textproduct: Philadelphia/Mt Holly

SYNOPSIS

As high pressure remains centered to our south through tonight, an arctic front then moves through our region during Thursday. The center of arctic high pressure builds into our area later Thursday night and Friday morning before shifting offshore by later Friday. An area of low pressure tracks to our south and east later Friday into Saturday, followed by a cold front later Sunday. High pressure builds in later Monday into Tuesday, then low pressure and a cold front may arrive later Tuesday into Wednesday.

NEAR TERM /THROUGH THURSDAY/

An expansive area of high pressure is over the Eastern US, which will result in tranquil conditions through tonight. Skies will be mostly clear outside of some thin passing high clouds and perhaps some stratus over the higher elevations in the Poconos. Temperatures drop into the low to mid 20s, with the warmer urban heat island of Philly hovering around 30.

For Thursday, a cold front will pass through the region, ushering in an arctic airmass. While the frontal passage will mainly be dry for most, there may be just enough moisture, lift, and instability to result in a few snow showers along and north of I-80 during the day. While this should generally be not impactful, the snow squall parameter is not zero, so a few squalls reaching the far northern part of our area cannot be ruled out. Otherwise for most, the main impact will be increasing winds as the front passes and in its wake as strong cold-air advection ramps up. Winds increase with gusts around 25-35 MPH expected out of the northwest. Temperatures will be in the mid to upper 30s and low 40s Thursday, but it will certainly feel cooler than that with a brisk northwesterly wind.

SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY NIGHT/

Arctic air settles in for the end the week and to start the weekend.

Following the passage of an arctic front, the center of arctic high pressure will gradually build into our region Thursday night. As this occurs, a gusty northwesterly wind will diminish through Thursday evening. Due to the strong cold air advection, temperatures will drop significantly Thursday night and bottom out in the teens (some single digits in the Poconos to far northwestern New Jersey) by daybreak Friday. There will be a wind chill factor as well, however this will be mostly during the evening as the winds are expected to mostly decouple during the night. Some record low temperatures could be challenged for December 5th, especially in the Lehigh Valley and Berks County. Record low temperatures are listed below in the climate section for reference.

The center of arctic high pressure across our area to start Friday morning shifts offshore by later in the day. A shortwave trough however arrives later in the day Friday and especially Friday night. This feature does not appear to sharpen a lot as it arrives given the more zonal flow aloft ahead of it. The deterministic and ensemble guidance varies with the northward placement of the precipitation shield. A cold/dry air mass is forecast to be in place, and this may hinder the northward extent of the precipitation shield. Surface low pressure is forecast to develop and track to our south as a result of the incoming mid level energy and it looks to be a quick mover. Given the current synoptic setup, the highest probability of precipitation remains across Delmarva to southeastern New Jersey. There is the potential for light snow accumulations especially the farther south one goes across the area, with some potential for a wintry mix or a change to rain closer to some of the coastal areas. Some guidance is a bit stronger and northward with the precipitation shield, which offers more snow. If a ribbon of stronger forcing develops (frontogenesis or strengthening frontogenesis), then a more expansive precipitation shield would be plausible and perhaps a banding feature north/northwest of the surface low. This would tend to occur if the mid level wave sharpens some more. The details will be determined by the track and strength of the system which will then determine the precipitation types and snow amounts. As of now though, snow amounts are an inch or less. Rather cold Friday with highs ranging from the mid to upper 20s far north to upper 30s far south, however the wind looks to be on the light side and therefore not much of a factor.

The system quickly departs Saturday with some decrease in the cloud cover. It should not be quite as cold Saturday, however still remaining below average.

LONG TERM /SUNDAY THROUGH WEDNESDAY/

Synoptic Overview...An upper-level trough is forecast to be across much of eastern Canada Sunday and Monday. This feature looks to extend south across much of the eastern U.S. especially as stronger shortwave energy travels along the southern parts of the trough into the Southeast U.S. The trough may weaken Tuesday into Wednesday, however this will depend on if a strong shortwave dives southeastward from the Midwest Monday night and Tuesday. At the surface, a cold front arrives later Sunday with high pressure sliding across our area Monday into Tuesday. Low pressure and its cold front may then arrive later Tuesday into Wednesday.

For Sunday and Monday...An upper-level trough across much of eastern Canada amplifies southward as shortwave energy rounds its base across the southern states. As this occurs, a cold front looks to cross our area later Sunday. The moisture with this looks to be rather limited and therefore much of the area should remain dry. However, this front will deliver another shot of cold air into our area for Monday. Temperatures are forecast to remain below average, with it being much colder/below average on Monday. As the axis of high pressure moves across our area Monday night, temperatures are forecast to be very cold with lows in the teens (single digits across the far north).

For Tuesday and Wednesday...Despite that the main upper-level trough may weaken or retreat more to our north Tuesday, a potentially strong shortwave trough diving southeastward from the Midwest should cross our area Tuesday night into Wednesday. This feature may end up supporting a clipper-type system at the surface, which then quickly crosses our area Tuesday night into Wednesday. The moisture is typically limited with these features, however if the mid level wave is stronger like some guidance suggests then forcing for ascent would be stronger. There is uncertainty with this given all the moving pieces within the parent upper-level trough, therefore did not deviate from the National Blend of Models (NBM) guidance. Cold/below average temperatures continue, however as of now a little moderating of the temperatures are forecast for Wednesday.

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. Northwest winds around 5-10 kt become west/southwesterly around 21z and gradually diminishing. High confidence.

Tonight...VFR. West/southwest winds 5 kt or less.

Thursday...VFR. Winds increasing with a frontal passage between 14z- 16z. West/northwest winds 10-20 kt with gusts around 25 kt. High confidence in prevailing VFR, moderate confidence in timing of frontal passage and winds increasing.

Outlook...

Thursday night...VFR. West-northwest winds diminishing.

Friday and Saturday...A period of sub-VFR conditions possible along with some rain/snow (highest probability currently south and east of KPHL) later Friday into early Saturday.

Sunday and Monday...Mostly VFR.

MARINE

No marine headlines expected through tonight. West winds around 10- 15 kt with seas 2 to 4 feet.

A Small Craft Advisory goes into effect for all marine zones beginning at 14z. West/northwest winds around 15-25 kt with gusts around 30 kt expected. Seas 3 to 5 feet.

Outlook...

Thursday night...Small Craft Advisory conditions in the evening, with winds and seas decreasing through the overnight.

Friday through Monday...The conditions are anticipated to be below Small Craft Advisory criteria.

CLIMATE

Very cold temperatures are forecast for early Friday morning which may challenge a few record lows.

Record Low Temperatures for December 5th...

SITERECORD/YEAR Allentown (ABE)11/1926 AC Airport (ACY)11/1966 AC Marina (55N)15/1901 Philadelphia (PHL) 13/1886 Wilmington (ILG) 12/1926 Reading (RDG) 12/1926 Trenton (TTN) 10/1926 Mount Pocono (MPO) -2/1971 Georgetown (GED) 14/1966

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 9 AM to 10 PM EST Thursday for ANZ430-431. Small Craft Advisory from 9 AM Thursday to midnight EST Thursday night for ANZ450>455.