textproduct: NWS Wilmington

WHAT HAS CHANGED

Forecast PoPs have been reduced this afternoon into this evening as various inhibiting factors are preventing widespread storms from forming. Models are trending cooler with the incoming Canadian airmass Monday night through Wednesday.

KEY MESSAGES

1) Deep Gulf moisture and a break in the ridge aloft will maintain good chances for showers and thunderstorms through Sunday.

2) A modified Canadian airmass should bring cooler temperatures beginning Tuesday.

DISCUSSION

KEY MESSAGE 1...Deep Gulf moisture and a break in the ridge aloft will maintain chances for showers and thunderstorms through Sunday.

Forecast convective development today has been scaled back significantly compared to what NBM and synoptic models were suggesting over the past 2-3 days. On a broad scale the culprit may be subsidence from persistent offshore convection. On a more local scale, persistent mid level clouds inland, subsidence behind a subtle shortwave lifting northward across North Carolina, or the lack of strongly convergent surface boundaries could be to blame. Forecast PoPs through early evening have been scaled back to only 30-40 percent which is still higher than most of the recent CAMs suggest.

For any storms that do manage to develop this afternoon, the airmass should still support locally heavy rain given slow cell motion and anomalously high precipitable water values near 2.25 inches.

CAMs suggest new showers and thunderstorms may begin to develop across the coastal waters around midnight. Mid level winds would tend to bring these northeastward where they could drop rain across portions of Brunswick and New Hanover counties late tonight into Sunday morning.

Sunday's daytime seabreeze front may be the focus for additional scattered showers and storms. The mid level flow will veer more westerly on Sunday as a shortwave moves eastward across the Carolinas. Bulk shear should also increase to 15-20 kt which may lead to multicell storms redeveloping along boundaries like the seabreeze front. Drier conditions developing aloft inland during the afternoon should tend to reduce coverage of showers or storms there.

KEY MESSAGE 2...A modified Canadian airmass should bring cooler temperatures beginning Tuesday.

The strongest Canadian high, 1030 mb, we've seen since last spring will move eastward across southern Quebec and New England Monday through Tuesday, pushing a cold front southward. Models are in excellent agreement showing the boundary will push across Cape Fear early Monday evening, racing southward past Myrtle Beach and Georgetown before midnight.

Modified Canadian air will spread southwestward behind the front Monday night, scouring out the excessive humidity and unseasonable warmth of the past week. Given the long path over 75-85 degree water off the East Coast, this won't feel like a crisp fall airmass although it will be cooler and drier.

Forecast highs Tue-Thu should hold in the 80s, close to normal for this time of year. The big improvement will be with nighttime lows: models have continued to trend lower each run and now show some potential for upper 50s in spots inland Tuesday night. Dewpoints in the 50s and 60s will feel very refreshing.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Scattered showers and thunderstorms are expected to develop across this afternoon in an unseasonably warm and humid airmass. Triggers for storm development should include the seabreeze front pushing inland from the beaches, and differential heating along the edges of the persistent mid level cloud deck inland. Coverage should average 30-50 percent of the area this afternoon which explains the PROB30 group for the coastal airports and the TEMPOs inland. Convection should diminish in coverage around sunset.

VFR conditions overnight will likely become interrupted by low stratus developing at KLBT and KFLO late tonight, mainly after 09z. This cloud cover should linger through 14-15z Sunday before mixing out into VFR conditions during the late morning. Along the coast, high-res models suggest marine convection streaming northeastward could affect the KILM airport after sunrise. Sunday's seabreeze front could also be a potential focus for new showers or storms affecting KMYR and KCRE mainly after 16z.

Extended Outlook... Scattered showers and thunderstorms bringing brief MVFR/IFR conditions are expected to continue into Saturday evening. Some showers are possible on Sunday, but coverage should be less than previous days. There is also a low potential for late night/early morning stratus or ground fog affecting KLBT and KFLO late Saturday night and again late Sunday night. VFR conditions should prevail Monday through Wednesday as a more stable and drier Canadian airmass builds in from the northeast.

MARINE

Tonight through Thursday...Aside from anticipated waves of showers and thunderstorms developing late tonight into Sunday morning, then additional isolated showers/storms Sunday afternoon along the seabreeze front, marine weather conditions won't be too bad for the next couple of days. Synoptic winds from the southwest should average 10-15 knots with seas averaging 2-3 feet in a mixture of wind chop and a 9 second southeasterly swell.

Big changes should arrive Monday evening as a cold front sweeps quickly across the area from the north. A surge of northeasterly winds should follow the the front, driven southward by the strongest Canadian high we've seen since last spring. Northeast winds 15-20 knots, occasionally reaching 25 knots at times, should develop Monday night and persist through Wednesday. There's better than 50-50 chances wind/wave conditions will necessitate a Small Craft Advisory beginning Monday evening.

ILM WATCHES/WARNINGS/ADVISORIES

NC...Coastal Flood Advisory until 1 PM EDT this afternoon for NCZ107. Coastal Flood Advisory from 10 PM this evening to 1 AM EDT Sunday for NCZ107. SC...None. MARINE...None.