textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Sussex and Warren County in NJ and Lehigh, Northampton, and Berks County in PA were added to the Winter Weather Advisory. Snowfall totals were bumped up slightly from the I-95/NJ Turnpike corridor on north and west. A widespread 2-5" is expected in those areas with diminishing amounts near the coast.

For the cold snap, temperatures have trended downward slightly, mainly on Tuesday Night. Monday Night and Tuesday Night will be the coldest stretch. Wind chills will be more severe on Monday Night, even with lower air temperatures anticipated on Tuesday Night.

KEY MESSAGES

1. A coastal low will slide by offshore today, bringing rain and snow to the area. Snowfall accumulations around 1 to 4 inches are expected, with amounts up to 5" possible, mainly in a thin corridor in central and northern New Jersey.

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

DISCUSSION

KEY MESSAGE 1...A coastal low will slide by offshore today, bringing rain and snow to the area. Snowfall accumulations around 1 to 4 inches are expected, with amounts up to 5" possible.

Quite a complicated forecast with today's event. The main changes were expanding the Winter Weather Advisory northwestward to now include Sussex and Warren County in NJ and Berks, Lehigh, and Northampton County in PA, beginning at 4 AM this morning, continuing through 8 PM tonight. Precipitation breaks out later this morning between 4 and 6 AM from southwest to northeast as a strong upper level jet moves over the area. CAMs have been hinting at a potential midday break in the snowfall until the surface low gets closer as it moves up the coast later today. The surface low passes by this afternoon and is progged to be east of the benchmark by this evening. The closer track appears to be coming to fruition however, which will bring some higher snowfall totals compared to what was previously thought a day or two ago.

Overall, our forecast was not changed too much. Expecting a widespread 2 to 5 inches from our western border to about I-95 and the NJTP. Lesser amounts south and east of the Turnpike in NJ, and south of I-95 over Delmarva. This is due to temperatures climbing into the mid to upper 30s today with warmer air aloft, which will result in more rain than snow at least through most of today. As the surface low pivots and flow turns more north/northwesterly, cold air will filter in, allowing for a changeover to snow in these locations, producing about an inch to an inch and a half or so over a few hours. Will have to watch this area if the changeover happens sooner as it wouldn't take much more to get to Advisory criteria. Thinking the changeover happens between 3 and 6 PM.

For areas within the Winter Weather Advisory, a widespread 2 to 5" is expected. For now, Carbon and Monroe were left out of the Advisory as amounts should stay below 3". The southern Poconos will get most of their snow this morning with less this afternoon given their proximity to the coastal low. Admittedly, our forecast is on the higher side compared to individual deterministic models. With yesterday's over-performance, the thinking is that models are generally too warm and underestimating the snowpack. For example, temperatures from the NBM at the surface initialized 5-10 degrees too high and had to be manually adjusted down. This would result in a colder solution, especially for areas along and north/west of the I-95 corridor who saw measurable snowfall.

Overall, this is a lower confidence forecast and the forecast could go wrong if the low tracks further or closer and if thermal profiles are just a bit colder or warmer in either direction. At this point, we'll see how it plays out. The last of the snow should move out quickly after 7-8 PM or so with temperatures quickly dropping overnight, and beginning a period of below normal temperatures. Any untreated surfaces could see some refreezing, becoming slick.

KEY MESSAGE 2...A prolonged period of below normal temperatures are expected this upcoming week, with single digit and below zero wind chills possible both Monday Night and Tuesday Night.

A deep trough sets up over the eastern US tonight, ushering in an arctic airmass. The stretch of below normal temperatures commences tonight, with lows in the mid to upper teens for most. The height of the cold airmass will be over our area on Monday Night 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 Monday night, this will result in wind chills between -10 and -15 in the southern Poconos, below near or zero over the Lehigh Valley, and in the single digits over the rest of the area. We almost certainly need some cold weather headlines, especially in the southern Poconos, but cannot rule them out for the rest of the region.

With the arctic airmass directly overhead on Tuesday, highs won't climb out of the 20s. With a steady westerly breeze, wind chills will be in the single digits and the low teens. Areas in the southern Poconos likely won't get above 0 even during the daytime.

High pressure will center itself overhead Tuesday night, allowing winds to diminish and skies to become mostly clear. We could see decent radiational cooling as a result, potentially leading to more widespread overnight lows in the single digits than the previous 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. These values will not warrant any cold weather headlines, but it will be bitterly cold nonetheless.

We break the cold snap on Wednesday as temperatures get toward more seasonable levels, and even potentially above normal on Thursday. However, a cold front moving through sometime late week will drop temperatures back to near or slightly below normal levels to end the work week. The pattern potentially turns more active next weekend, but way too early to know specifics.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

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

Today...MVFR conditions lowering to primarily IFR as snow develops during the morning, with even some rain/snow mix possible at the onset especially at KMIV/KACY. A brief improvement around midday between waves of precip is possible at the I-95 sites before falling again. Conditions likely begin to improve for all sites back towards MVFR/VFR by late in the afternoon as precipitation departs. Winds becoming north- northwest and increasing to around 10 knots. Low confidence in the details including the timing of precipitation changes.

Tonight...Primarily VFR, though KMIV/KACY could see some lingering restrictions during the early part of the night as the coastal low pulls away. Westerly winds around 10 knots. Moderate confidence. Outlook...

Monday through Tuesday...VFR. Wind gusts 20-30 kt during the day, 15- 20 kt at night.

Tuesday Night through Wednesday Night...VFR. No significant weather expected.

Thursday...Mainly VFR. Slight chance (10-20%) for a snow shower.

MARINE

Sub-SCA conditions expected today on the waters. Light winds this morning will increase to around 10-20 kt out of the north with 2 to 4 foot seas.

For tonight, winds increase a little bit further. Small chance (20- 30%) for SCA conditions on the lower ocean zones as wind gusts out of the W/NW could get near 25 kt at times, but did not have enough confidence to issue an SCA.

Outlook...

Monday though Tuesday...SCA conditions expected as wind gusts of 25- 30 kt expected. A Small Craft Advisory was issued for all marine zones. Seas 3 to 5 feet.

Tuesday Night through Wednesday Night...SCA may need to be extended as gusts hover near 25 kt but only took the SCA through Tuesday for now.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Weather Advisory until 8 PM EST this evening for PAZ060>062-070-071-101>106. NJ...Winter Weather Advisory until 8 PM EST this evening for NJZ001- 007>010-012-013-015>019. DE...Winter Weather Advisory until 8 PM EST this evening for DEZ001. MD...None. MARINE...Small Craft Advisory from 1 PM Monday to 6 PM EST Tuesday for ANZ430-431. Small Craft Advisory from 9 AM Monday to 6 PM EST Tuesday for ANZ450>452. Small Craft Advisory from 11 AM Monday to 6 PM EST Tuesday for ANZ453>455.