textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Wind Advisory expanded inland to include most of the forecast area this afternoon into tonight.

Coastal Flood Advisory issued for Ocean and Sussex (DE) Counties, where minor tidal flooding is anticipated along the back bays due to the strong southerly winds this afternoon into tonight.

Updated Aviation discussion for the 06Z TAFs.

KEY MESSAGES

1. We continue to monitor the potential for strong to severe thunderstorms this afternoon into tonight along and ahead of a strong cold front. Even outside of storms, strong winds are likely, especially closer to the coast. Localized flash flooding will also be possible, especially inland.

2. Some areas of minor coastal flooding this afternoon into tonight.

3. Cold and dry through mid-week before temperatures moderate through the weekend.

DISCUSSION

KEY MESSAGE 1...We continue to monitor the potential for strong to severe thunderstorms this afternoon into tonight along and ahead of a strong cold front. Even outside of storms, strong winds are likely, especially closer to the coast. Localized flash flooding will also be possible, especially inland.

A strongly amplified upper level trough across the central CONUS will turn negatively tilted as the day progresses. A strong mid latitude cyclone will continue to strengthen and move northeastward across the eastern Great Lakes and into southeastern Canada into tonight. This system will continue to push north towards James Bay in Quebec and will usher in a strong cold front with a sharp temperature gradient through the region late this evening.

A warm front will lift north of the area this morning, bringing the warm sector of the cyclone over the Mid Atlantic. This will usher in both warmer temperatures and higher dewpoints. The southerly flow associated with this system will transport a deep moisture plume into the region ahead of the frontal system. Guidance is depicting significant height falls along with a strong upper level jet with the right rear located right over the Mid Atlantic. The combination of strong warm air advection, rapidly falling surface pressure, and upper level dynamics will result in significant forcing for ascent across the region this afternoon and through the evening.

Low level wind shear will be quite strong just ahead of the frontal passage and the combination of dynamic lift, sufficient moisture, and modest instability (for mid March standards) will support organized convection. Instability will be the limiting factor, however the dynamics should play a strong enough role in overcoming that instability. High res guidance show sufficient low level support to see STP values greater than 0.5-1 during a brief window during the late afternoon/evening. It is worth noting that the outlook from the Storm Prediction Center continues the ENHANCED risk of severe weather in our County Warning Area.

Something to note, is that if the temperatures/dewpoints surge higher than initially forecast, instability will increase quite a bit. Guidance has a tendency to be too conservative with surface conditions in strong southerly flow regimes (as we will have during this event). Higher temperatures/dewpoints in the warm sector ahead of the cold front could support potential for some isolated supercells to develop. The primary threat remains wind, however the threat for tornadoes is increasing. Shear vectors are generally supportive of QLCS type tornadoes and these are often the hardest type of tornado to issue warnings for, as they can be very brief in nature. A potential limiting factor for this setup could be too much convection. If too much disorganized, benign convection occurs ahead of the line, it could weaken lapse rates and instability. But again, we'll have plenty of forcing and very strong wind fields to help recover the environment in the wake of such activity.

Finally, it should be noted that even outside of any thunderstorms the strong wind fields will support southerly winds gusting at least 35 to 45 mph this afternoon into tonight, locally up to 50 mph. The Wind Advisory was expanded to include most of the forecast area. The strong southerly winds will help usher in an unseasonably warm and humid airmass. High temperatures this afternoon and evening will be in the mid 60s to low 70s inland, and closer to 60 degrees near the coast and higher terrain.

In addition to the severe weather and strong wind threat, the showers/storms will likely be producing heavy rainfall as PWATs are progged to be 1.1-1.4" which is above the 90th percentile for early March. MMEFS guidance is showing modest rises in rivers, so we'll continue to watch for any potential hydro concerns, especially where any training of convection occurs.

KEY MESSAGE 2...Some areas of minor coastal flooding this afternoon into tonight.

The strong southerly winds developing this afternoon and evening are anticipated to result in some areas of minor coastal flooding. The most likely areas to experience impacts will be along the northern end of Barnegat Bay in Ocean County NJ, Rehoboth Bay and Little Assawoman Bay in Sussex County DE. These bays are particularly susecptible to flooding during strong southerly wind events, as the winds prevent water from draining normally during low tide. Thus, impacts may not follow typical astronomical tide cycles. These advisories are in effect from 2 PM today through 2 AM Tuesday, however extensions may be needed if impacts linger after winds shift offshore.

