textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Increasing confidence of snow squall potential this afternoon. Confidence remains high with possible rain or snow showers today, with isolated snow squalls possible. Confidence in the forecast and details later in the week with regard to possible coastal system remains low.

KEY MESSAGES

1) Snow squalls possible along a line of rain and snow showers moving through Sunday afternoon.

2) Dry weather expected Monday and Tuesday with seasonable temperatures.

3) Cold frontal passage is expected on Wednesday with a potential coastal storm Thursday into Friday. Impacts are very uncertain depending on how close the low tracks to the coast.

DISCUSSION

KEY MESSAGE 1...Snow squalls possible along a line of rain and snow showers moving through Sunday afternoon.

While the initial cold front will be through the region Sunday morning and cold air advection will be underway, it will not be until a secondary cold front crosses the region late Sunday afternoon that the colder air arrives. This front will be accompanied with a strong H5 trough with strong shortwave energy. As a result, can expect a broken line of rain and/or snow showers with its passage. Additionally, confidence has increased in the potential for snow squalls embedded in this broken line across eastern PA and portions of New Jersey, potentially impacting the I-95 corridor, including the Philadelphia metro. Modest instability (MUCAPE upwards of 100-200J/kg) and favorable lapse rates along with gusty winds will result in the potential for heavy bursts of wind- driven snow and reduced visibilities. While surface temperatures may remain above freezing for much of the afternoon, the onset of precipitation has the potential to wet bulb temperatures down to support snow showers, and thus snow squalls. Not expecting much in the way of accumulating snow however, though some spots well north and west of Philadelphia could pick up a quick coating. Will go ahead and bump up PoPs to slight chance and low- end chance in the afternoon for the region. A tight pressure gradient develops, and northwest winds will increase to 15 to 20 mph with 25 to 35 mph gusts by sunset. The gusty winds continue Sunday night, however some gradual diminishing should occur overnight. Some locations may see occasional wind gusts near or reaching Wind Advisory criteria (46 mph), but confidence and coverage remained too low at this time to issue the advisory.

KEY MESSAGE 2...Dry weather expected Monday and Tuesday with seasonable temperatures.

Upper level trough that has been situated over the Mid-Atlantic region departs Monday morning. This will give way to mainly zonal flow aloft through Tuesday. At the surface, broad and elongated high pressure will be situated over the Deep South on Monday before shifting offshore on Tuesday. All of this will result in a period of tranquil and dry weather with light winds under mostly clear skies. Temperatures will be near-normal with highs ranging in the upper 30s to mid 40s on Monday and low 40s to low 50s on Tuesday. Lows will mainly be in the 20s and 30s.

KEY MESSAGE 3...Cold frontal passage is expected on Wednesday with a potential coastal storm Thursday into Friday. Impacts are very uncertain depending on how close the low tracks to the coast.

A mid-level disturbance embedded in the upper flow will be tracking across the northern Great Lakes Tuesday night before tracking east into southern Quebec on Wednesday. Associated surface low will track well to our north, but its associated cold front is projected to cross through the Mid-Atlantic region sometime Wednesday night. This will welcome the return of an arctic airmass and much cooler temperatures later in the week. PoPs with this system are mainly in the 20-30% range for Wednesday into Wednesday night. As of now, precipitation during this timeframe looks to be mainly rain as temperatures ahead of the front will be well into the 40s and 50s.

As this initial system passes, the large scale upper air pattern is anticipated to become quite amplified. A strong upper ridge will be building over the Pacific Northwest as a deep upper trough over eastern Canada digs down toward the Gulf Coast on Wednesday. This upper trough will then gradually shift east toward the end of the week. As of now, forecaster confidence in a coastal storm developing somewhere near the Mid-Atlantic/Northeast region toward the end of the week has increased a bit, but still remains low as global and ensemble guidance becomes more aligned. In short, there remains a lot of uncertainty as a whole in the exact details with regard to where exactly the low develops, its strength, and its track. Ensemble guidance essentially puts the placement of the low anywhere over eastern Pennsylvania to offshore the Carolinas to as far east as Cape Cod. Some guidance even develops the low along the aforementioned cold front, which may play a key role in how this forecast pans out. A coastal low over or near our coast will result in higher impacts and more precipitation brought to our region. However, a more offshore low will result in lesser impacts and less precipitation. General timeframe looks to be as early as Wednesday night and as late as Friday. Considering the vast amount of uncertainty with how the pattern evolves, have stuck with NBM guidance which brings a 20-40% chance of light rain and/or light snow to the area through Thursday night. More details to come in the coming days once guidance comes into better agreement.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

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

Sunday...Mainly VFR. A few lingering light rain showers for MIV/ACY. NW winds increase to around 10 kt with gusts up to 20 kt in the morning, then increase further to 10 to 15 kt with 20 to 25 kt gusts in the afternoon. A brief SHRA and/or SHSN possible from around 19Z to 23Z. Cannot rule out the potential for and embedded snow squall during this time for I-95 terminals and ABE/RDG. Moderate confidence overall. Low confidence on snow squall potential.

Sunday night...Mainly VFR. Isolated rain or snow showers possible early. West-northwest wind gusts 25-35 knots, gradually diminishing overnight. Moderate confidence.

Outlook...

Monday through Tuesday night...VFR. No significant weather expected.

Wednesday through Thursday...Sub-VFR conditions expected at times. A chance of light rain on Wednesday with a chance of light rain and/or light snow on Thursday.

MARINE

Marine Dense Fog Advisory was cancelled after automated sensor and webcam observations indicated an improvement to visibilities with the passage of the frontal boundary.

A Small Craft Advisory remains in effect for the ocean waters early this morning. A brief lull in SCA conditions may occur this morning before winds and seas ramp back up during the day Sunday. Later this morning and into the afternoon behind the departing low, winds shift to the west-northwest at 15 to 20 kt with 25 to 30 kt gusts. SCA remains in effect for the ocean and Delaware Bay.

SCAs become Gale Warnings Sunday night for all marine zones due to a period of west to northwest wind gusts up to 40 knots (seas 4-7 feet).

Outlook...

Monday...Lingering SCA conditions probable with wind gusts up to 25- 30 kt diminishing during the afternoon. Seas up to 5 feet.

Monday night through Tuesday...No marine headlines expected at this time.

Tuesday night through Wednesday...SCA conditions possible with wind gusts up to 25 kt and seas up to 4-6 feet.

Thursday...No marine headlines expected at this time.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ430- 431-450>455. Gale Warning from 6 PM this evening to 6 AM EST Monday for ANZ430-431-450>455.