textproduct: Philadelphia/Mt Holly

SYNOPSIS

High pressure builds into the region later today before weakening and shifting to our south and east into Saturday. A weak area of low pressure tracks nearby later Saturday night and Sunday morning, with an arctic front crossing our area. The center of arctic high pressure slides to our south Tuesday before weakening as it shifts offshore Wednesday. A warm front may slide by to our north Wednesday into Thursday.

NEAR TERM /THROUGH TONIGHT/

Relatively quiet period in the near term now that lake-effect snow showers have mostly ended. The balance of today should be notably less brutal than yesterday, with winds mostly staying in the 10-15 mph range with notably lower and fewer gusts. With some sun and slightly warmer day-time highs compared to yesterday, it should feel considerably more comfortable outside.

Tonight, an approaching weak warm front will begin to spread clouds across the region. A few models suggest a little light precip may overspread the region before dawn Saturday, but right now it looks more likely that low-level dry air should prevail to make this nothing more than virga. With temps again falling well below freezing in most areas, this could end up being a little frozen or freezing precip if it was to materialize, but again, odds fairly strongly favor nothing reaching the ground. Will keep an eye on trends, however.

SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/

Temperatures below to well below average as another arctic blast arrives. A snow event is expected to occur for Saturday night and Sunday morning as the arctic blast arrives.

A robust closed low should be centered near the upper Great Lakes Saturday. This will drive surface low pressure eastward across Canada as the closed low pivots to the east. This will send a cold front into our area Saturday night. The weather conditions during the day Saturday look to be on the tranquil side, although it will continue to be chilly (below average). Clouds are expected to increase during the day, but moreso during the evening as our next system starts to arrive.

As the aformentioned closed low pivots eastward, the axis of it takes mostly more of a positive orientation. This will tend to result in a surface low staying weaker and farther south along a baroclinic zone. While there still remains some uncertainty with the track of this surface wave, the main drivers of this incoming precipitation event may be more tied to the mid to upper level dynamics. As the strong upper-level trough gets closer Saturday night and Sunday morning, a robust 250 mb jet moves across our area and about the southern half of our region gets into the right entrance region for a time. In addition, a zone of strengthening 700- 850 mb frontogenesis crosses about the southern half of our region. This may explain why some guidance quickly develops the precipitation shield over our area instead of moving one already developed into our area. An examination of some model guidance via BUFKIT, the omega fields (while not real strong) are nearly centered across the snow growth region along with high relative humidity. It appears as colder air gets drawn southeastward, snow ratios increase from northwest to southeast. All of this would point to a zone of potentially some increased snowfall intensity for a time early Sunday morning. There still remains some potential on the southern fringe of the precipitation shield that the precipitation starts as some light rain or a mix of rain/snow before changing to all snow. The model consensus puts the axis of the maximum precipitation amounts very close to the I-95 corridor. It is noted that the NBM probabilities have increased some particularly centered on the I-95 corridor. Still some wiggle room with this as some shift in the mid to upper level dynamics will pull or push the precipitation shield north or south some.

Overall though, looks like 1-3 inches of snow across nearly the entire area with an area of 4 inches possible. Total snowfall amounts should be held down some as this system is a quick mover. This is currently looking like a Winter Weather Advisory event for at least portions of our area. Still some time though and therefore after collaborating with our neighboring offices, held off on issuing any advisories at this time.

As the system pivots eastward and the surface low shifts farther offshore, arctic air will pour into our area as a west to northwest wind increases on Sunday. This will result in temperatures not going up all that much during the day Sunday (they may actually fall some during the afternoon), and with a gusty wind there will be a noticeable wind chill factor. Given the current forecast of total snowfall, the gusty winds will then result in some blowing snow. Air temperatures drop into the teens Sunday night with some single digits possible in the Poconos. A brisk wind will drive wind chills well down into the single digits to near zero through Sunday night with some below zero values mostly in the Poconos. If the wind remains strong enough combined with even colder temperatures, some areas may be very close to Cold Weather Advisory criteria later Sunday night and early Monday morning.