The Eastern Shore of Chesapeake Bay and upper portion of the tidal Delaware River (north of Philadelphia) could also experience some minor coastal flooding, though confidence is lower for these areas. We will continue to monitor trends in the coming tide cycles to determine if any additional advisories may be needed.

KEY MESSAGE 3...Cool and dry weather expected through Wednesday before moderating through the weekend.

A much colder airmass filters in for Tuesday and Wednesday as a deep upper trough will be situated over the eastern CONUS. At the surface, broad high pressure near the Gulf Coast on Tuesday will shift northeast over the Mid-Atlantic on Wednesday. This will yield dry conditions under mostly sunny skies with highs mainly in the mid 30s to mid 40s. However, the pressure gradient will be tight over the area, so post-frontal winds with gusts up to 30-35 mph are possible on Tuesday. Also, with drier air filtering in, MinRH values are expected to fall into the 25-35% range.

By late week and into the weekend, temps begin to moderate back to seasonable and eventually seasonably warm levels. At this point, the next chance for precipitation is expected to be Friday night into Saturday with the next disturbance tracking north of the area. Shower chances range from 20-40%.

AVIATION /08Z MONDAY THROUGH FRIDAY/

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

Rest of tonight...Lowering CIGs and VSBYs as showers develop across the terminals. The showers will move N/NE across the area then decrease by dawn. Mostly IFR or low-end MVFR conditions with the showers but not all of the time. LLWS with 60 to 65 kts of winds at 020 thru dawn. Low confid overall.

Monday... Improving conditions following the overnight showers but just how much is not certain. We'll use VCSH for much of the day but bring another period of SHRA for the afternoon many sites. Mostly MVFR conditions but IFR/LIFR at KACY near the cold ocean waters. Low confidence overall.

Monday night...A strong cold front will cross the terminals between 01Z (KRDG/KABE) and 03Z/04Z (KMIV/KACY). LIFR conditions with +SHRA/+TSRA and very gusty winds with the activity surface gusts 45kt to 55kt expected. LLWS with S/SSW winds 65 to 75 kts at 020 and closer to 90 kts at 030. Rapidly improving conditions following the front with VFR a few hours after the frontal passage. Low confid overall.

Outlook...

Tuesday through Tuesday night...VFR. Gusty west winds up to 30 kt possible during the afternoon, diminishing into the evening.

Wednesday through Friday...VFR. No significant weather expected.

MARINE

Southerly winds will continue to increase and bring seas of 5-7 feet through the remainder of the overnight period, with SCA in effect for ocean waters. Continued increase in southerly flow through today will bring gale force winds, especially late in the day and during the evening, with Gale Warning remaining in effect. Gusty thunderstorms also possible, mainly later in the day and during the evening. Winds start to diminish late in the night.

Outlook...

Tuesday through Tuesday night...SCA conditions likely. West winds around 20-30 kt with seas around 5-8 feet.

Wednesday...Lingering SCA conditions possible. Winds generally below 20 kt with seas around 3-5 feet.

Wednesday night through Friday...No marine headlines expected. Winds mostly below 15 kt with seas around 2-4 feet.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Wind Advisory from 2 PM this afternoon to 2 AM EDT Tuesday for PAZ054-055-060>062-070-071-101>106. NJ...Wind Advisory from 2 PM this afternoon to 2 AM EDT Tuesday for NJZ007-009-010-012>027. Coastal Flood Advisory from 2 PM this afternoon to 2 AM EDT Tuesday for NJZ020-026. DE...Wind Advisory from 2 PM this afternoon to 2 AM EDT Tuesday for DEZ001>004. Coastal Flood Advisory from 2 PM this afternoon to 2 AM EDT Tuesday for DEZ003-004. MD...Wind Advisory from 2 PM this afternoon to 2 AM EDT Tuesday for MDZ012-015-019-020. MARINE...Gale Warning until 2 AM EDT Tuesday for ANZ430-431-450>455. Small Craft Advisory until 6 AM EDT early this morning for ANZ450>455.