textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Decreasing likelihood of dense fog early this morning, though some patchy fog remains possible.

Snowfall forecast remains largely unchanged, with the highest totals near and north of the I-195 and PA Turnpike corridors.

Confidence is increasing in above normal temperatures for the upcoming week, breaking a prolonged cold spell.

KEY MESSAGES

1. Fog development remains possible early this morning leading to pockets of reduced visibility. As temperatures are below freezing, 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...An unsettled pattern takes hold for the upcoming week, with multiple chances for rain, and potentially some mixed precipitation in northern areas.

DISCUSSION

KEY MESSAGE 1...Fog development remains possible early this morning leading to pockets of reduced visibility. As temperatures are below freezing, with many locations falling into the 20s, freezing fog is a concern.

An area of high pressure is overhead early this morning, resulting in light and variable to calm winds. 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. One key part of this is that our dew points are higher than previous days. All of this is leading to the potential for patchy fog development. Another concern with the fog is this being freezing fog as temperatures this morning are 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 Mississippi River Valley will track east along the Gulf Coast states through the day today. 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 this afternoon. Temperatures will be warm enough for precipitation to be plain rain.

Going into tonight, low pressure tracks into the Southeast, then lifts north into the Mid-Atlantic area. 00Z model guidance has largely stayed in line with the low coming off of Cape Hatteras late Sunday night, then tracking out to sea on Monday.

QPF amounts largely remain the same as a result, as have 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, on the order of 1/2 to 3/4 inch, with locally as high as 1 inch. Where it will be snow, QPF should be on the order of around 1/4 inch.

Most guidance has come into two camps, either showing a narrow corridor of higher snowfall totals, mainly in the 1-2 inch range, with less than 1 inch elsewhere or light snowfall totals less than 1 inch everywhere. The 00Z GFS remains a notable outlier from both of these camps, with a much wider swath of 2-3 inches of snow. As a result, the likelihood of needing an advisory for any locations has decreased significantly, though can't quite be ruled out completely. Primarily stuck with NBM for Snow Ratios, along with WPC QPF, ending up with similar totals of 1 to 2 inches of snow for most of New Jersey north of I-195 and the Lehigh Valley, and less than an inch elsewhere.

KEY MESSAGE 3...Above normal temperatures are expected for most of the upcoming 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.

We start the week with temperatures near normal in the low to mid 40s, but temperatures get into the mid to upper 40s and low 50s from Tuesday through Friday. Highs Thursday and Friday may even touch 60 over Delmarva.

In terms of overnight lows, Monday Night will feature temperatures in the upper 20s and low 30s, with this being the last night below freezing for some locations. Tuesday Night through Friday Night will feature lows in the 30s, with areas south of I-78 likely staying above freezing.

After over two weeks now of snow on the ground, warmer temperatures will finally allow the snowpack to melt more quickly. However, mostly cloudy skies due to the unsettled pattern for this week will limit the snowmelt a bit.

KEY MESSAGE 4...An unsettled pattern takes hold for the upcoming week, with multiple chances for rain, and potentially some mixed precipitation in northern areas.

Several waves of low pressure will move through the area this week, bringing some much needed rain (and potentially some mixed precipitation up north).

The first system to move through will be the weakest and most areas will stay dry. A weak shortwave dives in from the northwest on Monday, potentially bringing some light precipitation to the southern Poconos and northern New Jersey. Added an area of 20% PoPs on Monday Night and the precipitation type remains a bit uncertain. The favorable timing (late Monday Night/early Tuesday morning) could result in some light freezing rain/drizzle or snow, especially in the higher elevations. It bears watching but expecting any precipitation to be light and relatively unimpactful.

The next system, and potentially more impactful comes Wednesday afternoon into Wednesday Night. Latest run of the NBM has a mainly rain event, but some of the 00z guidance again hints at mixed precipitation or even snow in the far reaches of our area - along and north of I-80. Stuck with the NBM for now, but certainly cannot rule out some freezing rain or snow, especially if the system comes in later and timing is more favorable. However, there is rather high confidence in this system being mainly rain for most of the area outside of far northern NJ and the southern Poconos. QPF amounts should be rather light - NBM Probability of 1 inch or more of QPF is less than 10% for the entire area. However, our area could certainly use some rain and this could be the first measurable rain for some in almost a month or more (given how cold it has been, precipitation type has been all snow).

Almost rinse and repeat for the end of the week as an area of low pressure likely slides by to the north. Again, this should mainly be an all rain event for most, if not all of the region given the track of the low. However, similar to the mid-week system, if more precipitation comes during the overnight hours, higher elevation areas in the southern Poconos and far northern NJ could see some snow or freezing rain. NBM Probability of QPF over 1 inch is a bit higher than the mid-week system, but still only around 20-30%. The details will be ironed out over the next couple days, especially once the timing comes more into focus.

Overall, a slew of systems will bring primarily rain to most of the area, with the potential for mixed precipitation in far northern New Jersey and the southern Poconos. With these systems though, we are not currently expecting moderate or major impacts. In fact, for most of the region, it will be more of a beneficial rain.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

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

Today...Mainly VFR conditions after any lingering MVFR conditions due to fog dissipate. Between 22z-00z, ceilings lower to MVFR as rain/snow arrives from south to north. East winds 5-10 kt. Low confidence.

Tonight through Monday...MVFR or IFR in RA or SN. Best chances for SN will be at KRDG/KABE/KTTN, RA and SN at KPNE/KPHL/KILG, and RA at KMIV/KACY. Conditions gradually improve on Monday as precipitation departs. Northeast winds around 5-10 kts. Low confidence. Outlook...

Monday Night...Primarily VFR, though some MVFR conditions possible (50%).

Tuesday through Tuesday Night...VFR. No significant weather.

Wednesday through Wednesday Night...Sub-VFR conditions expected with rain showers and low ceilings moving through.

Thursday...Primarily VFR, though some lingering MVFR/IFR conditions possible (30-40%).

MARINE

Sub-SCA conditions expected through today. West-southwest winds around 10 kts or less this morning, gradually shifting to be out of the east by midday. Seas 1 to 2 feet. SCA conditions possible by late Sunday night for wind gusts over 25 kt, with the passage of a front but the window for these elevated wind gusts will be short. VSBY restrictions in rain or snow Sunday night into Monday morning. By Monday morning, seas are expected to build to be in excess of 5 feet for coastal Atlantic waters. A Small Craft Advisory is in effect beginning at 6 AM Monday. Sub-SCA conditions are expected along the Delaware Bay.

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...

Monday Night through Tuesday...A Small Craft Advisory is in effect for all ocean zones. Seas of 4 to 6 feet expected. Winds will be relatively light out of the north and west - around 10 kt or less. Sub-SCA conditions expected on Delaware Bay.

Tuesday Night...Lingering 5 foot seas could result in an extension to the Small Craft Advisory on the ocean.

Wednesday through Thursday...No marine headlines expected.

EQUIPMENT

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 6 AM Monday to 6 PM EST Tuesday for ANZ450>455.