LONG TERM /MONDAY THROUGH THURSDAY/

Summary...Arctic air/bitter cold starts to ease during mid-week, then it turns much milder Thursday.

Synoptic Overview...An upper-level trough and its associated cyclonic flow continues across the East Monday into Tuesday, then this eases and becomes more zonal aloft Tuesday into Wednesday. The flow aloft then looks to back more Thursday ahead of a potentially strengthening upper-level trough from the Great Lakes to the lower Mississippi Valley. The center of arctic high pressure slides just to our south Tuesday before shifting offshore. A warm front may slide by to our north during Wednesday as low pressure gradually pivots across the upper Great Lakes region. The associated cold front may take some time to shift eastward Thursday and therefore be well to our west.

For Monday and Tuesday...An upper-level trough and its associated cyclonic flow remains in place during this time frame. It could be enhanced for a brief time Monday night or Tuesday as a stronger shortwave dives southeastward out of the Great Lakes and across our area. The center of arctic high pressure builds well south of our area by later Monday, and with strong low pressure in the Canadian Maritimes, a tightened pressure gradient will remain across our area Monday. This will keep a gusty west-northwest surface wind going Monday which will add to the already bitterly cold conditions. High temperatures on Monday should remain below freezing for nearly our entire area, and with the added wind, this will continue to drive down the wind chill. While the center of the high is well to our south Tuesday before starting to shift offshore, its axis extending northward slides over our area. This will result in much less wind on Tuesday. It will still be rather cold, however the harsh conditions will start to ease through the day. As the air mass undergoes some moderation, temperatures are not expected to be quite as cold Tuesday night compared to Monday night.

For Wednesday and Thursday...A pattern change should result in a surge of much milder air across our area as we get into Thursday. As surface high pressure becomes more anchored in the western Atlantic and we lose the upper-level trough, warm air advection will be underway. This should increase more as a warm front slides by to our north during Wednesday. A potentially strong upper-level trough then amplifies from the Great Lakes to the lower Mississippi Valley Thursday as low pressure is in the Great Lakes region. The amplification of this will determine how quickly a cold front shifts eastward. Southwesterly flow well ahead of this will boost the warm air advection, with much of our area is currently forecast to get into the 50s on Thursday.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of Today...VFR with SCT/BKN high clouds. West-northwest winds around 10 kt with isolated gusts up to 20 kt. High confidence.

Tonight...VFR. Mostly clear skies early, with increasing mid- high level clouds after midnight. Light and variable winds generally 3 kt or less. High confidence.

Saturday...VFR. A few scattered low clouds in the morning, otherwise mostly cloudy. South-southwest winds around 10 kt with occasional gusts up to 15-20 kt in the afternoon. Moderate-high confidence.

Outlook...

Saturday night and Sunday...A period of MVFR/IFR conditions with snow from about late Saturday evening through mid Sunday morning. The conditions improve to VFR thereafter. West- northwest winds increase during Sunday with gusts up to 30 knots.

Monday and Tuesday...VFR. West-northwest wind gusts up to 30 knots Monday, then much lighter on Tuesday.

MARINE

As of 12:00 PM, all remaining Small Craft Advisories have been allowed to expire as winds continue to diminish below 25 kt.

Westerly winds 10-20 kt for the bulk of the region through today, diminishing tonight, with seas 2 to 4 feet.

Outlook...

Saturday...The conditions are anticipated to be below Small Craft Advisory criteria.

Sunday...Small Craft Advisory conditions developing during the morning, then Gale force wind gusts probable late Sunday afternoon and especially Sunday night. Areas of freezing spray possible Sunday night.

Monday...Gale force wind gusts possible in the morning, otherwise Small Craft Advisory conditions probable. Areas of freezing spray possible during the morning.

Tuesday...Small Craft Advisory conditions should be easing.

PHI WATCHES/WARNINGS/ADVISORIES

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