textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Chances for significant snowfall with a late weekend storm system continue to drop.

KEY MESSAGES

1. Milder today but with a gusty wind, then a brief cool down Thursday and Friday. Generally milder temperatures return this weekend into next week.

2. We continue to watch a storm system for the late weekend and into early next week which could affect the area depending on its track.

DISCUSSION

KEY MESSAGE 1...Milder today but with a gusty wind, then a brief cool down Thursday and Friday. Generally milder temperatures return this weekend into next week.

In the wake of a cold front early this morning, the pressure gradient is forecast to tighten through the day and into tonight. This will result in a breezy to windy day with wind gusts peaking between 25-35 mph especially this afternoon. It will be a milder day, air temperature-wise, as the cold air advection is initially focused above the boundary layer. The more notable cold air advection arrives later this afternoon and especially tonight with a secondary cold front. Therefore, high temperatures for much of the area will be well into the 30s to low 40s. Some mid to upper 40s in Delmarva. As the boundary layer warms with cooling aloft, a fair amount of stratocumulus should develop especially along and north/west of the Fall Line. The gusty winds however will result in wind chills in the 20s and 30s through the day (teens in parts of the southern Poconos). It turns colder tonight with a breeze continuing, and this will drive wind chills down into the teens (single digits in the southern Poconos). A few snow showers will be possible in the southern Poconos tonight.

High pressure will bring generally fair but somewhat cooler conditions for Thursday and Friday (compared to Wednesday). Expect highs both days generally in the 30s with overnight lows in the teens and 20s. The airmass over the area moderates over the weekend as high pressure slowly shifts off to the east. Expect highs both Saturday and Sunday generally ranging from the 30s north to the middle to upper 40s south. However overnight lows, at least for Saturday night will be below freezing in the 20s. The overall moderating trend looks to continue into next week with the one caveat being the coastal storm system that could impact the area by later Sunday into Monday (see Key Message #2). Depending on the strength and track of the system it could cause temperatures to be colder than we're currently thinking if we get into heavier precipitation that results in dynamic and evaporational cooling. Regardless of what happens with the storm, it will have departed by next Tuesday with highs by then looking to be well into the 40s for most areas with even some low 50s over parts of Delmarva.

KEY MESSAGE 2...We continue to watch a storm system for the late weekend and into early next week which could affect the area depending on its track.

Model guidance continues to depict an area of low pressure affecting the eastern seaboard beginning later Sunday and continuing into Monday. There is still uncertainty in the intensity, track, and timing of this system. Most of the model guidance has been trending towards keeping the system farther south and east with less impact on our area however the GEM still depicts a stronger system farther north bringing the area widespread precipitation Sunday night into Monday. This is due to this model depicting better phasing between northern and southern streams of energy. The airmass in place prior to this system arriving will be relatively mild (highs Saturday in the upper 30s to upper 40s) meaning rain or snow changing to rain would tend to be favored with any precipitation. However if the GEM solution were to verify, this set up could "manufacture" just enough cold air for accumulating snow to occur across much of the CWA excluding most of the coast. This all said, the solution depicted by this model is a lower probability scenario based on the overall trends in deterministic and ensemble guidance. In fact the latest NBM probabilities for snowfall amounts of at least 2 inches are now only in the 5 to 15 percent range across the area...lowest over southern DE and coastal NJ and highest over the Poconos. This is lower than the prior model cycle. In terms of our forecast, we stayed largely with the NBM which has 30 to 60 POPs late Sunday into early Monday (lowest north, highest over southern DE). Most likely an precip would fall as rain near the coast and over southern DE with snow or some mixing with snow possible for a time farther north towards the urban corridor and points northward. However it's worth noting these areas farther north that have a better chance of seeing any precip fall as snow also have lower POPs only in the 30 to 40 percent range at this time. Hence the diminishing chances for a significant snowstorm across the area.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

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

Rest of today (through 00Z)...VFR. West to northwest winds 12-18 knots with gusts to around 25 knots this afternoon. BKN to SCT deck of stratocu around 4-5kft. High confidence.

Tonight...VFR. West-northwest winds 10-15 knots with gusts to around 20 knots. The wind gusts should drop off into the overnight hours. Moderate confidence.

Outlook...

Thursday...VFR with gusty NW winds.

Thursday night thru Sunday morning...VFR.

Sunday afternoon through Monday evening...Sub-VFR possible with rain S/E and snow N/W.

MARINE

Small Craft Advisory in effect for all marine zones today and tonight. As a cold front crosses our area early this morning, west to northwest winds will increase through the morning. The air mass is milder today, therefore mixing should be limited some over the waters. As colder air works in later this afternoon and especially tonight, stronger winds are expected with gusts to 30 knots.

Regarding the river/bay ice...Significant ice cover continues across many area bay waters and inland estuaries. This includes, but is not limited to the Delaware estuary. On rivers, the ice thickness is enough to cause some restrictions in the flow. Per satellite imagery and ice analysis data, the Delaware River is mostly ice covered from the entrance to Delaware Bay northward, or upstream, to at least Washington Crossing.

The most common ways river ice can break up is either through a thermal or mechanical means. We would like to see temperatures rise into the 40s for a few days, which parts of the region are forecast to get to around 40 degrees again on Wednesday afternoon, then again over the weekend and into next week. Significant rainfall, producing river rises, can also break up ice but that is not in the forecast this week.

As a result, ice cover will continue. There may be a pause in the ice growth during the daylight hours as temperatures rise to and above freezing, however temperatures at night will continue to support ice expansion.

Outlook...

SCA conditions are expected to continue Thursday before winds and seas subside Thursday evening. Beyond this time, conditions most likely stay below Small Craft Advisory levels through the weekend into early next week unless a coastal storm system to our south tracks farther north closer to our waters. This could result in increasing winds/seas later Sunday into Monday.

EQUIPMENT

The Fort Dix radar (KDIX) remains out of service. Work remains ongoing with progress obtaining and installing replacement parts.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST Thursday for ANZ430-431- 450>455.