textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No changes to the Cold Weather Advisory through Wednesday 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.

Low pressure north of the Great Lakes in central Canada tracks east, and this will drag a strong cold front through the region late this afternoon and early this evening. An upper trough with strong shortwave energy is also associated with this system, and will cross the region by this evening as well.

These shortwaves may touch off some snow showers across the southern Poconos and far northwest New Jersey, so will go ahead and add slight chance PoPs to the forecast to cover these snow showers. Some flurries are also possible through the Lehigh Valley and down to the I-95 corridor, so will add those to the weather grids as well.

Strong cold air advection will be underway tonight as winds turn northwest at 10 to 15 mph with gusts up to 20 mph. Though skies clear out, winds may be a bit too elevated for optimal radiational cooling conditions despite the widespread snowpack. Lows tonight will mostly be in the single digits to as low as 5 below in the southern Poconos, though if winds do diminish in some localized spots, then temperatures could drop much lower than that. Minimum wind chills will range from zero to as low as 10 below for most of the area and from 10 below to 15 below for the southern Poconos, northern New Jersey, and the Lehigh Valley. No changes to the Cold Weather Advisory that is in effect until 10 am Wednesday.

Sunny and cold on Wednesday with highs in the teens to low 20s.

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 Wednesday night through Saturday, 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 some and is overall not too different than the GEM. The latest EC is still farther east than these models. It is also 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 for snow amounts greater than 2 inches (plowable) ranges from around 60 percent near the coast to 50 percent near the I-95 corridor with lower probabilities N/W of the urban corridor. For 6+ inches, these probs are around 40 percent near I-95 up to 50 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 /19Z TUESDAY THROUGH SUNDAY/

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

Rest of today...VFR. BKN-OVC CIGs 040-080 from around 21Z to 00Z. W-SW winds 10 to 15 kt with gusts up to 20 kt. Some flurries or brief SHSN possible, mainly at KRDG/KABE, with no expected restrictions. Moderate confidence.

Tonight...VFR. Becoming SKC by 06Z. W winds around 10 kt. High confidence.

Wednesday...VFR. W-NW winds around 10 kt with gusts up to 20 kt, diminishing to 5 to 10 kt after 18Z. High confidence.

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.

MARINE

While there may be a lull in Small Craft Advisory conditions this afternoon, winds will ramp back up into this evening, so will make no changes to the SCA for the ocean. Southwest winds 15 to 20 kt with 25 to 30 kt gusts will turn northwest behind a cold front this evening, then those 25 to 30 kt gusts continue through late Wednesday morning. SCA for the ocean remains in effect until 10 am Wednesday and will hoist a new SCA for Delaware Bay from midnight to 7 am.

The Freezing Spray Advisory for the ocean that was set to expire at 1 pm has been extended until 10 am Wednesday. A new Freezing Spray Advisory is now in effect for Delaware Bay from midnight to 7 am Wednesday.

Outlook...

Winds ramp up again Wednesday night into 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 Wednesday for PAZ054-055- 060>062-070-071-101>106. NJ...Cold Weather Advisory until 10 AM EST Wednesday for NJZ001- 007>010-012>027. DE...Cold Weather Advisory until 10 AM EST Wednesday for DEZ001>004. MD...Cold Weather Advisory until 10 AM EST Wednesday for MDZ012-015- 019-020. MARINE...Small Craft Advisory from midnight tonight to 7 AM EST Wednesday for ANZ430-431. Freezing Spray Advisory from midnight tonight to 7 AM EST Wednesday for ANZ430-431. Small Craft Advisory until 10 AM EST Wednesday for ANZ450>455. Freezing Spray Advisory until 10 AM EST Wednesday for ANZ450>455.