textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The Cold Weather Advisory has been extended through Friday morning.

Regarding the weekend storm potential, there have continued be some fluctuations back and forth in forecast guidance regarding the storm track but generally speaking no major changes in overall forecast thinking. There will very likely be at least some impacts across portions of the area this weekend from a potentially major coastal storm. Impacts could be significant, especially near the coast.

KEY MESSAGES

1. A prolonged stretch of dangerous (and potentially record- setting) cold settles over the region thru the remainder of this week into the weekend, with daytime highs well below freezing and overnight lows in the single digits, resulting in below zero wind chills.

2. A rapidly deepening storm is expected to develop off the East Coast this weekend. While confidence in specific forecast details still remains rather low, the storm could bring impacts to mostly coastal areas and southern Delaware. These impacts include some accumulations of snow, strong winds and coastal flooding.

DISCUSSION

KEY MESSAGE 1....A prolonged stretch of dangerous (and potentially record- setting) cold settles over the region thru the remainder of this week into the weekend, with daytime highs well below freezing and overnight lows in the single digits, resulting in below zero wind chills.

Early this morning, temps continue to run a little above NBM guidance, but not quite as atrociously as they did last night. Overall, cold weather advisory working out. Given uncertainty about just how well guidance performs for tonight, decided to just extend the advisory as opposed to upgrading to an extreme cold warning, at least for now. If later shifts have more confidence, it may yet be upgraded.

The main change for later today through Friday is that we'll see another reinforcing shot of cold air move down from the northwest. This one looks like the strongest, with temps likely to get closest to zero or sub-zero across much of the region tonight and Friday night, with highs on Friday likely staying in the teens for most of the region. Winds may remain a factor overnight as far as how cold it gets, with somewhat stronger winds tonight possibly limiting radiational cooling again, while Friday night looks a little calmer and thus, more radiative. However, while tonight skies look relatively clear, high clouds may overspread the region by Friday night, limiting raditional potential. In any case, its cold, so advisory will at the least be extended probably through Friday night. With the coastal storm starting to influence the region thereafter, temps likely begin a bit of moderation, though increased winds may require additional cold weather advisory extensions through the weekend. By Monday night, guidance is more firmly in the sub-advisory camp as winds relax and air mass moderation increases, so by Monday we're probably done with cold weather advisories.

KEY MESSAGE 2: A rapidly deepening storm is expected to develop off the East Coast this weekend. While confidence in specific forecast details still remains rather low, the storm could bring impacts to mostly coastal areas and southern Delaware. These impacts include some accumulations of snow, strong winds and coastal flooding.

Forecast models continue to forecast cyclogensis occurring beginning Saturday off the southeast coast with rapidly deepening low pressure depicted to then move north and east Saturday night through Sunday. This will occur due to a potent upper level disturbance pivoting around the base of the long wave trough over the east interacting with the strong baroclinic zone along the coast.

In terms of impacts to our forecast area, this hinges on the exact track the storm takes which still remains uncertain at this time. Generally speaking, the latest 00z deterministic guidance has our forecast area near the N/W fringe of the storm's precip (snow) shield. The GFS has shifted slightly farther N/W with the storm track and precip shield compared to the 12z run. It offers light QPF for extreme E/SE NJ and south Delaware. The CMC has moved its QPF sharply S/E and now barely has any QPF for our area. The ECMWF has remained steady with some light totals for NJ/DE. It offers a few inches of snow for DE and 1 to 2 for Eastern NJ. Our present forecast will have some of this in our (ending 12Z Sunday) snow fcst. More snow could fall after 12Z Sunday. A decent amount of uncertainity remains.

Potential storm impacts include not just precipitation but also strong winds and coastal flooding as the storm will have a tight pressure gradient with a very strong wind field. Timing wise, the earliest this would arrive is late day Saturday with the brunt of the storm occuring Saturday night into Sunday if we get it. Given the very cold temperatures in place both at the surface and aloft, all snow is strongly favored in terms of precip type.

When compared to Wednesday, the latest (01Z) NBM probabilistic data has remained steady near the shore but trended down for places close to the Delaware Valley. For snow amounts greater than 2 inches (plowable), the range remains around 60-70 percent near the coast to 25-35 percent near the I-95 corridor with lower probabilities N/W of the urban corridor. For 6+ inches, these probs have dropped and are now around 20 percent near I-95 up to 35-50 percent near the coast.

