textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A Dense Fog Advisory is now in effect for the forecast area tonight through Wednesday morning. Rain impacts the area Wednesday afternoon and evening.

Cooler Thursday in the wake of a backdoor cold front.

KEY MESSAGES

1. Dense fog develops tonight, then rain impacts the area on Wednesday.

2. A storm system moves through later Thursday and Friday bringing rain. Some wintry mix/ice possible for mostly the I-80 corridor.

3. A significant winter storm possible Sunday, then much colder early next week.

DISCUSSION

KEY MESSAGE 1...Dense fog develops tonight, then rain impacts the area on Wednesday.

High pressure remains anchored off the Mid-Atlantic and Southeast coasts, and will slide out to sea during the day Wednesday. Meanwhile, low pressure over the Northern Plains tracks east. A warm front extending out from that low will lift north through the region late tonight through Wednesday.

Return flow continues behind the offshore high, and warm air advection and low level moisture continues to increase throughout the region tonight through Wednesday. Though the lingering snowpack continues to diminish, there is enough for fog to develop as that moisture passes over the snowpack. Surface dew points will remain in the mid 30s or so, then will rise into the 40s on Wednesday. Though there was some locally dense fog this (Tuesday) morning, that dense fog will be more widespread over the area. 12Z HREF indicating a 70 to 80 percent probability of visibilities less than 1/4 mile, and HRRR and 12Z/17 NAM also indicating widespread visibilities less than 1/4 mile. Therefore, a Dense Fog Advisory is now in effect for tonight through Wednesday.

With abundant low level moisture over the area, skies remain cloudy, and though dense fog will lift Wednesday morning, fog will remain over the area. Rain develops in the afternoon, then tapers off in the evening with that warm front lifting north through the region. Patchy fog redevelops Wednesday night.

High temperatures this afternoon were lowered by several degrees as widespread cloud cover and fog has kept temperatures lower than forecast. With increasing southwest flow, temperatures should be able to warm up a bit more on Wednesday.

KEY MESSAGE 2...A storm system moves through later Thursday and Friday bringing rain. Some wintry mix/ice possible for mostly the I-80 corridor.

A cold front should have settled southward as a backdoor front to start Thursday as high pressure builds across New England. This would lead to a northeasterly low-level flow and keep temperatures cooler. As of now, the southward push of colder air does not look to be very strong, however given cloud cover, incoming precipitation along with a northeast surface wind, high temperatures will be near or slightly below average. Given the setup, went closer to the NBM 10th percentile for temperatures. The milder air will remain south and west of the front, with the actual surface front looking to be positioned to our west and south. This setup should lock in low clouds and areas of fog, potentially locally dense especially where snow cover remains, Thursday with the fog itself improving with time.

A low pressure system is forecast to cross our area during Friday. One low looks to track across the lower Great Lakes with another one near the Mid-Atlantic. A zone of warm air advection ahead of this system and associated front will result in some overrunning precipitation. It looks like a quick moving system and it is possible that the energy with this becomes stretched out with time. A round of rain is looking likely Friday, and this may start as early as during the day Thursday. If the precipitation arrives fast enough, then a period of some mixed precipitation is possible from about I-78 northward and also some light icing possible especially for the I-80 corridor. While the temperatures should be warm enough for a mostly rain event, will need to keep an eye on the far northern zones especially for at least some light icing. Rainfall amounts, at this time, look to be around 0.50 inches across the region. Areas of fog will be possible as the milder and more moist air overruns some lingering chilly near-surface air.

KEY MESSAGE 3...A significant winter storm possible Sunday, then much colder early next week.

An active pattern looks to continue with much of the model guidance showing low pressure sliding west to east and exiting off the northern Mid-Atlantic coast later Sunday. This storm looks to deepen as it does so, and has the highest potential to be impactful across our region including the potential for an accumulating widespread snowfall. Many of the deterministic models show a storm, and at least some of the ensembles do as well however they offer some variety regarding the track/strength of the surface low. Both of the ECMWF and GFS AI models show a strong storm off the Mid-Atlantic coast. The question, as always, is the track of the surface low. There is still a chance the system shifts more southeastward and results in mostly a miss or a complete miss for our area. The amount of cold air in place ahead of this storm and especially during it will be important when it comes down to precipitation types and amounts, as strong dynamics may also come into play. While it remains way to early though for specifics, there remains some signal for a possible significant winter storm for the second half of this weekend.

In the wake of this storm, a much colder air mass looks to settle into our region early next week. High temperatures probably do not get out of the 30s both Monday and Tuesday (colder in the Poconos), with lows in the teens to even some single digits. Winds may be strong enough to result in bitterly cold wind chills, especially Monday night.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

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

Rest of today...LIFR/IFR will become MVFR, and possibly even VFR briefly by 00Z. SW winds 5 to 10 kt. Low confidence.

Tonight...VFR initially, then IFR/LIFR CIGs and VSBYs return with the potential for VLIFR conds from 09Z-12Z. SW wind 5 kt or less. Moderate confidence.

Wednesday...LIFR CIGs with IFR VSBYs through midday, then IFR conds in RA/BR/low CIGs in the afternoon. SW winds 5 to 10 kt. Moderate confidence.

Wednesday night...Sub-VFR conds in BR and low stratus. RA ends in the evening.

Outlook...

Thursday...MVFR or lower conditions, especially ceilings. Fog possible in the morning. Some rain arrives later.

Friday...MVFR or lower conditions due to low clouds and some rain. Some wintry precipitation possible in the morning, mainly for KRDG and KABE.

Saturday...Ceilings may improve to VFR.

Sunday...Sub-VFR conditions with snow/rain possible. Gusty northeast winds possible.

MARINE

Though southwest winds will average 5 to 10 kt, seas on the ocean will remain elevated through this evening. The Small Craft Advisory is now in effect until 10 pm this evening. Southwest winds will increase to 10 to 15 kt Wednesday afternoon, and will remain 10 to 15 kt through Wednesday night. After seas subside to 2 to 4 ft this evening, there will be a lull until seas build back up to 3 to 5 feet Wednesday afternoon. A new SCA may be needed for Wednesday afternoon and evening.

Conditions remain below SCA criteria on Delaware Bay tonight through Wednesday night.

VSBYs will be reduced to 1 nm or less tonight through Wednesday due to the development of widespread dense fog. Therefore, a Marine Dense Fog Advisory is now in effect for tonight through Wednesday. Patchy fog is likely Wednesday night, and that Marine Dense Fog Advisory may have to be extended.

Outlook...

Thursday through Saturday...The conditions should be mainly below Small Craft Advisory criteria. Some fog possible Thursday morning.

Sunday...Small Craft Advisory conditions probable. A period of gale force wind gusts possible, depending on the track and strength of low pressure.

EQUIPMENT

The KDIX radar remains out of service until further notice.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Dense Fog Advisory from 10 PM this evening to 10 AM EST Wednesday for PAZ054-055-060>062-070-071-101>106. NJ...Dense Fog Advisory from 10 PM this evening to 10 AM EST Wednesday for NJZ001-007>010-012>027. DE...Dense Fog Advisory from 10 PM this evening to 10 AM EST Wednesday for DEZ001>004. MD...Dense Fog Advisory from 10 PM this evening to 10 AM EST Wednesday for MDZ012-015-019-020. MARINE...Dense Fog Advisory until 6 PM EST Wednesday for ANZ430-431- 450>455. Small Craft Advisory until 10 PM EST this evening for ANZ450>455.