textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Confidence is increasing in an arctic airmass bringing dangerous cold on Friday Night with wind chills below zero.

Confidence continues to increase in significant impacts from a winter storm Saturday night through early Monday across the entire area. As such, a Winter Storm Watch has been issued for the entire region Saturday night through early Monday afternoon.

KEY MESSAGES

1. Prior to the weekend's winter storm, an arctic airmass potentially brings wind chills below 0 to nearly the entire region, possibly getting as low as -20 in the Poconos on Friday Night. Cold weather headlines are likely (60-70%).

2. A significant winter storm appears increasingly likely Saturday night through Monday morning. Substantial snow accumulations will be possible across much of the area. Some mixed precipitation is expected as well.

3. Bitter cold is expected to continue after the winter storm has passed, with temperatures unlikely to rise above freezing through Wednesday.

DISCUSSION

KEY MESSAGE 1. Prior to the weekend's winter storm, an arctic airmass potentially brings wind chills below 0 to nearly the entire region, possibly getting as low as -20 in the Poconos on Friday Night. Cold weather headlines are likely (60-70%). Following a brief warmup both Thursday and Friday, an arctic front will usher in a very cold airmass. The front comes through during the day on Friday, with strong cold-air advection setting up in its wake. 850 mb temperatures drop to -15 to -25 C on Friday Night. The result will be temperatures dropping from the mid to upper 30s Friday afternoon down into the single digits (or even below zero in the Poconos).

In addition to temperatures tanking, winds will pick up in the wake of the front, driven by a tight pressure gradient that sets up with a very strong incoming arctic high diving in from Canada into the northern Plains and low pressure departing. Post- frontal gusts will be around 20-25 MPH, gusting up to 30-35 MPH or so. The combination of single digit temperatures and wind gusts in the 20s brings wind chills below zero for most of the region. With temperatures colder and winds a bit stronger in the southern Poconos, wind chills could get as low as -20, which is guidance for an Extreme Cold Warning. Still too early to issue any headlines, but would put the probability of needing cold weather headlines around 60-70% or so.

Not much warming comes on Saturday as the arctic airmass settles over the northeastern US. Highs only get into the upper teens/low 20s regionwide, making it quite uncomfortable outside for any winter storm preparations. Layer up both Friday Night and Saturday ahead of the storm if out and about!

KEY MESSAGE 2...A significant winter storm appears increasingly likely Saturday night through Monday morning. Substantial snow accumulations will be possible across much of the area. Some mixed precipitation is expected as well.

A closed mid-level low off the southwestern coast of CA will gradually open and pivot northeastward, eventually phasing with the northern jet as it approaches our region. This will place our entire region in the left exit region of a potent trough.

Synoptic scale ascent is expected to begin overspreading the region by Saturday evening. As surface low pressure begins to approach from the southwest, precipitation is expected to begin overspreading the region from southwest to northeast Saturday night. Given the very cold airmass in place initially, precipitation is expected to start as all snow. As the vertical column moistens, snow rates may quickly become heavy, with rapid accumulations possible.

During the day on Sunday, additional surface cyclogenesis is expected off the Mid-Atlantic coast, with the surface low tracking northeastward. The primary remaining uncertainty with this forecast is the exact low track. In general, guidance continues to suggest a low track fairly to the coast, with the GFS as a relative outlier a little bit farther out to sea. Given this low track, it is becoming increasingly likely that sleet, freezing rain, and even plain rain will enter the picture for parts of the area during the day on Sunday. Right now, this appears most likely from about the Philadelphia metro and south, and perhaps farther north for the Coastal Plain. While this could decrease snow totals some, exactly where mixing occurs remains uncertain, and confidence remains high in significant snow accumulations. In addition, mixed precipitation would still cause significant issues. For this region, DESI probabilities of exceedance for warning criteria snow (5 inches) are over 90%. Depending on how soon, and to what extent, mixing occurs, amounts could exceed 10 inches for this area. Outside of the aforementioned areas, all snow is expected, with significant accumulations likely. DESI probabilities of exceedance for warning level snow (6 inches) across the remainder of our eastern PA and northern NJ counties are over 90%. In fact, probabilities of exceeding 10 inches are 70-90%.

