textproduct: Philadelphia/Mt Holly

SYNOPSIS

An area of low pressure tracks nearby tonight into Sunday morning, bringing wintry weather with an arctic front crossing our area. The center of arctic high pressure slides to our south Tuesday before weakening as it shifts offshore Wednesday. A warm front may slide by to our north Wednesday into Thursday, followed by a cold front Thursday night. High pressure returns to close out the week.

NEAR TERM /THROUGH SUNDAY/

An impactful winter storm will impact portions of the area tonight through Sunday morning.

What has changed: An initial band of mostly flurries and sprinkles has moved through the region as of 8 PM. There is a broad area of snow still to our west over Central PA and MD. Forecast remains on track with snow amounts and the timing of the main round of precipitation.

A potent closed upper-low currently centered just to the north of Lake Superior will shift southeastward across southern Ontario by Sunday morning, gradually opening into a positively tilted upper- trough during the day Sunday as it shifts eastward and approaches the coast. Significant height falls aloft will begin this evening as cyclonic flow overspreads the area. At the surface, weak low pressure will shift eastward across portions of Ontario and into Quebec tonight, with a potent arctic front passing through the region by early Sunday. Additional weak surface cyclogenesis is anticipated off the Mid-Atlantic coast. The low pressure will track quickly out to sea during the day on Sunday.

Ahead of the approaching trough, synoptic-scale ascent will overspread the region, and the main round of precipitation will spread west to east over the region. At this point, most areas west of I-95 will see precipitation start and stay as snow. For areas near or east of I-95, there could be a brief period of a rain/snow mix at the onset of the main round of precipitation, but it should change over to all snow quickly.

Precipitation intensity looks to maximize during the pre-dawn hours Sunday, and right now it appears that the heaviest snow will be near the I-95 corridor. In this region, forecast soundings indicate backing winds in the lowest levels, with substantial veering above 850 mb. Given the veering winds aloft which indicates warm air advection and strong vertical motion, as well as a deep and saturated DGZ, a period of heavy snow is expected. In addition, model guidance continues to suggest a zone of frontogenetical forcing in this vicinity. FGEN banding is likely to occur, and snowfall rates upwards of 1 inch per hour are a reasonable expectation within this zone. While the exact area in which this banding sets up is still somewhat uncertain, confidence has increased that it will be across portions of southeastern PA into central and southern portions of NJ. Even though the period of heaviest snowfall is not expected to last more than a few hours, it will not take long for substantial snow accumulation to occur with these conditions. As a result, confidence in warning criteria snow has increased, and a Winter Storm Warning has been issued for portions of southeastern PA and southern NJ, including the Philadelphia metro. Here, snow accumulation of 4-5" is expected, and locally higher amounts cannot be ruled out.

Across much of northern NJ, snow accumulations appear likely to fall in the 3-5" range. Confidence is a little lower across the Delmarva (especially southern portions of DE and the Eastern Shore of Maryland), as the window of all snow will be the shortest. Still, general accumulations of 1-3" are expected in these areas. Winter Weather Advisories remain in effect for these locations.

For the Poconos, dry air will work its way in faster, but 1-3" of accumulation is still expected. With advisory thresholds across these areas of 3", have elected not to issue any this cycle, though if amounts trend up it is possible that advisory issuance could be needed in future updates.

Snow will clear the area from west to east by early Sunday afternoon. Even some partial clearing of clouds is possible by evening. With that said, it will remain a very cold day across the entire area, with highs in the 20s northwest of I-95 and in the low 30s southeast. Many of these highs will be reached early in the day, with temperatures falling through the afternoon due to strong cold air advection taking place. In addition, northwest winds will increase as the snow ends, with gusts 25-35 mph expected. Areas of blowing snow are possible, though this potential will be tempered some as the snow-to-liquid ratios with this event will not be particularly high. Regardless, the very cold air combined with the breezy conditions will lead to wind chills in the single digits to low teens across the entire area by sunset.

SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/

In the wake of the snow, we see a high pressure system build in from the west with a very cold airmass arriving. Due to the high building in, we are dry Sunday night with decreasing cloud cover. The main story Sunday night will be the temperatures and the wind. A northwest wind will usher in colder air. Cold air advection will allow stronger winds aloft to mix down along with a tight pressure gradient Sunday night which all leads to gusty winds developing. Wind gusts Sunday night are forecast to be upwards of 25-35 mph. The combination of these winds and temperatures in the teens for many will result in wind chill values in the single digits for most and in the negatives for the Poconos. If the wind remains strong enough with even lower temperatures, some areas may reach Cold Weather Advisory criteria Sunday night into early Monday morning.

