textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Gale Watch issued for all marine zones beginning Monday morning through early Tuesday morning.

Increasing threat for severe weather Monday into Monday night.

KEY MESSAGES

1. There is an increasing potential for strong to severe thunderstorms Monday along and ahead of a strong cold front. Localized flash flooding will also be possible.

2. There will be a brief return to much colder conditions heading into the middle of next week.

DISCUSSION

KEY MESSAGE 1...There is an increasing potential for strong to severe thunderstorms Monday along and ahead of a strong cold front. Localized flash flooding will also be possible.

Mid level ridging over the weekend will give way as a deepening surface low pressure system develops on the lee side of the Rockies Sunday afternoon. The upper level associated trough rapidly turns negatively tilted and leads to a strong mid latitude cyclone developing over the Great Lakes. This system will continue to push north towards James Bay in Quebec and will usher in a potent cold front over the region.

A warm front will push north through the region Monday morning bringing the warm sector of the cyclone over the Mid Atlantic. This will usher in both warmer temperatures and higher dew points. The southerly flow associated with this system will transport a deep moisture plume into the region ahead of the frontal system. Medium range 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 and upper level dynamics will promote lift across the region on Monday.

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 will support organized convection Monday. Instability will be the limiting factor however the dynamics should play a strong enough role in overcoming that instability. We're just getting into the window of some regional and high res guidance, which show sufficient low level support to see STP values greater than 0.5-1 during a brief window Monday late afternoon/evening. Will continue to monitor the increasing potential for severe weather with the current threats being strong to damaging winds and we can not rule out the potential for tornadoes to develop. It is worth noting that the latest outlook from the Storm Prediction Center has expanded the ENHANCED risk of severe weather farther east into our County Warning area. Finally, it should be noted that even outside of any thunderstorms the strong wind field will support southerly winds gusting at least 35 to 45 mph Monday. This will help usher in an unseasonably warm and humid airmass.

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. Longer range MMEFS guidance is showing modest rises in rivers so we'll continue to watch for any potential hydro concerns.

KEY MESSAGE 2...There will be a brief return to much colder conditions heading into the middle of next week.

As a strong cold front shifts offshore during Monday night, strong cold air advection develops. Model guidance is highlighting the potential for snow on the back side of the front however we anticipate that the most likely solution is that as the cold air arrives, it will also be ushering in quite dry air and so while there may be a few flurries or light snow showers, any impactful accumulation is not expected.

A much colder air mass settles in for Tuesday with temperatures struggling to reach 40 degrees in some areas. A gusty westerly wind will also add a chill factor with wind chill values down into the upper teens to low 20s early Tuesday morning and 20s and 30s during the afternoon. Still chilly on Wednesday however less wind is forecast.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

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

Tonight... VFR. Winds become light/variable overnight before switching to the east late. High confid.

Sunday...VFR with E/SE winds increasing to around 10 kts with a few gusts up to 20 kts. High confid.

Outlook...

Sunday night...Conditions drop below VFR, possibly below MVFR, with rain enveloping the region.

Monday and Monday night...Conditions may improve to VFR at times as rain departs, but more showers and t-storms with potential sub-VFR conditions are likely at times especially later in the day before conditions improve later at night. Gusty southwest winds may approach 30-35 kts with gusts 35-40 kts possible at night behind fropa, not inclusive of stronger gust potential in storms.

Tuesday through Thursday...VFR prevails with diminishing winds.

MARINE

Winds and seas will continue to diminish tonight with Sub Small Craft Advisory level conditions expected through the day Sunday.

Outlook...

Gale Watch issued beginning Monday morning through early Tuesday morning.

Small craft conditions ramp up to near gales late Sunday night into Monday morning, with gales more likely Monday night before ramping back down on Tuesday. Rain also likely Sunday night, with showers and possibly gusty thunderstorms expected Monday into Monday night.

Sub-SCA winds may return by late Tuesday night or Wednesday, but seas may remain elevated. Sub-SCA conditions prevail Thursday.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Monday morning through late Monday night for ANZ430-431-450>455.