textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes at this time.

KEY MESSAGES

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.

2. Potential winter storm Sunday into Monday.

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 tonight. A round of rains and higher elevation wintry precipitation is expected overnight. Start time for the wintry precip looks more likely to occur after midnight, but left inherited Winter Weather Advisory as-is. Still expect snow accums will be in the less an inch with ice of 0.1 to 0.2 inches possible.

A zone of warm air advection ahead of this system and associated front will result in some overrunning precipitation. It looks like a quick moving system, and it is possible that the energy with this becomes elongated with time. Periods of rain will occur, with the bulk of it falling tonight through Friday morning. Precip looks to come in two main rounds, one late this afternoon and evening across Delmarva and southern NJ, then a second round overnight through Friday morning focused on the northern half of the forecast area (though eventually spreading south). Rainfall amounts look to range from around a third to two thirds of an inch across the region, with some locally higher amounts possible. Some fog will likely as the milder and more moist air overruns some lingering chilly near- surface air, especially in areas that still have a snow pack.

There looks to be a south to north temperature gradient across our area Friday due to the placement of a warm front. Near 60 in Delmarva, but 40s for bulk of the region, with 30s in the area covered by the Winter Weather Advisory.

KEY MESSAGE 2...Potential winter storm Sunday into Monday.

The model guidance continues to show low pressure moving off the Southeast or Mid-Atlantic coast later Saturday night or Sunday, then deepen as it moves out to sea by later 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 also shows the varying solutions, although the GFS ensemble mean has shifting closer to the coast. The timing of short wave energy from the Midwest and also the West today 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.

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, the only change from the NBM was to increase the wind and winds gusts Sunday into Monday mostly closer to the coast and over the marine areas.

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 /18Z THURSDAY THROUGH TUESDAY/

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

Rest of Today... Generally MVFR cigs and VFR vsbys until close to 0Z, when lowering cigs and vsby creep into the area from SW with light rain showers. Winds east-northeast 5-10 kts. Moderate confidence.

Tonight... Lowering cigs and vsbys in rain/drizzle/mist, especially after 6Z, when IFR/LIFR will prevail. Most rain likely occurs late in period. Winds east-northeast 5-10 kts. Moderate confidence.

Friday...Very low cigs and low vsby will result in IFR/LIFR/VLIFR conditions at all terminals thru most of the day. Drizzle likely common after rain in the morning dwindles. East-northeast winds 5-10 kts may become north late. Moderate confidence.

Outlook...

Friday night...IFR/MVFR conditions improve to VFR.

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

Winds and seas generally remaining below SCA levels tonight. Seas will be mostly less than 5 ft on the ocean and 1 to 2 ft for Delaware Bay. Showers are expected along with more areas of fog. Marginal SCA possible on Friday with seas creeping up to 5 ft, but low confidence so will hold off on headline for the time being.

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 for the rest of the week.

Outlook...

Friday night and 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 from 7 PM this evening to noon EST Friday for PAZ054-055. NJ...Winter Weather Advisory from 7 PM this evening to noon EST Friday for NJZ001. DE...None. MD...None. MARINE...None.