As the high pressure system moves eastward, we stay mainly dry Monday with the wind decreasing as the pressure gradient lessens. There is the signal for a piece of upper-level energy to rotate into the region during the second half of Monday. Some guidance brings the potential for some snow showers to parts of the area late Monday afternoon into Monday night. There is also the potential for the high that builds in to suppress this activity north. At this time, kept POPs around 25% or less across the Poconos, Lehigh Valley, Berks County, and northwestern NJ. We do see more cloud cover filter in through the day due to that piece of upper- level energy moving through. Highs on Monday are in the low to mid 30s for most with the higher elevations in the 20s.

Monday night is another cold night with temperatures in the teens for many. One difference from Sunday night is the wind will be much less at generally 5-10 mph, so wind chill values will be in the low to mid teens for most with single digit values in the higher elevations. For Tuesday, it is a dry day with the high pressure system in place across the region. Highs on Tuesday are in the mid 20s to mid 30s.

LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/

By Tuesday night, temperatures are slightly higher with overnight lows in the 20s for most and teens for the higher elevations. Winds continue to remain at around 5-10 mph, so wind chill values are in the teens and low 20s for most.

Our weather pattern starts to change by Wednesday which results in more mild air filtering in. Our high pressure system starts to move offshore Wednesday which gives us more of a southwest wind that ushers in warmer air. Highs on Wednesday are in the upper 30s to upper 40s.

A more amplified pattern then looks to take shape for the end of the week. A deepening low pressure system near the Great Lakes region Thursday moves northeast into Quebec Friday. This low pressure system will drag a cold front through the area Thursday into Thursday night. The precipitation type with this cold front will be rain as temperatures will be mainly in the upper 40s to upper 50s. Come Friday, high pressure builds back in with temperatures are in the 40s that persist into Saturday as well.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

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

Tonight...VFR conditions to start, but will gradually fall to IFR and LIFR as precipitation begins. Under intense snow bands (most likely for KILG, KPHL, and KPNE) VLIFR conditions will be possible. Snow will spread from west to east by 06Z. Precip may begin as a brief period of rain at the onset, but should quickly changeover to snow thereafter. Light winds early, becoming northwest around 5-10 kt late. Low confidence overall.

Sunday...Lingering MVFR conditions possible in the morning with periods of light snow. Expect conditions to lift to VFR by 15Z at all terminals as precipitation ends from west to east. Northwest winds around 10-15 kt with gusts up to 25-30 kt in the afternoon. Moderate confidence.

Outlook...

Sunday night through Monday...VFR expected. Wind gusts up to 20-25 kt possible. A slight chance of a snow shower at KRDG and KABE on Monday.

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

Thursday...Primarily VFR. A slight chance of rain showers in the afternoon.

MARINE

No marine headlines tonight through early Sunday morning, though a period of rain changing to snow is expected tonight into Sunday morning. Winds pick up quickly beginning after daybreak Sunday morning, ramping up to gale-force by the afternoon and evening, continuing into the overnight hours. Gale Warnings are in effect for Sunday and Sunday night. Areas of freezing spray possible Sunday night.

Outlook...

Monday...Gale-force wind gusts in the morning with a period of Small Craft Advisory conditions afterward. Areas of freezing spray possible during the morning.

Monday night...Small Craft Advisory conditions could linger into the night with westerly winds 15-20 kt and gusts perhaps near 25 kt. Seas 2-4 feet.

Tuesday through Tuesday night...No marine headlines are currently anticipated with winds remaining below 25 kt and seas below 5 feet.

Wednesday...Small Craft Advisory conditions possible with southwesterly wind 15-20 kt with gusts perhaps near 25 kt. Seas 2-4 feet.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Weather Advisory until 1 PM EST Sunday for PAZ101-103- 105. Winter Weather Advisory until 7 AM EST Sunday for PAZ060>062. Winter Storm Warning until 1 PM EST Sunday for PAZ070-071-102- 104-106. NJ...Winter Weather Advisory until 1 PM EST Sunday for NJZ009-010- 012>016-020>022-025>027. Winter Weather Advisory from 1 AM to 1 PM EST Sunday for NJZ023-024. Winter Weather Advisory until 7 AM EST Sunday for NJZ001-007- 008. Winter Storm Warning until 1 PM EST Sunday for NJZ017>019. DE...Winter Weather Advisory until 1 PM EST Sunday for DEZ001. Winter Weather Advisory from 1 AM to 1 PM EST Sunday for DEZ002>004. MD...Winter Weather Advisory from 1 AM to 1 PM EST Sunday for MDZ012-015-019-020. MARINE...Gale Warning from 7 AM Sunday to 7 AM EST Monday for ANZ430- 431-453>455. Gale Warning from 11 AM Sunday to 7 AM EST Monday for ANZ450>452.