textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Threat has diminished for significant snow on Sunday. High confidence in a prolonged stretch of below normal temperatures from Sunday through Wednesday, with an increasing likelihood of needing cold weather headlines Monday Night and possibly Tuesday Night as wind chills could fall below zero or even below -10 in the southern Poconos.

KEY MESSAGES

1. Blustery and cold through Friday morning. Wind chills in the single digits in many areas and below zero in the southern Poconos by Friday morning.

2. Multiple rounds of precipitation possible this weekend, some of which could fall as snow. Watching an area of low pressure slide by off the coast. However, the risk for a significant snow event on Sunday is decreasing.

3. Prolonged period of below normal temperatures expected from Sunday into the middle of next week.

DISCUSSION

KEY MESSAGE 1...Blustery and cold through Friday morning. Wind chills in the single digits in many areas and below zero in the southern Poconos by Friday morning.

Cold air advection is underway, and a much colder night is on tap for tonight. Gusty winds as well, with northwest winds remaining sustained at 15-25 mph and gusts of 30-35 mph, with temps falling well below freezing. This will bring wind chills into the single digits for much of the area, and slightly below zero in the southern Poconos. Wind or cold thresholds will not be met, but it will still be quite cold, especially for the Friday morning bus pickups.

KEY MESSAGE 2...Multiple rounds of precipitation possible this weekend, some of which could fall as snow. Watching an area of low pressure slide by off the coast. However, the risk for a significant snow event on Sunday is decreasing.

A longwave trough sets up over the Eastern US, with a series of shortwaves moving over the area, resulting in some unsettled weather this weekend. The first system looks rather weak and should move through on Saturday. The change in the forecast has been the onset time of precipitation moving up, coinciding with an increase in synoptic lift with an area of 500 mb positive vorticity advection and low-level weak warm-air advection setting up. Precipitation now looks to start in the early morning hours of Saturday, primarily as snow, but changeover to rain gradually occuring from SE to NW. The changeover to rain will speed up after daybreak, and will eventually transition to all rain for everyone except the northern Lehigh Valley and southern Poconos, where snow could hang on longer. Precipitation should be rather light though. Snowfall amounts will be anywhere from a dusting up to an inch. Nothing more than a trace for areas along and south/east of I-95. QPF amounts are less than a tenth of an inch across the region.

The second system to watch will be a coastal low developing off the coast of the Outer Banks and lifting off to the northeast on Sunday. Bottom line up front, the threat for a significant snow event has decreased. The synoptic pattern and associated surface pattern just do not quite match up well for a big snow event. Low pressure slides by well offshore, with some light snow expected from the I-95 corridor on south and east during the day on Sunday. The GFS and GEFS ensembles have trended towards the rest of the global guidance which represent a more offshore solution. If you read this discussion yesterday, you'll know it is not a surprise at all. NBM Probabilities of Advisory level snow (2-3" or more) with this forecast cycle were around 20-25% at the immediate coast, 10-20% from the NJ Turnpike on south and east, and 5-10% for the rest of the area. Current forecast has 0.5-1.5" for the coast and southern Delaware with a coating to an inch up to the I-95 corridor. Cannot rule out a few flakes north and west of I-95 on Sunday, but nothing more than trace accumulation. Precipitation will move out on Sunday night.

KEY MESSAGE 3... Prolonged period of below normal temperatures expected from Sunday into the middle of next week.

A deep trough sets up over the eastern US, ushering in an arctic airmass. High pressure at the surface will be in control keeping things dry. Several days of below normal temperatures begin on Sunday, continuing through Wednesday, before some moderation to near normal later in the week. The height of the cold airmass will be over our area Monday Night through Tuesday Night, with both nights featuring wind chills in the single digits and below zero. As it stands now, it is becoming increasingly likely that Monday Night will be the coldest night, with Tuesday Night just a touch warmer. Cold Weather Headlines may be needed on one or both nights. Highs on Tuesday continue to trend colder, with current highs projected to be in the upper teens and low 20s.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

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

Tonight...Mostly VFR, although there is potential for MVFR ceilings, as guidance indicates cloud heights lowering to around 3,000-3,500 feet overnight. The best chance of MVFR ceilings is at RDG/ABE, which is included in the TAF. West to northwest winds remaining around 15-20 kts with gusts up to 30 kts through 06Z, then winds lessening a little toward dawn. High confidence in prevailing conditions, but lower confidence and lower probability in any MVFR ceilings occurring.

Friday...VFR. West wind gusts up to 20-25 kts to start, gradually diminishing through the day and shifting more southwesterly.

Outlook...

Friday Night...VFR. No significant weather.

Saturday through Sunday...Primarily VFR, though sub-VFR conditions possible (30-50%) with periods of rain and snow moving through.

Sunday Night...VFR. No significant weather.

Monday...VFR. Wind gusts out of the west around 15-25 kt.

Monday Night through Tuesday...VFR. No significant weather.

MARINE

Small Craft Advisories are no longer in effect, as the Gale Warnings have taken over. Winds increasing this evening, with gale force gusts 35 to 40 kts anticipated to develop for several hours into the overnight period before diminishing into Friday morning. Seas 4-7 feet. Freezing spray is possible (20-40% chance) tonight through Friday morning as air temperatures fall into the mid to upper 20s. Gale Warning in effect for all marine zones from 6 PM this evening through 10 AM Friday. Thereafter, winds should fall off pretty quickly, ending up below SCA levels by evening.

Outlook...

Friday Night through Saturday Night...SCA conditions expected with seas 4 to 6 feet.

Sunday through Sunday Night...No marine headlines expected.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Warning until 10 AM EST Friday for ANZ430-431-450>455.