textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Confidence is increasing that there will be a swath of 2-4 inches of snow accumulation today near and north of I-78, with locally higher amounts possible. We've expanded the Winter Weather Advisory to include Western Chester, Western Montgomery, Upper Bucks Counties in PA and Hunterdon County in NJ.

For Sunday, a slight increase in snow totals near the coast, though nothing significant.

High confidence in a period of below normal temperatures Sunday Night through Wednesday, with no significant changes in that portion of the forecast. Cold Weather Headlines may be needed, especially on Monday Night.

KEY MESSAGES

1. Periods of snow are expected through this afternoon, primarily northwest of I-95. Snow amounts will be highest near and north of I-78.

2. A coastal low slides by offshore on Sunday, bringing some snow, mainly to areas along and south/east of I-95. The highest snow amounts will be concentrated near the coast.

3. 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...Periods of snow are expected through this afternoon, primarily northwest of I-95. Snow amounts will be highest near and north of I-78.

The longwave trough over central portions of the US will continue to shift eastward with time resulting in height falls and enhanced southwesterly flow aloft continuing to overspread the region this morning. The surface low pressure is presently centered over the Great Lakes and will also gradually shift eastward, passing to the north of the region this evening.

Precipitation will continue to spread from southwest to northeast across the area as we head into the daytime hours. It should be all snow northwest of I-95, and perhaps for the I-95 corridor itself. Warming low-level temperatures are expected to lead to any precipitation for I-95 and points southeast changing to rain by mid-morning. Northwest of I-95, and especially north of I-78, guidance indicates that cold air will remain in place until precipitation departs. Snow coverage and intensity is expected to maximize after daybreak with the arrival of stronger forcing and through the morning hours, before precipitation tapers off from southwest to northeast during the early afternoon hours.

Overall, the most significant change with this forecast cycle is increasing confidence in the placement of the axis of the highest snowfall totals. Guidance has come into better agreement in a slight northwest shift of the highest totals, north of I-78, where 2-4 inches are expected, with localized amounts up to 5 inches possible. As the stronger frontogenetic forcing arrives later in the morning hours, snowfall rates in excess of 1" per hour will be possible at times (20-40% chance), particularly north of the Lehigh Valley, which could make travel difficult. Winter Weather Advisories are in effect until 4 PM.

Northwest of I-95 amounts generally up to about an inch or two are possible with little to no accumulation expected near and south of I-95.

KEY MESSAGE 2...A coastal low slides by offshore on Sunday, bringing some snow, mainly to areas along and south/east of I-95. The highest snow amounts will be concentrated near the coast.

An area of low pressure develops off the coast of the Outer Banks on Saturday Night and lifts northeast. The low looks to pass by close enough to bring some snow to the region, mainly along and south/east of the I-95 corridor. The latest trend with the 00z guidance was a slight jog northwest with the track of the low, which would bring more snow to the region. However, will not deviate much from the previous forecast to avoid wind-shield wiping the forecast plus the setup is not overall conducive for a significant snow event. The deterministic forecast is more in line with HREF and NBM. Thinking around 1-2" from I-95 on south and east. Cannot rule out some amounts higher than 2", especially near the immediate coast. However, there could be some mixing issues in the coastal areas as well at onset as temperatures will be above freezing in the low- levels. This may be offset though by higher QPF compared to inland areas and heavier snowfall rates. Both NBM/HREF probability of 2" or more (Advisory criteria) is around 20-30% for the coastal counties with 10-20% near I-95. So, we may need winter weather headlines near the coast, however the theme with this event remains unchanged as it should just be a rather light snowfall with insignificant snowfall totals.

Snowfall totals drop off quickly getting into Pennsylvania and northern NJ. Far western areas such as the southern Poconos and Berks County may not see any snowfall at all.

In terms of timing, precipitation looks to start early Sunday morning continuing through Sunday evening. However, precipitation should be rather light overall and snowfall rates unimpressive.

KEY MESSAGE 3...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 on Sunday Night, ushering in an arctic airmass. The stretch of below normal temperatures commences that night, 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, where 850 mb temperatures will be around -15 to -20 C. This will result in wind chills Monday Night between -10 and -15 in the southern Poconos, below zero over the Lehigh Valley, and in the single digits over the rest of the area. We 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.

The airmass begins to moderate on Tuesday Night ahead of a potential mid-week clipper system, though wind chills should still be in the single digits for most and below zero in the southern Poconos. Increasingly unlikely we'll need headlines on Tuesday Night, but it will be bitterly cold.

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.

AVIATION /14Z SATURDAY THROUGH WEDNESDAY/

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

Today...MVFR/IFR ceilings at KRDG and KABE along with snow, heavier at times, with IFR/LIFR visibilities. A period of snow or rain/snow mix at KTTN to KPNE/KPHL to KILG with MVFR to IFR visibilities at times. Little or no precipitation at KMIV/KACY. After the precipitation departs by later this afternoon, ceilings should improve back towards VFR. Light and variable to calm winds becoming south-southwest 5-10 knots. Some gusts up to 20 knots possible at KMIV and KACY this afternoon. Low confidence.

Tonight...Mainly VFR. Light snow may begin to move into KMIV and KACY after 08Z, potentially resulting in MVFR conditions (20-40% chance). Westerly winds 5 knots or less. Low confidence.

Outlook...

Sunday...MVFR/IFR conditions expected, mainly for the I-95 and South Jersey terminals with periods of snow possible (50-80%). Highest chance to see restrictions will be KMIV/KACY. KRDG/KABE may actually stay VFR as the precipitation shield likely stays off to the east.

Sunday Night...Primarily VFR, though KMIV/KACY could see some lingering restrictions as a coastal low pulls away.

Monday...VFR. West/southwest wind gusts 20-30 kt.

Monday Night...VFR. No significant weather.

Tuesday...VFR. West/northwest wind gusts 20-30 kt.

Tuesday Night...VFR. No significant weather.

Wednesday...VFR. South/southwest wind gusts 15-25 kt.

MARINE

A Small Craft Advisory remains in effect for all NJ and DE Atlantic Coastal Waters through 7 PM Saturday.

Southwest winds of 15-20 kt with gusts near 25 kt continue into the day Saturday before becoming more westerly and lessening in the evening. Seas of 4-6 feet continue into Saturday before diminishing as well. Sub-SCA conditions expected for tonight with westerly winds around 5-10 kts and seas of 2-4 feet.

Sub-SCA conditions expected on the Delaware Bay throughout the period though some gusts of around 20-25 kts remain possible through midday Saturday.

Outlook...

Sunday through Sunday Night...No marine headlines expected. Rain/snow expected on the coastal waters.

Monday through Tuesday...SCA conditions likely (60-70%) with wind gusts out of the west/southwest around 25-30 kt and seas near 5 feet.

Tuesday Night...No marine headlines expected.

Wednesday...SCA conditions possible (50%) as gusts out of the south/southwest get near 25 kt.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Weather Advisory until 4 PM EST this afternoon for PAZ054-055-060>062. Winter Weather Advisory until 4 PM EST this afternoon for PAZ101-103-105. NJ...Winter Weather Advisory until 4 PM EST this afternoon for NJZ001-007-008. Winter Weather Advisory until 4 PM EST this afternoon for NJZ009. DE...None. MD...None. MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ450>455.