While this event remains a few days out, confidence is higher than usual for this range in significant impacts across the region. Specifics will continue to be refined as the event approaches, but confidence is high enough in warning level snow to issue a Winter Storm Watch for the entire area, starting at 7 PM Saturday and continuing through early Monday afternoon. Stay tuned for future updates as this has the potential to be a very impactful storm for much of the area.

KEY MESSAGE 3...Bitter cold is expected to continue after the winter storm has passed, with temperatures unlikely to rise above freezing through Wednesday.

After the low pressure system departs on Monday, high pressure will build into the region. Initially, a fairly tight pressure gradient is expected with strong cold air advection and breezy conditions Monday afternoon into Tuesday. Temperatures are expected to remain below freezing across the entire area through Wednesday. In fact, many locations may only rise into the low- mid 20s. Monday night and Tuesday night are of particular concern, with low temperatures in the single digits areawide. Given the expected breezy conditions, wind chills could be below zero across the entire area during the nighttime hours, and only rise into the low-mid teens during the day. This type of prolonged cold could be quite dangerous, particularly if spending extended periods of time outdoors without proper clothing. One final note is that with significant snow cover potentially in place, winds could cause some blowing and drifting of the snow, and temperatures could potentially be even colder than currently forecasted. Trends will be monitored in the coming days.

AVIATION /15Z THURSDAY THROUGH MONDAY/

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

Today...VFR. Cloud cover clears out in the morning with scattered afternoon clouds. Winds generally out of the west/southwest around 5-10 kt to start, increasing to around 10-15 kt with gusts 20-30 kt or so out of the west/northwest. Gusts diminishing quickly between 21z-23z. High confidence in prevailing VFR, moderate confidence in timing and magnitude of wind shifts and increases.

Tonight...VFR. Westerly winds 5 to 10 kt. High confidence.

Outlook...

Friday through Friday Night...VFR. Wind gusts 20-30 kt out of the northwest expected.

Saturday...VFR. No significant weather.

Saturday night...VFR early, with sub-VFR conditions becoming increasingly likely with time as snow moves into the area from southwest to northeast.

Sunday through Sunday night...Sub-VFR conditions expected in periods of snow. Some mixed precipitation will also be possible at KMIV and KACY.

Monday...Sub-VFR conditions expected in periods of snow early, with conditions improving late.

MARINE

As of 11 AM, all remaining Small Craft Advisories have been cancelled. No other marine headlines are currently in effect.

Sub-SCA conditions expected from this afternoon through Friday morning. Winds out of the west around 10-20 kt and seas 2 to 4 feet.

Outlook...

Friday afternoon through Saturday...SCA conditions expected (90-100%) as wind gusts out of the northwest get to 25 to 30 kt. Seas 3 to 5 feet. Will wait for current SCA to expire before issuing a new one.

Saturday night...SCA conditions likely to develop late with northeast wind 20-25 kt and gusts to near 30 kt. Seas 3-6 feet.

Sunday through Monday...SCA conditions likely, with periods of gales possible. Sunday into Sunday night, northeast wind 20-30 kt with gusts as high as 35 kt and seas increasing to 5-9 feet. Monday, wind shifts to northwesterly, remaining 20-30 kt with gusts as high as 35 kt and seas 5-9 feet. Visibility restrictions are likely in heavy precipitation Sunday, with gradual improvement on Monday.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Storm Watch from Saturday evening through Monday afternoon for PAZ070-071-101-102-104. Winter Storm Watch from late Saturday night through Monday afternoon for PAZ054-055-060>062-103-105-106. NJ...Winter Storm Watch from Saturday evening through Monday afternoon for NJZ016>019-021>025-027. Winter Storm Watch from late Saturday night through Monday afternoon for NJZ001-007>010-012>015-020-026. DE...Winter Storm Watch from Saturday evening through Monday afternoon for DEZ001>004. MD...Winter Storm Watch from Saturday evening through Monday afternoon for MDZ012-015-019-020. MARINE...None.