textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The Cold Weather Advisory has been extended through Thursday morning.

There's increasing confidence there will 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 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 remains low, the storm could bring at least portions of the area moderate to significant impacts from snow, strong winds, and coastal flooding.

DISCUSSION

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

High pressure will build across the area today. The arctic cold continues, but at least the sun will be out today. The winds will relax a bit and should be mostly around 10 mph today and then around 5 mph tonight. Overall, the net effect of the colder temps and lower winds will be wind chill tonight close to what they are this present night. Therefore, we extended the Cold Weather Advisory to cover the upcoming night. Lowest wind chills will be -15 to -18 for the Pocono Plateau, -10 to -15 for the next set of counties S/E of the Poconos and 0 to -8 (or so) the remaining areas. (If temps lower or winds increase Extreme Cold Warning may be needed).

The very cold temperatures and bitter wind chills will persist the remainder of the week and beyond. This will occur due to a deep upper level trough centered over Eastern Canada with repeated re-enforcing shots of arctic air swinging in from the north and west around this feature. For Thursday night and Friday night, expect morning low temperatures in the single digits to negative single digits with each morning actually looking slightly colder than the previous one due to these re- enforcing shots of arctic air. More importantly, there will be at least some winds through most of this period with early morning wind chills still expected to be sub-zero region wide with negative teens N/W of the I-95 corridor to even around -20 in the southern Poconos. Some moderation in temperatures may occur Sunday into early next week however temperatures still look to be well below average.

Overall, it is exceedingly rare to get this combination of length and magnitude of an arctic airmass for this area and it should be taken seriously for those who have to venture out in the cold for extended periods of time.

KEY MESSAGE 2...A rapidly deepening storm will develop off the east coast this weekend. While confidence in specific forecast details remains low, the storm could bring at least portions of the area moderate to significant impacts from 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 still hinges on the exact track the storm takes which remains uncertain at this time. The latest GFS trended west a little. However, the latest GEM trended east, like the latest EC. It is worth noting that there continues to be spread in the GFS ensemble members (the GEFS) with the ensemble mean a bit farther east than the deterministic . So this all said, it boils down to whether the storm tracks farther northward along the coast before turning east (bringing the area greater impacts) vs. being quicker to turn east meaning the track would stay farther away bringing more limited impacts. Potential impacts include not just heavy 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.

The latest NBM probabilistic data has trended downward a bit. For snow amounts greater than 2 inches (plowable), the range is from around 50 percent near the coast to 40 percent near the I-95 corridor with lower probabilities N/W of the urban corridor. For 6+ inches, these probs are around 30 percent near I-95 up to 40 percent near the coast. These probabilities seem reasonable at this point. Regardless of snow amounts, the storm will likely track close enough to bring the area increasing winds Saturday night into Sunday morning. 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 will also help cause water to pile up along the coast.

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

Overall, it will continue to bear watching to see how things shake out over the next couple days and how guidance handles the features in the wake of our recent storm. Forecast details should come into clearer focus as we go through the week.

AVIATION /07Z WEDNESDAY THROUGH SUNDAY/

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

Today... VFR. Few clouds this morning the some low clouds (CIGS 060- 070) after 19Z/20Z N/W and 20Z/21Z S/E. West to Northwest winds up to around 10-12 knots. High confid.

Tonight... VFR. Evening low clouds (CIGS 060-070) then decreasing overnight. West to Northwest winds lowering to around 5 knots. High confid.

Outlook...

Wednesday night through Friday night...Generally VFR.

Saturday through Sunday...Potential for increasing winds and sub VFR conditions arriving late Saturday continuing into Sunday depending on track of coastal storm. Highest chance of impacts KACY, lowest KRDG, intermediate at KPHL.

MARINE

The latest push of cold air across the waters continues to produce SCA winds by-in-large with a few Gale gusts earlier. The winds will subside the rest of the night as high pressure ridges into the area. The SCA for Delaware Bay is in effect until 7AM. For the ocean, conditions will remain SCA thru abour 10AM. Freezing spreay is continue to be an issue until the winds subside. Fair weather today and tonight.

Outlook...

Winds ramp up again Thursday with SCA conditions probable. The winds should come down a bit Friday into early Saturday but will still be pretty close to SCA levels.

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

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 Thursday for PAZ054-055- 060>062-070-071-101>106. NJ...Cold Weather Advisory until 10 AM EST Thursday for NJZ001- 007>010-012>027. DE...Cold Weather Advisory until 10 AM EST Thursday for DEZ001>004. MD...Cold Weather Advisory until 10 AM EST Thursday for MDZ012-015- 019-020. MARINE...Small Craft Advisory until 7 AM EST this morning for ANZ430- 431. Freezing Spray Advisory until 7 AM EST this morning for ANZ430- 431. Small Craft Advisory until 10 AM EST this morning for ANZ450>455. Freezing Spray Advisory until 10 AM EST this morning for ANZ450>455.