textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Confidence continues to increase with the area being affected by an impactful winter storm this weekend.

KEY MESSAGES

1. Bitterly cold conditions continue into Wednesday.

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

DISCUSSION

KEY MESSAGE 1...Bitterly cold conditions continue into Wednesday.

As an upper-level trough relaxes across the Northeast through tonight, the center of arctic high pressure builds right over our area. As this occurs, the gusty breeze this afternoon will diminish this evening with many locations having the wind decouple overnight. This along with a mainly clear sky and a very dry air arctic mass will result in an excellent setup for radiational cooling. This cooling will also be enhanced some for areas where snow cover remains. As a result, went a few degrees lower than NBM for tonight which may not be cold enough for some areas. This results in low temperatures overnight between 0F and +10F across the area. The wind chill will become less of a factor as the wind itself diminishes, and therefore no Cold Weather Advisories are anticipated for tonight. The bitter cold continues to start Wednesday morning, however southerly flow and warm air advection increases during the day on the backside of departing high pressure. It will therefore not be as harsh Wednesday afternoon compared to today, with high temperatures getting into the 30s to even some low 40s across the coastal plain.

KEY MESSAGE 2...There is an increasing potential for the area to be affected by an impactful winter storm late Saturday through Sunday night, however quite a bit of uncertainty 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 still located way out over the Arctic and the Pacific Ocean respectively, so it'll likely take at least a couple more days for guidance to have a better handle and depiction on this system. However, almost all available guidance continues to place an area of low pressure developing near Texas on Friday, tracking eastward along the Gulf Coast on Saturday and then near the North Carolina coast on Sunday. Beyond this point though, the spread amongst forecast guidance varies. So simply put, there remains a bit of uncertainty regarding the details of how this system will pan out for the East Coast (and Mid-Atlantic). The main question...is just how close does the low track to our region? This comes down to a strong and broad arctic high that is projected to be located to our north. As much of the guidance shifted northward last night, some of the latest suite of guidance now shows a slightly stronger high and a bit more confluence. This would keep the storm track a bit more suppressed and result in a sharp northern edge to the precipitation shield. Still to early for the little details, but suppose this is just typical guidance waffling back and forth, so without a definitive northern or southern trend, the forecast track largely remains unchanged at this point in time.

In fact, snow probabilities of the NBM have actually gone up quite a bit, now showing a 70-90% chance of 2+ inches of snow and a 60-80% chance of 6+ inches of snow for areas south of Philadelphia, with probabilities gradually falling off further north. Likewise, the probabilistic Winter Storm Severity Index (WSSI-P) has increased further with now almost our entire area expected to observe at least Moderate Level winter storm impacts. As noted with the previous shift, these probabilities are higher than the prior forecast cycle.

So while this system still around 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. Considering that the pieces of energy that will drive this storm are still thousands of miles away, that means it's still way too early to have much confidence on how much snow will fall at any given location. Users are recommended to keep a close eye on the forecast over the coming days.

AVIATION /19Z TUESDAY THROUGH SUNDAY/

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

Rest of this afternoon...VFR. West-northwest winds 10-15 knots with gusts 20-25 knots, diminishing to 5-10 knots after 21z. High confidence.

Tonight...VFR. Westerly winds 5-10 knots, becoming light and variable to calm. High confidence.

Wednesday...VFR. Light and variable to calm winds becoming southerly 4-8 knots during the morning, then 10-15 knots with some gusts to 20 knots in the afternoon. Moderate confidence.

Outlook...

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

Saturday through Sunday...VFR conditions early on Saturday, likely to become sub-VFR by Saturday night and into Sunday. Periods of snow likely.

MARINE

Small Craft Advisory conditions continue into this evening, then winds will diminish as high pressure builds across our area during tonight. The stronger winds look to linger longer across the northern to central New Jersey Atlantic coastal waters therefore the advisory has been extended until 10 PM tonight. Southerly flow then increases during Wednesday, especially in the afternoon and another round of Small Craft Advisory conditions may therefore develop. Any light freezing spray will end into this evening as the winds diminish.

Outlook...

Wednesday night through Thursday...SCA conditions likely with wind gusts up to 30 kt and seas of 4-7 feet. Conditions slowly diminish on Thursday.

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

Friday night through Sunday...Marine headlines likely. SCA conditions probable, possible gales at times.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ430- 431-453>455. Small Craft Advisory until 10 PM EST this evening for ANZ450>452.