Regardless of snow amounts, we have growing confidence the storm should at least track close enough to bring the area increasing winds Saturday night into Sunday. Our current forecast has N/NE winds forecast to be 15 to 25 gusting 25 to 35 mph inland Sunday with winds 25 to 35 gusting up to 50 mph along the coast. Winds could even end up a bit stronger than this if a more N/W track with the storm occurs. This could lead to some damage and power outages and may also help cause water to really pile up along the coast leading to the coastal flooding concerns if winds end more towards the NE vs. the north.

By Sunday evening, the storm should be rapidly pulling away to the northeast with generally tranquil but cold weather for early next week.

Overall this remains a very challenging forecast. While many of the latest operational models seem to be pointing to a near miss for the region, we can't ignore the possibility that a small change or track could bring larger impacts to the area. We continue to advise remaining preparaed and paying attention to future forecasts.

AVIATION /07Z THURSDAY THROUGH MONDAY/

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

Today...VFR. Northwest wind increasing to 10-15 kt with gusts 20-25 kt after 15Z. High confidence.

Tonight...VFR. Winds remaining northwest but diminishing to 5-10 kts. High confidence.

Outlook...

Friday thru Sat morning: VFR expected.

Sat afternoon thru Sunday: VFR for KRDG/KABE. Lower conditions possible for the Delaware Valley sites with snow possible. Low CIGS/VSBYS with some snow KACY/KMIV. Confid Low for SE areas and medium/high for NW sites.

Sunday night thru Tuesday: VFR expected. Windy Sun night.

MARINE

Small Craft Advisory and Freezing Spray Advisory have been expanded to all marine zones and extended thru tonight. Conditions will be admittedly marginal, with west to northwest winds gusting right around the 25 kt threshold, but with the exacerbating factor of freezing spray, it seems reasonable to hoist both headlines for the near-term duration. Conditions may relax for Friday so held off on further extension at this time.

Outlook...

Late Saturday through Sunday...Winds ramp up due to coastal storm with Gale force winds probable and even the potential for storm force winds, especially over our ocean zones. Seas could exceed 10 feet Sunday.

Sunday night through Monday...Winds/seas will trend down through this period but there could still be Gale force winds into early Monday with conditions likely to still be at Small Craft Advisory levels even late day Monday.

CLIMATE

Potentially record-breaking cold is expected later this week with a chance for some of our climate sites to set record low temperatures AND record low maximum temperatures.

Record Low Temperatures and Record Low High Temperatures for:

January 29

Site Low/Year Low Max Temperature/Year ABE -9/1963 12/1977 ACY 0/1966 18/1977 PHL -5/1963 18/1977 ILG 0/1961 18/1966 RDG -4/1987 18/1902 TTN 0/1873 17/1977 MPO -21/1988 7/1966 GED -6/1966 18/1966 55N 7/1966 22/2014

January 30

Site Low/Year Low Max Temperature/Year ABE -5/1928 14/1934 ACY -3/2014 19/1965 PHL 7/2019 18/1965 ILG 3/2014 16/1934 RDG -1/2014 9/2013 TTN 4/2014 12/1873 MPO -15/1965 7/1934 GED -5/2014 24/2010 55N 8/1935 18/1934

January 31

Site Low/Year Low Max Temperature/Year ABE -9/1948 14/2019 ACY 0/1948 20/2019 PHL 3/1948 18/2019 ILG 3/1948 19/2019 RDG -5/2019 14/2019 TTN -1/1920 16/2019 MPO -15/1908 4/2019 GED 1/1948 23/2019 55N 4/1948 19/2019

Not all records will be challenged or broken, but some could fall over the coming days and nights.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Cold Weather Advisory until 10 AM EST Friday for PAZ054-055- 060>062-070-071-101>106. NJ...Cold Weather Advisory until 10 AM EST Friday for NJZ001- 007>010-012>027. DE...Cold Weather Advisory until 10 AM EST Friday for DEZ001>004. MD...Cold Weather Advisory until 10 AM EST Friday for MDZ012-015- 019-020. MARINE...Small Craft Advisory until 6 AM EST Friday for ANZ430-431- 450>455. Freezing Spray Advisory until 6 AM EST Friday for ANZ430-431- 450>455.