textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

1. Patchy dense fog is expected through Friday with intervals of mainly light rain/showers.

2. Turning very mild this weekend and especially the first half of next week.

DISCUSSION

KEY MESSAGE 1...Patchy dense fog is expected through Friday with intervals of mainly light rain/showers.

Front stalled mostly to our south through the period will keep the weather cool and murky, with weak waves of low pressure bringing several periods of mainly light rain. The first will arrive early this morning, with a second wave arriving tonight. Between these rainy intervals, mist/drizzle and perhaps some dense fog will develop as the cool but very moist air mass lingers across the region with relatively light winds. The one thing in our favor is that winds may be just strong enough to prevent widespread dense fog, especially as remaining snow melts and ground temps approach air temps, but the risk will continue. For the time being will continue dense fog advisory near the coast thru 10 AM, though it looks questionable at this moment.

Friday will feature another day of dreary conditions as a backdoor front will have pushed through from the north and east and then remain stalled for a time over Delmarva. Expect lots of clouds with some fog, drizzle or a few showers as low clouds and moisture remain trapped below an inversion. Most of our area should see highs in the 40s.

KEY MESSAGE 2...Turning very mild this weekend and especially the first half of next week.

A much warmer air mass is forecast to overspread our area this weekend and especially the first half of next week. In addition, there will be some opportunities for showers and perhaps a thunderstorm across the area.

A shortwave trough is forecast to slide along the northern side of a building ridge that becomes anchored/centered in the western Atlantic through Friday. The trough should exit the New England coast Friday morning, however strong high pressure across eastern Canada is forecast to build southward. As this occurs, the ridge amplifies west and northward aloft. This setup forces a boundary to our south and west with an east to northeast low- level flow. This will drive a colder air mass across our region along with more cloud cover and perhaps some fog. This cold air damming scenario will likely lead to a strong temperature gradient, especially on Saturday as the wedge of colder air starts to erode from the south. As a result, the NBM has continued a cooling trend for Friday and therefore used a NBM/NBM10 blend for temperatures to get it a bit colder. Given the continued cooling, may have to watch Friday morning across the I-80 corridor for some light freezing rain if some precipitation is around.

Another upper-level disturbance tracks to the north of our area Saturday night, with a trailing cold front moving through. This looks to bring additional showers and perhaps a thunderstorm to our region, although the trends in the latest model guidance looks to be lowering with the coverage of showers. As of now, did not make any changes to the NBM PoPs for Saturday night. An increase in the low- level warm air advection ahead of a weakening cold front looks to erode the wedge of colder air from south to north, therefore high temperatures Saturday for much of our area look to occur during Saturday night. Used a NBM/NBM10 blend for temperatures Saturday into Saturday night to show a slower erosion of the low-level colder air. We become more embedded within the warm sector Sunday and especially early next week with surface high pressure anchored in the western Atlantic and a south to southwest flow. This should result in a much warmer air mass surging into the region. Highs Sunday and Monday are forecast to be in the 60s for much of the area, then into the low to mid 70s for Tuesday and Wednesday. The flow may be light enough and also more southerly though Sunday and early next weak, which would introduce significant cooling along the coast with perhaps even a robust sea breeze racing inland. Some guidance is faster with a cold front and ripple of low pressure Wednesday, and if this occurs then temperatures may not get into the 70s.

AVIATION /06Z THURSDAY THROUGH MONDAY/

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

Thru 12Z this morning...Becoming IFR/LIFR as dawn approaches with patchy fog and off and on light rain moving in. Winds generally light/variable, though a light easterly flow may be favored at times. Moderate confidence in extent of restrictions with low confidence in timing of how long it takes for conditions to drop.

Today...IFR/LIFR conditions expected through the day. Light to moderate rain expected through the morning, with light showers, low clouds, and fog taking over for the afternoon. Light winds around 5 kt expected out of the east. Moderate confidence in extent of restrictions, with low confidence in timing and if there is any improvement through the day.

Tonight...IFR/LIFR conditions likely continue with some rain returning. East-northeast winds around 5 kt. Low confidence overall.

Outlook...

Friday...Sub-VFR conditions due to low clouds and possible fog. A few showers possible.

Saturday...Sub-VFR ceilings may try and improve. Some showers, especially at night, with some thunder possible.

Sunday...Conditions may improve to VFR. A few showers still possible.

Monday...Mainly VFR.

MARINE

Winds and seas will remain mainly below SCA conditions through tonight. A slow moving front will keep clouds and showers on the waters during the period. Also, with the cold waters and humid airmass, fog is expected to continue. A Dense Fog Advisory continues into the morning and may be extended thru the day. As a weak wave passes later today into tonight, winds may approach SCA criteria on our northern ocean waters, but at this time confidence is low so held off on any SCA.

Outlook...

Friday...Northeast winds should be decreasing, and as of now the conditions are anticipated to be below Small Craft Advisory criteria. Some fog possible.

Saturday and Sunday...A period of Small Craft Advisory conditions possible late Saturday into early Sunday.

Monday...The conditions are anticipated to be below Small Craft Advisory criteria

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...Dense Fog Advisory until 10 AM EST this morning for NJZ013-014- 020-022>027. DE...Dense Fog Advisory until 10 AM EST this morning for DEZ003-004. MD...None. MARINE...Dense Fog Advisory until 10 AM EST this morning for ANZ430-431- 450>455.