textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A Cold Weather Advisory was expanded to include Sussex and Morris counties in New Jersey. Widespread wind chills between 0 and -15 degrees are expected tonight into Tuesday morning near and northwest of the I-95 corridor. Also, there is increasing potential for an impactful winter storm this weekend.

KEY MESSAGES

1. Black ice possible on untreated road surfaces tonight and early Tuesday morning.

2. A prolonged period of below normal temperatures are expected this week, with single digit and below zero wind chills possible both tonight and Tuesday night.

3. There is signal for an impactful winter storm to impact portions of the area this weekend, however quite a bit of uncertainty remains.

DISCUSSION

KEY MESSAGE 1...Black ice possible on untreated road surfaces tonight and early Tuesday morning.

Temperatures tonight and early Tuesday morning will fall into the single digits and teens. Any snowmelt from this 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, with single digit and below zero wind chills possible both tonight and Tuesday night.

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 tonight through Tuesday night, where 850 mb temperatures will be around -15 to -20 C, resulting in low temperatures at the surface into the teens and single digits. Combined with breezy winds expected tonight, this will result in wind chills early Tuesday morning between -10 and -15 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, the Cold Weather Advisory for Monday night into Tuesday morning was expanded to include Sussex and Warren counties in New Jersey. This includes most areas near and northwest of the I-95 corridor as well as much of central New Jersey. Wearing appropriate layers of clothing will be 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. Should be able to see decent radiational cooling as a result, potentially leading to more widespread overnight lows in the single digits than Monday night. While temperatures may be colder, the calmer winds will make the wind chills less severe, though still in the single digits for most and below zero in the southern Poconos. As of now, these values do not warrant any cold weather headlines, but localized areas may reach criteria. It will be bitterly cold nonetheless.

We briefly break the cold snap on Wednesday and Thursday as temperatures get back toward more seasonable levels with highs 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. Another round of cold weather headlines may be warranted in the coming days.

KEY MESSAGE 3...There is signal for an impactful winter storm to impact portions of the area this weekend, however quite a bit of uncertainty remains.

Signals of 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 a few more days for guidance to become better aligned. In the meantime, almost all available guidance depicts an area of low pressure developing over the Southern Plains on Friday before tracking eastward into the Deep South on Saturday. Beyond Saturday, its uncertain where the track of the low may go. The main player here is the Arctic high to the north and how strong the high becomes. Some guidance depicts a very strong high near 1044mb, which results in a suppressed storm track keeping any significant precipitation to the south. Whereas, a weaker high would result in a more northerly storm track and potentially bringing significant accumulation to the area.

Considering that the system is 5-6 days out, the signal for a winter storm to impact at least portions of the area do appear to be stronger than usual. In fact, snow probabilities of the NBM show a 40-60% chance of 2+ inches of snow and a 20-40% chance of 6+ inches of snow for areas south of Philadelphia. Probabilities do lessen further north. Even the Probabilistic Winter Storm Severity Index (WSSI-P), depicts a 40- 60% chance of moderate level winter storm impacts to the south of Philadelphia. Obviously a lot can change between now and the weekend, but users should keep a close eye on the forecast over the coming days.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

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

Tonight...VFR. A dry cold front will bring an increase in winds for the early overnight then winds will become less gusty after midnight. High/medium confidence.

Tuesday...VFR. West/northwest wind gusts 15-25 kt.

Outlook...

Tuesday night through Friday...VFR. No significant weather expected.

Friday night through Saturday...Sub-VFR conditions possible, especially on Saturday. A chance of light snow on Saturday.

MARINE

The Small Craft Advisory remains in effect through Tuesday for all marine zones. WSW winds of 15-20 kts with gusts up to 25 kts turn WNW overnight and may gust up to 30 kts early Tuesday morning. Seas 4-6 feet throughout the period.

Outlook...

Tuesday night through Wednesday...No marine headlines expected although conditions will be marginal late Wednesday afternoon.

Wednesday night through Thursday...SCA conditions probable for Wednesday night with gusts up to 25 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 from 2 AM to 10 AM EST Tuesday for PAZ054-055-060>062-101>106. NJ...Cold Weather Advisory from 2 AM to 10 AM EST Tuesday for NJZ001-007>010-012-013-015. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST Tuesday for ANZ430-431- 450>455.