over any remaining areas of snow pack. The higher dewpoints and winds should work to quickly erode any remaining snow, but something worth mentioning.

SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/

Low pressure moving through the Great Lakes Thursday night will lift into eastern Canada on Friday. A deep H5 trough will dig down into the Tennessee Valley and the Mid-Atlantic, and a strong cold front will cross the area on Friday. Strong warm air advection develops ahead of the cold front Thursday night through Friday morning, then strong cold air advection develops behind the departing cold front. This results in a non-diurnal temperature curve in which temperatures will rise Thursday night, and then temperatures fall during the day Friday as the cold front works its way through the region.

In terms of sensible weather, the two main impacts from this system are moderate to heavy rain and gusty to potentially damaging winds. The southerly pressure gradient tightens over the area between high pressure offshore, low pressure passing north of the area, and high pressure building into the central United States. In addition, a 60 to 70 kt low level jet will move across the region late Thursday night through Friday morning. For the most part, can expect south winds 15 to 25 mph with 30 to 40 mph gusts late Thursday night through Friday morning, though gusts up to 50 mph are likely for the southern Poconos, the Lehigh Valley, the Delaware Valley, most of New Jersey and coastal Delaware. There should be a lull in the winds late Friday morning, then west to northwest winds ramp up Friday afternoon and evening once again with 30 to 40 mph gusts for most of the area, and gusts up to 50 mph for those same areas. Winds then diminish late Friday night. A Wind Advisory is now in effect for Thursday night through Friday night for most of the area, though will note a lull in the strongest winds. The overall trend is showing stronger wind gusts, so will follow suit accordingly.

Rain develops Thursday evening, and then the heaviest rain develops late Thursday night through Friday morning in association with the aforementioned low level jet. There should be a lull in the rain Friday morning, but then the cold front passes through the region late Friday morning through Friday afternoon. While surface based instability will be minimal, there will be some 200 to 300 J/kg of ML MUCAPE ahead of the front and 65 to 75 kt 0-6 km Bulk Shear with the passage of the front. This should be enough to produce a few elevated thunderstorms across the region. Will go ahead and add slight chance for thunderstorms to the forecast Friday morning. A few stronger storms could produce locally damaging wind gusts, but chances in this happening are quite low, but not zero. A line of heavy rain showers will develop with the passage of the cold front. Temperatures then crash as strong cold air advection develops.

Low temperatures Thursday night will occur Thursday evening, and will range from the low to mid 40s, though perhaps a few degrees cooler in the southern Poconos and a few degrees warmer in southern Delmarva. Highs on Friday will occur in the morning, and will generally be in the low to mid 50s north and west of the Fall Line and in the upper 50s across southern New Jersey, the Delaware Valley, and Delmarva. Temperatures will fall into the upper 30s and low 40s by Friday afternoon. Lows Friday night will be in the 20s.

Storm total rainfall is trending up for the northern half of the forecast area and trending down for the southern half, and generally 1 to as much as 2 inches across the southern Poconos, northern New Jersey, and the Lehigh Valley and 1/2 inch to 1 inch across southern New Jersey, the Delaware Valley, and Delmarva. Widespread mainstem flooding is not a concern, but minor flooding and urban and poor drainage flooding is possible due to the heavy rainfall and melting snow.

Dry and colder on Saturday with highs in the upper 30s to low 40s.

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

Although several frontal boundaries will impact the area during the Long Term period, most of these fronts will be fairly benign. High pressure moves offshore Saturday night and then a cold front passes through on Sunday. No significant weather with its passage. Warmer ahead of the front with highs in the 40s, then colder on Monday with highs in the 30s.

Another cold front passes through on Tuesday with a mix of rain and snow, with rain to the south and snow to the north and a mix in between.

Conditions dry out on Wednesday. Near normal temperatures Tuesday and Wednesday.

AVIATION /02Z THURSDAY THROUGH MONDAY/

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

Tonight...VFR is anticipated to prevail, but there is the potential for sub VFR conditions developing after 03Z as some patchy fog and/or low stratus may develop. The greatest chances of this occurring is where thicker snow pack remains, mainly for TTN/ABE. Have maintained TEMPO groups for 3SM vsby at the more climo favored fog sites...RDG, ABE, TTN. Winds light (under 5 knots) and variable, but generally favoring a NW direction until late at night when they should veer to NE or become calm. Low confidence regarding cigs/visbys with moderate confidence in the wind forecast.

Thursday...Potential for sub VFR conditions to linger into the morning. There is a low probability that some fog lingers through the day, but confidence on this isn't high enough to include in the TAFs. Aside from that, prevailing VFR conditions expected. Winds initially light/variable to calm, then settling out of the SE/S around 5-10 knots after 15Z. Moderate confidence in prevailing VFR, high confidence in the wind forecast.

Outlook...

Thursday night...MVFR to IFR conditions developing by 06Z in RA and low clouds. S winds increasing to 15 to 25 kt, with some gusts near 35 kt possible after 06Z. Potential for LLWS despite the strong surface winds due to a 60+ kt low level jet.

Friday...MVFR to IFR in the morning in RA. An isolated TSRA possible around midday. VFR in the afternoon. S-SW winds 15 to 25 kt with 35 to 45 kt gusts, turning W-NW in the afternoon.

Friday night...VFR. W-NW winds 15 to 20 kt with gusts up to 30 kt, diminishing to 5 to 10 kt late.

Saturday through Monday...VFR. No significant weather.

MARINE

The Small Craft Advisory was allowed to expire as scheduled. Winds and seas look to remain at below advisory levels through Thursday. Variable winds 5-10 kts overnight, settling out of the southeast to south and increasing to 10-20 kts by Thursday afternoon. Seas 2-4 feet.

Outlook...

Thursday night...Gale Warning now in effect. S-SW winds 20 to 33 kt with 35 to 45 kt gusts. Seas building to 6 to 9 feet. Visibility restrictions in rain and fog.

Friday...Gale Warning now in effect. S-SW winds 20 to 35 kt with 35 to 45 kt gusts in the morning, then winds diminish a bit around midday, then turn W-NW and increase to 20 to 30 kt with 35 to 40 kt gusts in the afternoon behind a departing cold front. Visibility restrictions in rain and fog in the morning. A isolated thunderstorm is possible in the late morning/early afternoon with the passage of the cold front.

Friday night...Gale force winds in the evening, diminishing after midnight. SCA conditions for the rest of the night.

Saturday...Sub-SCA conditions.

Saturday night through Sunday night...SCA conditions expected with wind gusts to 30 kt and seas 4 to 7 feet.

Monday...Lingering SCA conditions possible.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Wind Advisory from 1 AM Friday to 1 AM EST Saturday for PAZ054- 055-060>062-070-071-101>106. NJ...Wind Advisory from 1 AM Friday to 1 AM EST Saturday for NJZ001- 007>010-012>027. DE...Wind Advisory from 1 AM Friday to 1 AM EST Saturday for DEZ001- 004. MD...None. MARINE...Gale Warning from 6 PM Thursday to 1 AM EST Saturday for ANZ430-431-450>455.