textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Wind Advisory issued for southern and central New Jersey and southern and central Delaware for Monday afternoon and Monday night.

Gale Watch upgraded to Gale Warning for all waters Monday into Monday night. Small Craft Advisory issued for tonight for ocean waters.

Confidence in potential severe weather continues to increase for Monday into Monday night.

KEY MESSAGES

1. There is an increasing potential for strong to severe thunderstorms Monday into Monday night 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. 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 into Monday night 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.

Mid level ridging today will give way as a deepening surface low pressure system develops on the lee side of the Rockies this 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 Monday evening.

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. 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. High res guidance show sufficient low level support to see STP values greater than 0.5-1 during a brief window Monday 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 dewpoints surge higher than initially forecast, instability will increase quite a bit, and this would lead to the potential for some isolated supercells to develop. The primary threat remains wind, however the threat for tornados 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 are brief in nature.

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 into Monday night, locally up to 50 mph. This will help usher in an unseasonably warm and humid airmass. A Wind Advisory has been issued for locations mainly southeast of I-95 and US 13 just for the synoptic flow winds late Monday and Monday evening.

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.

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 /18Z SUNDAY THROUGH FRIDAY/

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

Rest of today...VFR with mid-level clouds. E/SE winds increasing to around 10 kts with a few gusts up to 20 kts. High confid.

Tonight...Lowering CIGS then VSBYs with rain showers and fog. VFR early then IFR developing at most TAF sites by around midnight. LLWS with S/SE winds 40 to 50 kts at 020. A thunderstorm can't be completely ruled out late tonight, but kept out of TAFs for now. Medium confid.

Monday...Intervals of IFR to LIFR with showers, gusty southerly winds and LLWS from S at 50-55 kts at 020. Thunderstorms possible, especially in the afternoon, but kept out of the morning period for the time being. Medium confidence.

Outlook...

Monday night...Showers and t-storms with potential sub-VFR conditions are likely at times in the evening before conditions improve later at night. Gusty southwest winds may approach 30-35 kts with gusts 35-45 kts possible, not inclusive of stronger gust potential in storms.

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

MARINE

Sub-SCA rest of the day. Building southerly flow will bring seas of 5-7 feet tonight, with SCA in effect for ocean waters. Continued increase in southerly flow on Monday will bring gale gusts, especially late in the day and during the evening, with Gale Warning now in effect. Gusty thunderstorms also possible, mainly later in the day and during the evening. Winds start to diminish late in the night.

Outlook...

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...Wind Advisory from 2 PM Monday to 2 AM EDT Tuesday for NJZ012>014-016>027. DE...Wind Advisory from 2 PM Monday to 2 AM EDT Tuesday for DEZ002>004. MD...None. MARINE...Gale Warning from 6 AM Monday to 2 AM EDT Tuesday for ANZ430- 431-450>455. Small Craft Advisory from 6 PM this evening to 6 AM EDT Monday for ANZ450>455.