textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Dense Fog Advisories have been replaced with a Special Weather Statement that is in effect until 9 AM this morning.

The risk for some strong to severe thunderstorms has increased some Wednesday night.

KEY MESSAGES

1. Areas of patchy dense fog expected this morning and possibly again tonight. Near record warmth today.

2. Warmth and increasingly more humid conditions Wednesday with record high temperatures possible.

3. A strong cold front crosses our region late Wednesday night and Thursday morning with showers and some potentially strong to severe thunderstorms.

DISCUSSION

KEY MESSAGE 1...Areas of patchy dense fog expected this morning and possibly again tonight. Near record warmth today.

High pressure situated over the western Atlantic continues to dominate. Dense fog has developed over the Atlantic coastal and Delaware Bay waters where water temperatures are much cooler than land areas. This fog has at times moved inland, but hasn't advected as far inland as much as forecast guidance had originally suggested. That is until the last hour or so, where fog has developed over southern New Jersey and has slowly been spreading northeastward up the Delaware River Valley and across the Pine Barrens. Given that coverage in fog is relatively shallow and rather sporadic in coverage, have opted to cancel the Dense Fog Advisories and replace it with a Special Weather Statement for patchy areas of dense fog through 9 AM today. After the morning fog burns off, another day with plentiful sunshine is expected with highs in the mid to upper 70s for most locales, 60s in the Poconos and 50s along the coast. Temperatures may approach records in some spots (see Climate Section below for more info).

Another round of fog and mist is possible again this evening into early Wednesday morning, but confidence is occurrence is low. Latest HREF guidance suggests that this slug of marine fog will likely remain mostly over the waters or right along the coastal strip.

KEY MESSAGE 2...Warmth and increasingly more humid conditions Wednesday with record high temperatures possible.

Southerly flow increases on Wednesday as a strong cold front approaches from the west and the pressure gradient tightens. South to southwest winds will increase with gusts 20-25 mph. Warm air will remain in place, and a more humid airmass spreads into the region with dew points rising into the upper 50s to low 60s. High temperatures are forecast to be in the mid to upper 70s for many places, although still much cooler along the coast due to a southerly wind off the cold ocean. See the Climate section below for the records. The warmth could be muted some however if thicker cloud cover arrives faster during the day Wednesday. Some showers should start to develop during the course of the day, especially later in the afternoon across our western zones.

KEY MESSAGE 3...A strong cold front crosses our region late Wednesday night and Thursday morning with showers and some potentially strong to severe thunderstorms.

Low pressure will track to our north Wednesday and Wednesday night. This pulls a strong cold front across our region late Wednesday night and Thursday morning. Showers develop with the arrival and passage of the front. A warm and moist environment will be in place ahead of the cold front and some thunder will be possible with the front or with a pre-frontal zone of forcing. The timing at night into the early morning hours however is not ideal for supporting thunder given the loss of diurnal heating, especially as the upper- level trough is forecast to lag well to our west. However given the mild temperatures ahead of the front, with temperatures potentially not dropping off as much, instability could be sufficient for some thunderstorms. Some model forecast soundings show a decent amount of especially elevated instability. Ahead of the upper-level trough, very strong deep-layer southwesterly flow is forecast with around 80 knots at 500 mb and 50-60 knots at 850 mb. This will result in strong shear profiles that would favor line segments with locally strong to damaging wind gusts. Given the timing however, the greater risk for severe thunderstorms looks to be just to our west where stronger forcing coincides with greater instability with strong shear. This convection may tend to weaken as it shifts east late Wednesday afternoon and night and arrives into our western zones. This is a strong cold front though, so will have to monitor how organized the convection gets to our west as that will determine its eastward longevity and therefore severity. Given the setup, added a slight chance to chance of thunderstorms from roughly along and north/west of I-95 Wednesday night.

The cold front clears our region by later Thursday morning, then strong cold air advection develops throughout the day. It looks like the daily high temperature will occur just after midnight, then temperatures Thursday fall into and through the 50s in the morning to the 40s by late afternoon as strong northwest winds develop behind the front.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

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

Today...Any restrictions are expected to improve by 13-15Z after BR/FG dissipates. VFR then expected the rest of the day with mainly clear skies. South-southwest winds around 5-10 kt. A seabreeze should develop and turn winds south-southeast at KMIV and KACY in the afternoon. High confidence, with lower confidence in timing of sea breeze wind shift.

Tonight...Prevailing VFR expected for most terminals. The exception is near KMIV and KACY where another round of marine fog, mist, and stratus is possible. Confidence is not high enough to include in TAFs at this time. South winds around 5 kt or less. Low-moderate confidence.

Outlook...

Wednesday...VFR with increasing clouds during the day, then areas of sub-VFR conditions in showers mainly at night. A few thunderstorms possible at night. Southwest winds may gust to 20 knots.

Thursday..Sub-VFR conditions in showers in the morning, then VFR. Southwest winds becoming northwest with gusts up to 30 knots.

Friday...VFR overall. Some rain or snow showers possible mainly north and west of the I-95. Southerly winds may gust to 25 knots.

Saturday...VFR.

MARINE

Marine Dense Fog Advisories remain in effect for Atlantic Coastal waters including the Delaware Bay until 11 AM this morning.

South-southwest winds around 10-15 kt are expected through tonight with seas around 2-3 feet. Areas of dense marine fog expected through this morning, becoming patchy this afternoon. Another round of dense marine fog possible tonight into early Wednesday morning. Outside of fog, fair weather is expected.

Outlook...

Wednesday...Small Craft Advisory conditions developing. Fog possible in the morning.

Thursday...Small Craft Advisory conditions probable

Friday...Small Craft Advisory conditions mainly at night.

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

CLIMATE

Near record high temperatures are forecast for March 10th and 11th. Records for our climate sites are listed below:

Record High Temperatures: March 10 Site............................Record/Year Allentown (ABE).....................80/2016 AC Airport (ACY)....................81/2016 AC Marina (55N).....................68/2000 & 2016 Georgetown (GED)....................80/2006 Mount Pocono (MPO)..................69/1977 Philadelphia (PHL)..................82/2016 Reading (RDG).......................79/2016 Trenton (TTN).......................81/2016 Wilmington (ILG)....................80/2016

Record Warmest Low Temperatures: March 11 Site............................Record/Year Allentown (ABE).....................44/2006 AC Airport (ACY)....................48/1955 AC Marina (55N).....................51/2016 Georgetown (GED)....................55/1955 & 1986 Mount Pocono (MPO)..................47/1955 Philadelphia (PHL)..................54/1955 Reading (RDG).......................51/1955 Trenton (TTN).......................51/1955 Wilmington (ILG)....................48/1955

Record High Temperatures: March 11 Site............................Record/Year Allentown (ABE).....................73/2021 AC Airport (ACY)....................79/1967 AC Marina (55N).....................70/1879 Georgetown (GED)....................77/2021 Mount Pocono (MPO)..................66/1977 Philadelphia (PHL)..................74/2021 Reading (RDG).......................77/2021 Trenton (TTN).......................73/2021 Wilmington (ILG)....................74/2021

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Dense Fog Advisory until 11 AM EDT this morning for ANZ430-431- 450>455.