textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes from the previous forecast.

KEY MESSAGES

1. Arctic high pressure will keep the region cold through Tuesday, then several weak waves of low pressure will bring spotty wintry precipitation during this time.

2. A significant warm-up is expected beginning mid-week and continuing through the weekend with several chances for showers.

DISCUSSION

KEY MESSAGE 1...Arctic high pressure will keep the region cold through Tuesday, then several weak waves of low pressure will bring spotty wintry precipitation during this time.

Arctic high pressure centered over the eastern Great Lakes will build over northern New York state by this afternoon, then slides offshore tonight and will be anchored over the western Atlantic south of the Canadian Maritimes on Tuesday. Highs today will be some 10 degrees or more below normal, topping off in the low to mid 30s.

Meanwhile, low pressure will organize and develop over the Southern Plains, and a warm front will develop out ahead of it. As the high moves offshore, winds will turn to more of a south-southeast flow, pulling that warm front north through the Mid-Atlantic and Delmarva Peninsula. Some shortwave energy will ride on this front, and this will touch off some light snow this afternoon that will develop over Delmarva, and possibly even into far southeast New Jersey. A coating of light snow is possible over the eastern shores of Maryland and Delaware. Any snow will taper off this evening.

The warm front then lifts north late tonight through Tuesday. With temperatures below freezing, snow develops initially over Delmarva. Temperatures trends indicate a fairly quick changeover to plain rain over southern Delmarva. Farther north, precipitation will be snow, becoming a wintry mix of snow, rain, and freezing rain, quickly changing to plain rain over southern New Jersey. However, this wintry mix will persist over southeast Pennsylvania and northern New Jersey through Tuesday morning. This will result in not only light snowfall, generally less than an inch, but also some light ice accretion, generally a glaze to less than 1/10 inch. A Winter Weather Advisory will likely be needed for areas north and west of Philadelphia where the ice will fall, as this will mostly be during the Tuesday morning commute.

By Tuesday afternoon, the wintry mix will change to plain rain throughout, with highs getting into the 40s.

KEY MESSAGE 2...A significant warm-up is expected beginning mid-week and continuing through the weekend with several chances for showers.

By mid-week, a pattern change is imminent as upper troughing develops over the western CONUS and upper level ridging sets up over the eastern CONUS. By Friday, troughing out west will dig further south into western Mexico which will allow the ridge over the East to further amplify through the weekend. At the surface, a large Bermuda high will be in control.

As a result, temperatures late week into the weekend are forecast to be as much as 10 to 20 degrees above normal. On Wednesday, temps will be in the low 50s to low 60s with temps on Thursday mainly in the upper 50s to upper 60s; even pushing low 70s across the Delmarva. For Friday, the temperature forecast is a bit more complicated as guidance indicates a back-door front dropping in from New England. This would knock temperatures down a bit compared to Thursday with highs only in the low 50s to low 60s, but still above average nonetheless. For Saturday and Sunday, the back-door front lifts back north putting the region firmly back into the warm sector. Highs over the weekend should mainly be in the 60s and 70s. Coastal areas though will likely only be in the 50s thanks to chilly Atlantic water temperatures.

Overall, the large scale pattern does not point to anything significant occuring on the horizon. For Wednesday through Friday, the main feature will be a boundary that will extend from the Ozark region to New England. This boundary will fluctuate north and south from time to time while several weak disturbances transverse along it leading to several chances for showers (~30-60%). A slightly stronger wave is progged to pass through Thursday night, where some rumbles of thunder may be possible. All in all, beneficial rainfall considering the ongoing drought conditions. Shower chances continue into the weekend, but do not look more than a 20-40% chance as the jet stream will be well to our north.

AVIATION /12Z MONDAY THROUGH FRIDAY/

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

Today...VFR. NE winds 5 to 10 kt, becoming LGT/VRB in the 16Z to 19Z timeframe, then becoming S to SE less than 10 kt. High confidence.

Tonight...VFR for most of the night, then MVFR CIGs possible after 06Z, followed by lower CIGs and MVFR VSBYs in FZRA/SN prior to 12Z. High confidence through 06Z, then low confidence.

Outlook...

Tuesday...Sub-VFR conds in SN/RA/FZRA in the morning, changing to plain RA in the afternoon.

Outlook...

Tuesday night...Sub-VFR likely with periods of rain.

Wednesday through Wednesday night...Sub-VFR conditions possible with a chance of rain showers.

Thursday through Thursday night...Sub-VFR conditions probable. A chance of rain showers and a slight chance for thunder.

Friday...Prevailing VFR with a slight chance for rain showers.

MARINE

Sub-Small Craft Advisory today through Tuesday. NE winds 10 to 15 kt this morning, turning E to SE late this afternoon through Tuesday. Seas 2 to 4 feet.

VSBY restrictions possible in snow over the Delaware ocean waters this afternoon, then in rain late tonight through Tuesday.

Outlook...

Tuesday night...Small Craft Advisory conditions possible. Southwest winds around 10-15 kt with seas up to 5 feet. Rain likely.

Wednesday through Thursday night...No marine headlines expected. Winds less than 15 kt and seas around 3-4 feet. A chance for rain showers.

Friday...Small Craft Advisory conditions possible. Northeast winds around 15-20 kt with seas up to 5 feet. A slight chance for rain showers.

PHI WATCHES/WARNINGS/ADVISORIES

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