textproduct: Philadelphia/Mt Holly

SYNOPSIS

A cold front will cross the region and move offshore overnight, ushering in arctic high pressure with much colder and drier conditions through Tuesday. A quick moving low pressure system and its associated fronts will impact the region on Wednesday, then a clipper system will impact the area Thursday night into Friday. Arctic high pressure will then return into next weekend, resulting in a prolonged period of well below normal temperatures.

NEAR TERM /THROUGH MONDAY/

A mid-level shortwave axis associated with a closed low over portions of the Hudson Bay and Quebec will shift southeast to the north of the region into tonight. At the surface, a strong cold front will pass through the region tonight into early Monday morning.

It still appears that the front itself will be mostly dry for our area, with limited moisture and the primary synoptic scale ascent displaced to the north. With that said, it is not entirely out of the question that a few flurries could occur tonight, primarily across the Poconos. The main impacts will be the dramatic temperature difference and wind changes. Guidance tends to have a low bias with arctic cold air advection regimes even if the trough isn't very amplified (as this one doesn't appear to be), so have included wind speeds and gusts on the higher end of guidance. Even so, should stay well below wind advisory thresholds. The peak gusts in our region will likely be in the 25 to 35 mph range, with gusts as high as 40 mph possible for the Poconos.

Low temperatures tonight look to be in the upper 10s to lower 20s across most of eastern PA and northern NJ, and in the mid to upper 20s elsewhere.

Flow aloft will weaken into Monday as another impulse approaches the Eastern Seaboard to the south of the area by evening. At the surface, strong high pressure will build in from the Great Lakes region. A weak area of low pressure associated with the impulse to the south of the area is expected to develop near the coast Monday, but the strong high should keep it and most of the associated precipitation to the south. The far northern part of the precipitation shield could potentially make it as far north as portions of the eastern shore of Maryland and far southern Delaware, where it would fall as snow. PoPs are in the 20-30% range for this area during the late morning and into the afternoon hours. Little to no accumulation would be expected with any snow given its very light nature.

High temperatures on Monday will be quite chilly. Low-mid 20s are expected across the Poconos and into far northwestern NJ. Near- freezing temperatures can be expected across the remainder of eastern PA and NJ away from the coastal plain, where they could perhaps reach the mid 30s.

SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/

The main story for the short term period Monday night through Tuesday will be the continued well below normal temperatures (on the order of 10-20 degrees below normal) following a cold frontal passage late tonight (Sunday night).

A split flow pattern will remain in place through early in the new week, with the main feature of interest being a trough axis passing to our south Monday night. Rather cold and dry arctic high pressure will build in from the Great Lakes through Tuesday morning. The high will then shift offshore during the day Tuesday, with some south to southwesterly return flow developing later in the day.

The coldest night of the week and likely so far this season for most areas will be Monday night into Tuesday morning. As the high pressure builds in over the area maximizing subsidence, skies should clear out enough to give us decent radiational cooling conditions. However, there will be considerable mid level cloudiness dissipating early in the night. The lingering mid level moisture will probably inhibit any extreme bottoming out of temperatures despite dewpoints near 0 degrees, especially with a lack of snowpack. Forecast low temperatures range from the single digits for the Lehigh Valley, NW NJ, and southern Poconos, to the low to mid 10s in most other areas, and closer to 20-25 degrees for the immediate coastal areas. Fortunately, winds will be light to calm, so the wind chill will not be much of a factor and no cold weather headlines are anticipated.

These temperatures are anomalously cold, but are currently anticipated to fall just shy of record values. For more details on daily records, see the Climate section below

As the high pressure shifts offshore on Tuesday, we will start to develop some return flow and airmass modification into the afternoon hours. High temperatures will rebound a few degrees, mainly in the low to mid 30s and closer to 40 degrees near the coast. Increasing clouds through the day. With the high pressure offshore for Tuesday night, we will keep a light south to southwest breeze near 5-10 mph much of the night. This combined with broken cloud cover will significantly limit radiational cooling, so low temperatures should range mostly from the mid 20s to low 30s.

LONG TERM /WEDNESDAY THROUGH SATURDAY/

The pattern from the middle of the week into next weekend remains cold with below normal temperatures, but becomes more active across the Eastern US with several areas of low pressure and their associated fronts taking aim at the area.

Guidance has come into better agreement with Wednesday's system as a rather strong shortwave tracks through the Great Lakes with an upper level low meandering within the vicinity of the Hudson Bay. At the surface, low pressure will track out of the Great Lakes and across southern Quebec into Wednesday night. As it does so, its associated cold front will cross through the Mid-Atlantic region. This will be the next opportunity for much of the area to recieve some kind of precipitation ahead of FROPA on Wednesday. With the region located well south of the low center, its likely that this precipitation will be mostly in the form of rain, although some snow is likely in the southern Poconos.

