textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No changes to the Cold Weather Advisory and associated forecast for very cold temperatures to dominate for the next week. There remain concerns that a coastal storm may impact the area next weekend.

KEY MESSAGES

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

2. We are monitoring the potential for a coastal storm next weekend. While forecast confidence regarding specific impacts remains low, there are concerns the storm may track close enough to the region to bring at least some impacts to portions of our forecast area.

DISCUSSION

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

An upper-level trough axis will pivot across our area this evening and this will increase the cold air advection. For tonight, temperatures drop into the single digits to low teens by daybreak with the passage of a sharp upper-level trough axis and the aforementioned uptick in cold air advection. Strong high pressure centered in the Ohio Valley and departing low pressure will keep a tight pressure gradient in place, therefore a gusty west-northwest breeze will continue through at least the first half of the night. The winds should then diminish some overnight. This will drive wind chill values down below zero for nearly the entire area, getting as low as -15F in the southern Poconos. Given such low wind chills, a Cold Weather Advisory remains in effect for the entire region starting this evening. High temperatures on Tuesday only get into the teens to low 20s with bitterly cold wind chills, especially during the morning hours, and there should be a little stratocumulus cloud development.

A reinforcing shot of cold air comes on Tuesday Night as a cold front sweeps through. It will be moisture starved though and not expecting any precipitation. The bigger impact will be temperatures taking even more of a tumble. While the Cold Weather Advisory expires Wednesday morning, some sort of headline will be needed Wednesday Night through Friday Night. The question remains whether it would be a Cold Weather Advisory or Extreme Cold Warning. The current forecast has daytime highs only in the mid to upper teens each day with temperatures possibly getting above 20 (but not much higher) south and east of I-95. Overnight lows on Wednesday Night and especially Thursday Night will either be low single digits or below zero region wide. Factor in an elevated wind with a modest pressure gradient overhead, and wind chills end up well below zero for most of the area Wednesday night through Friday night to as low as the negative teens and 20s near and especially NW of the I-95 corridor (below -20 values over the Poconos). This will be bone chilling cold that will have impacts to energy infrastructure and could result in frostbite and hypothermia within minutes if not layered up.

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...We are monitoring the potential for a coastal storm next weekend. While forecast confidence regarding specific impacts remains low, there are concerns the storm may track close enough to the region to bring at least some impacts to portions of our forecast area.

Deterministic and ensemble guidance continue to show the potential of some sort of system developing off the Mid-Atlantic coast next weekend. The latest 12z forecast guidance is still is all over the place but there is fair agreement in some sort of storm developing. The 0z ECMWF had been very bullish with this storm giving the area a direct hit but the latest run has backed off. In fact latest EC is pretty similar to GFS while 12z GEM is very bullish. It should be noted there's also alot of spread in the GFS ensemble members (GEFS).

In terms of probabilities and potential impacts....the latest NBM has 24 hour probabilities of snowfall > 4 inches around 20 to 25 percent across most of the area. In terms of timing, if this storm affects us it could arrive as early as late day Saturday and then last through Sunday. It should also be noted that this storm may have significant winds with it such that even if the center of the storm tracks well offshore, coastal areas could still have strong winds.

Overall, it bears watching to see how things shake out over the next few days and how guidance handles the features in the wake of yesterday's storm. What there is high confidence in is that there will be a cold airmass in place, sufficient for snow at least at the storm's onset if we get it. It will generally stay very cold through next weekend into at least the beginning of next week.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

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

Tonight...VFR. West-northwest winds around 15 knots with gusts to 25 knots this evening, then diminishing to 5-10 knots overnight. High confidence.

Tuesday...VFR. West to southwest winds 10-15 knots. Some gusts up to 20 knots possible. Moderate confidence.

Outlook...

Tuesday night through Friday...Generally VFR.

Saturday...Potential for increasing winds and sub VFR conditions arrived during the latter part of the day or at night depending on track of coastal storm.

MARINE

A Gale Warning remains in effect through tonight as colder air surges into our area with gusts 35-40 knots at times. Seas will remain elevated although gradually subside through Tuesday given the stronger offshore wind component. Freezing spray will occur as even colder air settles in combined with the gusty winds and elevated seas. This will result in light to moderate ice accretion rates at times and the Freezing Spray Advisory goes through Tuesday morning.

Outlook...

Tuesday Night...SCA conditions probable. Wind gusts around 25-30 kt and seas of 3-6 feet. Freezing Spray also expected.

Wednesday through Wednesday Night...SCA conditions early Wednesday, then no headlines anticipated for a time late day before winds back up again Wednesday night and likely reach SCA conditions once again.

Thursday through Friday Night...SCA conditions probable Thursday (80-90%) with a chance for gales (30-40%). Wind gusts up to 25-30 kt through the period with seas getting near 5 feet.

Saturday...The conditions should be sub SCA for a time during the day but then will likely ramp up again at night due to potential coastal storm.

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...Gale Warning until 6 AM EST Tuesday for ANZ430-431-450>455. Freezing Spray Advisory until 8 AM EST Tuesday for ANZ430-431. Freezing Spray Advisory until 1 PM EST Tuesday for ANZ450>455.