textproduct: NWS Wilmington

WHAT HAS CHANGED

Forecast rain chances have been reduced by 10-20 percent for both Saturday and Sunday, but the likelihood for isolated heavy rainfall amounts still continues. Forecast low temperatures for Tuesday night and Wednesday night have been adjusted downward 2-3 degrees.

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.

This morning's 12z CHS weather balloon measured a precipitable water of 2.35 inches. That's one-third of an inch above the 90th climatological percentile and near the daily max for this time of year. Deep Gulf moisture will continue to stream northeastward across the eastern Carolinas through Sunday morning between a mid level ridge east of the Bahamas and a trough over the Mid- Mississippi Valley.

Surface-based CAPE near 2000 J/kg, forecast 0-6 km bulk shear around 10 knots, and little-to-no dry air aloft argues for single-cell storm development on Saturday with heavy rain potential. Some loose storm organization could occur along the seabreeze front and any outflow boundaries, but severe weather risk should remain very low. HREF ensemble 24-hr max precip of 3-5 inches implies there will be a few isolated heavy totals within the sea of 0.25 to 0.50 inch weekend amounts.

The upper trough producing the break in the ridge should slide eastward across the Carolinas on Sunday, 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.

850 mb temps in the +18C to +19C range should continue through the weekend. This is at or above the 90th climatological percentile for mid September. If it weren't for the increased cloudiness and convection expected this weekend we'd probably be looking at Heat Advisory-level conditions.

As it stands now, forecast highs in the upper 80s to around 90 coupled with dewpoints well into the 70s should yield heat indices in the 100-103 range through Monday.

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 /18Z FRIDAY THROUGH WEDNESDAY/

Dense tropical moisture streaming across the Carolinas should yield scattered showers and thunderstorms this afternoon. Coverage should be greatest in the vicinity of the seabreeze front near or just west of KCRE and KMYR through 21z. Scattered showers and storms inland will develop in loose clusters moving northeastward with the mid level winds. All of this convective activity will be accompanied by brief MVFR/IFR conditions, and should tend to dissipate this evening with the loss of sunlight.

Late tonight there is a moderate potential for LIFR ceilings to develop over KFLO and KLBT mainly after 10z, lingering through 13-14z much like we saw this morning. These low stratus clouds should lift into a cumulus cloud deck with bases 2500-3500 feet AGL after 14-15z Saturday with another round of showers and thunderstorms anticipated to develop during the heat of the day.

Extended Outlook...Scattered showers and thunderstorms bringing brief MVFR/IFR conditions are expected Saturday afternoon and again Sunday afternoon. 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 Tuesday night...Models continue to consistently show a cold front stalling across interior North Carolina tonight, then lifting back north on Saturday with no direct impact on our weather or winds this far south. Light to moderate southwesterly winds are expected to continue through the weekend along with multiple waves of showers and thunderstorms Saturday through Sunday night.

Drier weather should develop on Monday 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 from 9 PM this evening to 1 AM EDT Saturday for NCZ107. SC...None. MARINE...None.