textproduct: Philadelphia/Mt Holly

SYNOPSIS

High pressure will shift to our south and east today. A weak area of low pressure tracks nearby tonight into Sunday morning, 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.

NEAR TERM /THROUGH SUNDAY/

High clouds spreading overhead early this morning though radiational cooling has allowed many areas to get quite cold , with mid-teens here at the office. This is definitely the quiet before the storm, however.

For the balance of today, not much will happen, with just plenty of mid and high clouds and a milder southwesterly breeze. After the chilly start, we'll likely rebound into the 40s across much of our region, perhaps even approaching 50 across the southern Delmarva, but likely staying in the 30s north of I-78 and west of I-287.

The weak wave of low pressure which will bring snow to the region by this time tomorrow morning is still well to our west- northwest over the western High Plains of Montana. At the surface there actually is very little reflection of a low pressure, as the forcing for this system is nearly 100 percent aloft, driven by a strong shortwave trough, vort max and jet streak heading southeastward across the Plains and Midwest today before crossing our area on Sunday. That's not to say there isn't a surface component to our incoming weather, namely the arctic front plunging southeastward across the Upper Midwest presently. This front will also reach our region early Sunday, just in time to have the cold air meet the forcing aloft. Guidance is relatively stable at present, bringing light precip into the area very late today, then a much more focused band of precip develops later this evening and continues into early Sunday. As mentioned, it will warm up decently today, so especially from I-95 south and east, precip stands a good shot at starting as rain before transitioning to snow late this evening into the overnight. The heaviest snow will definitely occur after midnight Saturday night, the least impactful time we can think of, so this system at least has that going for it. As the system starts to pull eastward early Sunday, we should see a fairly quick end to the snow very late tonight into Sunday morning from northwest to southeast. That having been said, lingering instability may cause spotty snow showers to linger into the afternoon, though any additional accumulation should be negligible. With the arctic front having moved through, gusty northwest winds will take over, possibly causing some blowing and drifting snow along with falling wind chills, but the worst of that will occur Sunday night.

For this package, main changes to the forecast were a modest uptick in snowfall, which resulted in an expansion of the winter weather advisory northward and southward. Both expansions are still a little marginal, 2-4 inches along the I-78/I-287 corridors (criteria is 3 here) and 1-3 inches in Sussex County Delaware (criteria is 2 here), but enough confidence is present to bring them into the fold. Across a large central portion of the advisory, areas along and southeast of I-95 in PA, NJ DE and MD, we're now expecting a wide area of 3-5 inches, and there are some snowier models suggesting a stripe of warning criteria (which is mostly 5 inches in this area, though 6 in central NJ) could occur. Not quite sold on that yet given potential for lost QPF from the transition from rain to snow, but its definitely on our minds for potential changes. For now however, kept advisory level forecast across this region, though high-end for sure.

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. Due to uncertainty, PoPs were kept below mentionable at this time. 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 FRIDAY/

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. By Friday, temperatures are in the 40s for most.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

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

Today...VFR but with slowly thickening and lowering cigs. South-southwest winds around 5-10 kt. Moderate-high confidence.

Tonight...Snow will envelop northwestern terminals in the evening, with precip starting as rain PHL south and east, but transitioning to snow overnight from northwest to southeast. Thus, expect any lingering VFR early to fall to IFR if not LIFR or even briefly VLIFR across the board by 6Z or shortly thereafter. Winds light/variable. Moderate confidence.

Sunday...Conditions improve to VFR. West-northwest winds increase with gusts up to 30 knots by afternoon.

Outlook...

Sunday night...VFR. Northwest winds gust up to 20 knots.

Monday...VFR. West-northwest winds gust up to 20 knots.

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

MARINE

Marginal Small Craft Advisory conditions expected to develop for a time around midday today for the northern two ocean zones, with gusts up to 25 kts. Gusts should stay lighter elsewhere. Otherwise, no marine headlines through early Sunday, though a period of rain changing to snow is expected tonight into Sunday morning. Winds pick up quickly Sunday afternoon, with gales possible by day's end.

Outlook...

Sunday night...Gale force wind gusts probable. Areas of freezing spray possible.

Monday...Gale force wind gusts possible in the morning, otherwise Small Craft Advisory conditions probable. 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 from 7 PM this evening to 1 PM EST Sunday for PAZ070-071-101>106. Winter Weather Advisory from 7 PM this evening to 7 AM EST Sunday for PAZ060>062. NJ...Winter Weather Advisory from 7 PM this evening to 1 PM EST Sunday for NJZ009-010-012>027. Winter Weather Advisory from 7 PM this evening to 7 AM EST Sunday for NJZ007-008. DE...Winter Weather Advisory from 7 PM this evening to 1 PM EST Sunday for DEZ001-002. Winter Weather Advisory from 1 AM to 1 PM EST Sunday for DEZ003-004. MD...Winter Weather Advisory from 7 PM this evening to 1 PM EST Sunday for MDZ012-015-019-020. MARINE...Gale Watch from Sunday afternoon through Monday morning for ANZ430-431-450>455. Small Craft Advisory until 6 PM EST this evening for ANZ450- 451.