Temperatures for Wednesday will be close to normal, with highs in the 40s to near 50 degrees in most areas and nighttime lows in the upper 20s to mid 30s. Following the frontal passage, temperatures are expected to dip back a bit below normal for Thursday as some weak ridging at the surface returns. Aloft, the Mid-Atlantic will remain nestled well under the deep trough, yielding the continuation of cool weather.

Beyond Thursday and continuing into next weekend, guidance diverges significantly with the timing, position and strength of features. As of now, it does appear that will be some sort of 'clipper-like' system that will move into the region Thursday night into Friday. This system looks to be a rather quick mover, but temperatures will be cool thanks to the frontal passage the day prior. This could result in a period for light snow and/or light rain across the area, but ultimately comes down to timing and the track of the low. For this reason, kept NBM guidance which depicts a 30-40% chance of light rain/snow across much of the area.

Heading into next weekend, an arctic outbreak with well below normal temperatures looks to be on the horizon beginning next weekend. A few consecutive days of temperatures on the order of 10-20 degrees below normal are looking increasingly likely. The exact magnitude of the cold still is a little uncertain at the moment, but looking like roughly 4 days or so of highs near to below freezing and lows in the 10s to low 20s across the board beginning Saturday and into early the following week. Persistent west to northwest winds near 10-15 mph will result in wind chills as low as the single digits at night. Some snow is possible sometime Saturday as well, but this will depend on the amplitude and timing of the trough axis, and if any surface low pressure can consolidate.

AVIATION /19Z SUNDAY THROUGH FRIDAY/

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

Through 00Z...VFR. SCT/BKN mid and high clouds. West-southwest wind 5-10 kt. High confidence.

Tonight...VFR. FEW/SCT high clouds. LGT/VRB wind to start will become north-northwest 10-15 kt with gusts near 20 kt with cold frontal passage between 06-08Z. High confidence overall, moderate confidence in timing of the front and associated windshift.

Monday...VFR. BKN high clouds. North-northeast wind 10-15 kt with gusts to 20-25 kt, decreasing to near 10 kt by late afternoon. High confidence.

Outlook...

Monday night through Tuesday night...VFR. no significant weather. LLWS possible Tuesday night.

Wednesday through Friday...Restrictions possible (30-50% chance) in low clouds and rain/snow. LLWS possible on Wednesday.

MARINE

Small Craft Advisory will be in effect beginning at 1 AM Monday for all ocean zones as well as the Delaware Bay.

Winds generally westerly and increasing to 10-15 kt this evening, becoming northerly around 20-25 kt with gusts around 30 kt overnight and into the day Monday. Winds are expected to be slightly lower over the Delaware Bay, but still expect frequent gusts to around 25 kt. Seas increase from 2-3 feet this evening to 4-6 feet overnight and remain elevated through Monday.

Outlook...

Monday night...Advisory conditions expected to continue into the night for much of the Atlantic coastal waters. North to northeast winds diminishing to around 10-20 kts, but seas will remain near 4-6 feet.

Tuesday...No marine hazards expected.

Wednesday through Thursday...Advisory conditions likely. Winds increasing to near 20-30 kts and seas building 5-8 feet. A chance of rain.

Friday...No marine hazards expected. A chance of rain and snow.

TIDES/COASTAL FLOODING

Now several days removed from the Full Moon phase that occurred on December 4th, expect one more round of spotty minor tidal flooding with this morning's high tide cycle. This is primarily for the NJ and DE coasts and lower Delaware Bay.

No tidal flooding is expected elsewhere.

CLIMATE

Very cold Monday Night into Tuesday Morning. At this point, we aren't forecasting to break any record lows. Below are the record lows for December 9.

SITERECORD/YEAR Allentown (ABE) 7/2002 AC Airport (ACY) 6/1968 AC Marina (55N) 5/1876 Philadelphia (PHL) 6/1876 Wilmington (ILG) 13/1960 Reading (RDG) 0/1989 Trenton (TTN) 12/2002 Mount Pocono (MPO) -1/1902 Georgetown (GED) 10/1976

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 1 AM to 6 PM EST Monday for ANZ430- 431-450. Small Craft Advisory from 1 AM Monday to 1 AM EST Tuesday for ANZ451. Small Craft Advisory from 1 AM Monday to 5 AM EST Tuesday for ANZ452>455.