textproduct: NWS Wilmington

WHAT HAS CHANGED

Synoptic models (GFS, 12km NAM, ECMWF) and their associated MOS output maintain very high shower/thunderstorm chances today. However the latest suite of mesoscale/convection allowing models show very little convection developing.

While we have large, uncapped CAPE within an unseasonably warm airmass with near-record high precipitable water values, outside of this afternoon's seabreeze front and differential heating developing around the edges mid level cloud patches, it's hard to pinpoint obvious convective triggers. An upper disturbance lifting northeastward across the area should move up into central North Carolina around 2 pm, missing peak heating this far south.

For the forecast update I've blended high-res models into today's hourly PoP forecast which reduces values by roughly 10-20 percent. The expectation is there still will be scattered slow- moving thunderstorms this afternoon with locally heavy rainfall, but coverage will be less than was earlier thought. The area I'd pinpoint for the highest rainfall potential is the seabreeze convergence zone from Georgetown across inland Horry County to near Whiteville this afternoon.

KEY MESSAGES

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

2) Well-above normal temperatures and high humidity will maintain daytime heat indices at or above 100 degrees through Monday.

3) A modified Canadian airmass should bring near-normal temperatures beginning Tuesday.

DISCUSSION

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

Deep Gulf moisture in place over the Carolinas and above-normal temperatures will produce widespread showers and thunderstorms this afternoon. High resolution models have maintained poor consistency each hour. Surface convergence along the sea breeze is likely to be the main initiator of convection after noon. A few isolated showers are possible in coastal southeast NC this morning due to onshore flow. Weak shortwave energy lifting northward through the Carolinas suggests that some development is possible during the afternoon for inland areas in additional to surface convergence along the sea breeze. This lift is weak and would favor the NC/SC border and points northward into the Sandhills.

Weak shear should keep the potential for severe weather low. Some heavy rain is possible given the near record PW and slow storm motions.

Drier air moves into the mountains and western Piedmont on Sunday, but the moisture-laden atmosphere will largely remain in tact near the coast. The upper trough producing the break in the ridge should slide eastward across the Carolinas, perhaps enhancing bulk shear to around 20 knots. Multicells should become possible, perhaps bringing better potential for gusty winds. The upper trough should shift offshore Sunday night ahead of rising mid and upper level heights spreading eastward from the Mid South.

KEY MESSAGE 2... Well-above normal temperatures and high humidity will maintain daytime heat indices at or above 100 degrees through Monday.

Above-normal temperatures continue to sit near the 90th percentile for mid September. Deep moisture should produce widespread cloudiness in addition to convection this weekend. These factors will keep heat indices below the 105 degree threshold for heat advisories.

Ensemble spread for 850 temperatures remains low through Monday, the latest forecast keeps forecast highs in the upper 80s to around 90 through Monday. With dewpoints well into the 70s, heat indices will remain in the 100-103 range through Monday. Monday is expected to be the warmest of these days with decreased cloudiness.

NWS ILM has never issued a Heat Advisory later than September 5 since the introduction of VTEC product codes twenty years ago. (and that latest advisory was earlier this month)

KEY MESSAGE 3...A modified Canadian airmass should bring near-normal temperatures beginning Tuesday.

Models have all come into line over the past 24 hours showing the strongest Canadian high we've seen since last Spring moving across New England Tuesday into Wednesday. This should push a cold front southward across the eastern Carolinas Monday night, allowing a modified Canadian airmass to spread across the area Tuesday into Wednesday. This won't be crisp, dry Canadian air by any means given its long passage across water temps in the 70s and 80s, but it should bring cooler air across the area knocking temperatures back down closer to normal for mid September.

Forecast lows in particular look refreshing with 60s expected for all but east-facing beaches Tuesday night and Wednesday night. Some inland locations could conceivably dip to 60 degrees.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Rest of Tonight...The 12 UTC Aviation forecast is an update to the previous forecast as the restrictions are underway inland. The remainder of the forecast remains intact.

Mixing should maintain VFR for coastal terminals, but can't rule out some brief MVFR CIGs. Light boundary layer winds, wet soils, and enhanced low level moisture should produce IFR across a large portion of inland terminals. Expect some CIGs to reach LIFR by sunrise. Light wind should mix-out restrictions a couple of hours after sunrise.

Saturday... Isolated showers near the NC coast may product brief restrictions at ILM this morning (around sunrise). As the sea breeze deepens during the early afternoon, convection should mature near the coast and gradually push inland through the afternoon. Additional development is possible inland during the early afternoon. Most areas are expected to see impacts from thunderstorms by the end of the day as showers become widespread.

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

Southerly flow will continue to supply a warm and humid air mass through the weekend. This moisture-laden air mass will bring multiple rounds of showers and thunderstorms both Saturday and Sunday night. Southerly winds around 10-15 knots. The sea breeze will produce localized enhancement of gusts to 20 knots each afternoon.

Drier weather should develop on Monday evening as high pressure strengthens aloft over the Carolinas. The strongest Canadian high we've seen since last Spring should move across New England Tuesday and Wednesday, pushing a cold front southward across the Carolina coastal waters Monday night. Northeast winds behind the front should increase to a solid 20 knots offshore and 15-20 knots nearshore Tuesday into Tuesday night. Seas within 20 miles of shore could build to 4-6 feet with dominant wave periods near 6 seconds.

ILM WATCHES/WARNINGS/ADVISORIES

NC...Coastal Flood Advisory until 1 PM EDT this afternoon for NCZ107. SC...None. MARINE...None.