textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

There is the potential for freezing fog tonight.

Models continue to trend north the storm Sunday night into Monday, resulting in slightly higher snowfall totals across the region.

KEY MESSAGES

1. Fog development is expected tonight leading to pockets of reduced visibility. As temperatures fall below freezing tonight, with many locations falling into the 20s, freezing fog is a concern.

2. Low pressure moves through the Mid-Atlantic Sunday night through Monday, bringing rain and snow to the area.

3. Above normal temperatures return to the area by the middle of next week.

4. Several waves of low pressure may affect the area late next week.

DISCUSSION

KEY MESSAGE 1...Fog development is expected tonight leading to pockets of reduced visibility. As temperatures fall below freezing tonight, with many locations falling into the 20s, freezing fog is a concern.

An area of high pressure has been building into the region today and will move overhead tonight. As a result, winds will lessen to light and variable or even calm at times tonight. There will be warmer air moving northward aloft while at the surface we remain colder, especially helped by the snowpack still on the ground. This will result in a prominent inversion which is shown in model soundings. Low-level moisture looks to get trapped below this inversion overnight. One key part of this is that our dew points are higher than previous days. All of this is leading to the potential for fog development overnight into Sunday morning. The exact extent of the fog is low confidence but probabilistic guidance shows a 40-60% probability of dense fog developing across parts of the area. Right now, the fog is kept at patchy with the potential to be locally dense in spots. However, if conditions continue to favor fog development, it has the potential to be more widespread. Another concern with the fog is this being freezing fog as overnight lows are expected to be mainly below freezing and in the 20s for most. Freezing fog can result in a light glazing of ice on untreated surfaces, particularly elevated surfaces.

KEY MESSAGE 2...Low pressure moves through the Mid-Atlantic Sunday night through Monday, bringing rain and snow to the area.

Low pressure currently over the Southern Plains will track east along the Gulf Coast states tonight through Sunday. A warm front will develop out ahead of this low, and on this warm front, overrunning precipitation west of the area will lift into Delmarva, extreme southern New Jersey, and the Delaware Valley late Sunday afternoon. Temperatures will be warm enough for precipitation to be plain rain.

Going into Sunday night, low pressure tracks into the Southeast, then lifts north into the Mid-Atlantic area. 12Z model guidance has continued the trend with a slightly more northern track with the low coming off of Cape Hatteras late Sunday night, then tracking out to sea on Monday.

With this northern shift, QPF amounts have trended higher, as has snowfall amounts. Given that surface temperatures will generally be in the low to mid 30s across northern New Jersey and the Lehigh Valley, can expect snow ratios there to be on the order of 10:1. Farther south, ratios will be lower, as surface temperatures will be in the mid 30s or so, and just above freezing. The highest QPF will be across the southern half of the forecast area, where precipitation will be mostly rain, and where it will be snow, QPF should be on the order of around 1/4 inch. Farther south, QPF will be on the order of 1/2 to 3/4 inch, with locally as high as 1 inch.

12Z/14 NAM indicating on the order of 3 to 5 inches of total snow for this event, mostly across central and southern New Jersey and into the Delaware Valley, while the 12Z/14 GFS is mostly 2 to 4 inches in the same area. 12Z/14 ECMWF and the ECMWF-AI are a bit lower, generally 1 to 3 inches. The Canadian only has less than an inch of snow, mainly across the northern half of the forecast area.

Using a 1:1 blend of NBM:NBM10 for Snow Ratios, along with WPC QPF, generally ended up with 1 to 2 inches of snow for most of New Jersey north of I-195, the Lehigh Valley, and the Delaware Valley, and less than an inch elsewhere. Should these totals hold out, a Winter Weather Advisory may be needed for portions of New Jersey Sunday night and Monday morning.

KEY MESSAGE 3...Above normal temperatures return to the area by the middle of next week.

High pressure builds into the Eastern Seaboard on Monday, then establishes itself offshore Tuesday through Wednesday. Return flow sets up at the surface and zonal flow sets up aloft. This allows for warm air advection to develop, and temperatures will finally push to above normal levels. Normal highs for this time of year range from the mid 30s in the southern Poconos to the low 40s in the Lehigh Valley to the mid 40s along the I-95 corridor, and areas southeastward. High temperatures Tuesday through Thursday will mostly be 10-15 degrees above this range, from the mid 40s or so in the southern Poconos and Lehigh Valley to the upper 40s and low 50s along the I-95 corridor, Delaware Valley, and southern New Jersey. Highs will be in the mid and upper 50s in southern Delmarva. Lows for the most part will be down into the low to mid 30s. Although relatively warm for this time of the year, it will also remain cloudy. The temperatures should allow for some of the abundant snowpack to start to melt, but it may not melt as much as it could due to the cloudy skies.

KEY MESSAGE 4...Several waves of low pressure may affect the area late next week.

Conditions then turn somewhat active for the middle to the end of next week. The first wave of low pressure will pass through Pennsylvania and New Jersey late Wednesday and Wednesday night. Precipitation at this point looks to be plain rain along and south of the I-95 corridor and a wintry mix of rain and snow, possibly mixed with some freezing rain, across the southern Poconos, far northwest New Jersey, and the Lehigh Valley.

Another system may affect the area for the end of the work week.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

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

Tonight...VFR conditions begin the period, before MVFR/IFR conditions develop between 03Z-08Z as fog develops. Winds become light and variable tonight. There is a 40-60% probability of IFR or lower conditions due to fog. However, the exact extent of the fog is low confidence.

Sunday...Sub-VFR through around 15Z due to fog. A period of VFR conditions possible during the afternoon hours. Between 21z-00z, ceilings lower to MVFR as rain/snow arrives from south to north. East winds 5-10 kt. Low confidence.

Outlook...

Sunday Night through Monday...MVFR or IFR in RA or SN. Best chances for SN will be at KRDG/KABE/KTTN, RA and SN at KPHL/KILG, and RA at KMIV/KACY.

Monday night through Tuesday night...VFR.

Wednesday through Thursday...Sub-VFR possible from time to time, mostly in RA.

MARINE

Sub-SCA conditions expected through Sunday with west- southwest winds around 10-15 kt decreasing to under 10 kt tonight. Winds gradually shift overnight to be out of the east by Sunday under 10 knots. Seas 1 to 2 feet.

Regarding the river/bay ice...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 Delaware Memorial Bridge upstream, to at least Washington Crossing, though we are seeing improvement with warmer temperatures that will only get better with warmer days ahead.

According to latest analysis from the National Ice Center, Delaware Bay is actually mainly ice-free with less than 10% coverage of ice.

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 once again this weekend with some areas getting into the 50s early 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 gradually decline over the next week or so and we are already seeing major improvement in Delaware Bay.

Outlook...

Sunday night through Monday...SCA conditions possible late Sunday night for wind gusts over 25 kt, but the window for these elevated wind gusts will be short. There is a better chance for SCA conditions, mainly due to elevated seas in excess of 5 feet, on Monday. VSBY restrictions in rain or snow Sunday night into Monday morning.

Tuesday...Lingering SCA conditions possible due to elevated seas.

Wednesday through Thursday...Sub-SCA conditions expected.

EQUIPMENT

KDIX will remain out of service through at least Sunday, February 15.

KACY ASOS will remain out of service through Tuesday, February 17.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...None.