textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No changes to the Cold Weather Advisory headlines for this morning. Confidence continues to increase in the area being affected by an impactful winter storm this weekend.

KEY MESSAGES

1. Black ice is possible on untreated road surfaces this morning.

2. A prolonged period of below normal temperatures are expected this week into this coming weekend, with single digit and below zero wind chills expected for this morning and again for Tuesday night into Wednesday morning.

3. There is an increasing potential for the area to be affected by an impactful winter storm late Saturday through Sunday, however quite a bit of uncertainty in the forecast details still remains.

DISCUSSION

KEY MESSAGE 1...Black ice is possible on untreated road surfaces this morning.

Temperatures early this morning will fall into the single digits and teens. Any snowmelt from yesterday afternoon may refreeze on untreated road surfaces, resulting in the potential for black ice. Use caution if traveling and allow extra time to reach your destination.

KEY MESSAGE 2...A prolonged period of below normal temperatures are expected this week into this coming weekend, with single digit and below zero wind chills expected for this morning and again for Tuesday night into Wednesday morning.

A deep trough is setting up over the eastern US, ushering in an arctic airmass. The height of the cold airmass will be over our area through Tuesday and Tuesday night, with 850 mb temperatures as low as -15 to -20 C, resulting in low temperatures at the surface into the teens and single digits. Combined with breezy winds, this will result in wind chills early this morning around -10 in the southern Poconos, near or below zero northwest of the I-95 corridor and into central New Jersey, and in the single digits over the rest of the area. As a result, a Cold Weather Advisory remains in effect overnight until 10 AM Tuesday and includes most areas near and northwest of the I-95 corridor as well as much of central New Jersey. Wearing appropriate layers of clothing is essential, particularly if needing to spend any periods of time outside while traveling.

High pressure will center itself overhead Tuesday night, allowing winds to be light under clear skies. We should be able to see decent radiational cooling as a result, leading to more widespread overnight lows in the single digits compared to this morning. The winds won't be as strong as early this morning but we could still see wind chills getting down to near or below zero north and west of the I-95 corridor which may necessitate more cold weather headlines. Bottom line, it will be bitterly cold.

We briefly break the cold snap on Wednesday and Thursday as temperatures get back toward more seasonable levels with highs generally in the 30s and 40s, respectively. However, another arctic front moving through late week will bring a return to colder conditions by Friday. Highs will return to the 20s and 30s for most areas with lows in the single digits and teens by Friday night. Yet another round of cold weather headlines may be warranted in the coming days. The very cold conditions are expected to continue next weekend with highs generally in the teens and 20s and lows generally in the single digits and teens.

KEY MESSAGE 3...There is an increasing potential for the area to be affected by an impactful winter storm late Saturday through Sunday, however quite a bit of uncertainty in the forecast details still remains.

Signals in the upper level pattern by late week and the weekend becomes quite active between the northern and southern stream jets. The key elements embedded in these features are currently located over the Arctic and the Pacific Ocean, so it'll likely take at least a couple more days for guidance to have a good handle on this system. However, almost all available guidance depicts an area of low pressure developing near Texas Friday before tracking eastward along the Gulf coast into Saturday. Beyond Saturday there still remains forecast uncertainty regarding the details of how this system will play out for the east coast. However, at this time most forecast guidance tracks the low north and east towards the North Carolina coast and then off to the north and east from here by the Sunday into Monday time frame. This track puts the mid Atlantic region on the northern side of the system with the main question then becoming just how close to us does the low track. There will be a very strong and broad arctic high to our north and some of our prior guidance had suggested this may be strong enough to suppress the system just south of the area. But over the past 12 to 24 hours there's been a general trend northward in guidance suggesting the precipitation shield will make it into our area. In fact, snow probabilities of the NBM now show a 60-70% chance of 2+ inches of snow and a 40-60% chance of 6+ inches of snow for areas roughly along and south of the I-95 corridor with decreasing probabilities farther north. These probabilities are higher than with the prior forecast cycle. So while this system still is about 5 days out, the signal for a winter storm to impact at least parts of the area is higher than usual. The timing of this would be as early as very late day Saturday continuing through Sunday and potentially even lingering into Monday depending on the system's speed. We will stress though that even with this trend in the guidance, most east coast snowstorms typically drop their heaviest snows in swaths 50 to 100 miles wide or less. And considering that the pieces of energy that will drive this storm are still thousands of miles away, that means it's much too early to have much confidence on how much snow will fall at any given location. Users should keep a close eye on the forecast over the coming days.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

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

Today...VFR. West- northwest wind 10-15 kt with gusts around 20 kt, gradually becoming more westerly and decreasing to 5-10 kt after 21-22Z. High confidence.

Tonight...VFR. West wind generally around 5 kt in the evening becoming light after 06Z. Calm periods possible. High confidence.

Outlook...

Wednesday through Friday night...VFR. No significant weather.

Saturday...Primarily VFR. Sub-VFR conditions possible with a slight chance (20-30%) of snow.

MARINE

The Small Craft Advisory remains in effect through Tuesday for all marine zones. Generally expect NW winds around 15 to 20 gusting 25 to 30 knots. Also the wind combined with the very cold air temperatures and water temperatures below 45 degrees will result in freezing spray being an issue through Tuesday.

Winds and seas are expected to be below Small Craft Advisory levels Tuesday night.

Outlook...

Wednesday through Thursday...SCA conditions probable for Wednesday late day into into Wednesday night with gusts of 25-30 kt and seas up to 5-6 feet. Conditions slowly diminish on Thursday.

Thursday night through Friday...No marine headlines expected.

Friday night through Saturday...Marine headlines likely. SCA conditions probable at this time.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Cold Weather Advisory until 10 AM EST this morning for PAZ054- 055-060>062-101>106. NJ...Cold Weather Advisory until 10 AM EST this morning for NJZ001- 007>010-012-013-015. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ430- 431-450>455.