textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A Small Craft Advisory has been issued for the Coastal waters off New Jersey and Delaware.

Potential for snowfall continues to increase Sunday into Monday, however exact amounts remain uncertain due to a large range of potential outcomes.

KEY MESSAGES

1. Periods of rain through today. Light wintry precipitation may cause travel hazards through this morning, mostly near and north of the I-80 corridor.

2. A winter storm Sunday into Monday will bring potential for moderate to heavy snowfall.

DISCUSSION

KEY MESSAGE 1...Periods of rain tonight and Friday. Light wintry precipitation may cause travel hazards tonight and Friday morning, mostly near and north of the I-80 corridor.

Low pressure moving into the western Great Lakes region will cause a surface warm front to move towards the area today. A round of rain and higher elevation wintry precipitation is expected to overspread the area this morning. We'll continue with the Winter Weather Advisory until noon as the main batch of precipitation is yet to arrive across the area and temperatures will remain cold enough to produce some wintry mix. We haven't changed our thinking with respect to snow or ice totals with a few tenths of an inch of snow possible and ice of around a tenth or an inch most areas. The ice total near Mount Pocono may be closer to 0.2 inches.

There looks to be a pronounced south to north temperature gradient across our area today due to the placement of a warm front. High temperatures will be near 60 over Delmarva, but 40s for bulk of the region, with mid/upper 30s for the areas covered by the Winter Weather Advisory.

KEY MESSAGE 2...A winter storm Sunday into Monday will bring potential for moderate to heavy snowfall.

Potential for widespread accumulating snowfall continues to increase, however confidence remains low on the forecast snowfall amounts with the winter storm anticipated to impact the region Sunday into Monday. Forecast totals and timing should come into better focus by late tonight.

The model guidance remains in good consensus showing low pressure moving off the Southeast or Mid-Atlantic coast late Saturday night or Sunday, then strengthening as it moves out to sea by late Sunday into Monday. The variability in the model guidance continues to be high, with solutions ranging from an intense surface low tucked close to or on the Mid-Atlantic coast to a weaker and farther south and east surface low. The ensemble guidance continues to produce widely varying solutions, and model suites did not change or trend in any particular direction with the overnight runs. The exception being that the ECMWF has come on board with at least light snow accumulations for the entire area. Long story short, we are confident that this low pressure system will bring precipitation to our region Sunday into Monday (80-90% probability of precipitation), however the details in track and intensity of the storm remain unclear. These details will dictate the outcome of snowfall amounts, coastal flooding, and winds.

The timing of short wave energy from the Midwest today and also the West Coast will determine the amplification of the upper- level trough as it shifts eastward. Some guidance is phasing this energy with more of a neutral to negative tilt of the trough, thus a much stronger storm close to the coast. While the uncertainty remains high, the outcome may become more in the middle with a deepening storm offshore but not as quick as the more robust guidance. May also have to watch an inverted surface trough to the northwest of the surface low. This trough is more evident especially on the weaker and farther south/east model solutions. These different solutions are to blame for the wide range in snowfall amounts, despite being within 72 hours of the storm.

If the system is much stronger and closer to the coast, more intense dynamics would lead to more snow (heavier precipitation rates), while weaker and farther offshore solutions would favor lighter snow amounts with even precipitation potentially starting as rain. The air mass ahead of this system is forecast to not be all that cold, therefore boundary layer temperatures will come into play regarding rain versus snow and thus accumulations. Even a weaker storm will still result in an increasing northeasterly low-level wind into our coastal areas especially. This would increase the risk for at least some coastal flooding Sunday into Monday, and this would increase even more if the storm ends up being stronger (i.e. stronger onshore winds).

Given the uncertainty, our latest forecast stuck closely to the NBM. We did increase the wind and winds gusts Sunday into Monday, mainly closer to the coast and over the marine areas. Most of the storm is within the QPF/snow amount period now. The initial storm total snowfall forecast is generally a widespread 2-6" across most of the area, but again these amounts are still highly uncertain and will likely change in future updates as the aforementioned meteorological forecast details become more clear. Don't focus on the specific details of our forecast just yet.

In the wake of this storm, a colder air mass settles into our region Monday and Tuesday, with high temperatures mostly in the 30s (colder in the Poconos) and lows in the teens and 20s. It then turns milder for the second half of next week.

AVIATION /07Z FRIDAY THROUGH TUESDAY/

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

Today...Low pressure and a warm front will track across the Northern Middle Atlantic region today. Low CIGS are expected with the present prevailing MVFR expected to lower to IFR in many places and LIFR possible too. VSBYs will lower as a more steady rain arrives towards dawn and continues thru the morning. The rains will taper off from SW to NE thru the afternoon, but lingering drizzle is possible. A little freezing rain or snow may be found across the far NW counties. East to Southeast winds around 10 to 15 knots expected. Low/medium confid.

Tonight...Most of the rains will be done by evening with perhaps some lingering drizzle possible. Not enough confid in placement for inclusion in TAFs attm. Lower CIGS are expected to remain in place early with some improvement possible after midnight. Low/medium confid overall. West to Northwest winds 5 to 10 knots much of the time.

Outlook...

Saturday...VFR, then ceilings lower at night.

Sunday and Monday...Sub-VFR conditions with rain/snow, then conditions should improve during Monday. Northeast to north/northwest winds increase and become gusty.

Tuesday...VFR. Gusty northwest winds possible during the day.

MARINE

We issued a Small Craft Advisory earlier as it now appears that seas will remain around 5 ft the rest of today and into the overnight for the ocean waters. Sub-SCA expected for Delaware Bay. Rains and areas of fog will be across the waters much of today before diminishing tonight. Winds will favor E/SE directions today before switching around to W/NW for tonight.

Regarding the river/marine ice, Delaware Bay continues to be mainly ice free outside of shallow near-shore areas. Ice coverage remains around 10-30% from the the Delaware Memorial Bridge up to the Commodore Barry Bridge. North of that up through Philadelphia, ice coverage is around 30-70%.

Ice coverage will continue to decline with warmer temperatures through Saturday.

Outlook...

Saturday...The conditions are anticipated to be below Small Craft Advisory criteria.

Sunday and Monday...Gale force winds probable with hazardous seas, with a period of storm force wind gusts possible. Wind should lower to Small Craft Advisory criteria later Monday and especially Monday night. The magnitude of the winds though will depend on the track and strength of low pressure.

Tuesday...Lingering Small Craft Advisory conditions possible.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Weather Advisory until noon EST today for PAZ054-055. NJ...Winter Weather Advisory until noon EST today for NJZ001. DE...None. MD...None. MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ450>455.