textproduct: NWS Wilmington

WHAT HAS CHANGED

Forecast rain chances have been increased by ~20 percent both Saturday and Sunday afternoons, largely driven by NBM. Forecast high temps Saturday through Tuesday have been increased by 1-2 degrees over the previous forecast. This increases forecast heat indices by a few degrees through the period too.

KEY MESSAGES

1) Shower and thunderstorm chances will increase this weekend as a front approaches the area.

2) Heat and humidity will build and heat indices could exceed 100 degrees yet again.

DISCUSSION

KEY MESSAGE 1...Shower and thunderstorm chances will increase this weekend as a front approaches the area.

A shallow flow of Atlantic moisture should spread across eastern South Carolina on Thursday. This could lead to scattered showers breaking out during the day. Dry air aloft within the upper ridge should tend to limit coverage to 20-40 percent.

The persistent mid and upper level ridge over the Carolinas should begin to break down this weekend as a trough digs into the mid Mississippi valley. This should result in a deeper flow of Gulf moisture and precipitable water values should rise to 2.0-2.2 inches. An approaching synoptic cold front should stall out before reaching this far south, but an increasingly uncapped atmosphere and differential heating/seabreeze boundaries may provide enough support for numerous showers and storms inland on Friday, then across the entire area Saturday and Sunday. Three-day QPF is 0.50 to 1.00 inch with the obvious inclusion of higher amounts in heavier storms.

It's worth mentioning the increase in forecast rain chances in official forecasts is driven largely by NBM output. MOS forecasts based on the GFS and ECMWF remain a good 20-30 percent lower for PoPs through the weekend.

The very humid airmass will continue on into early next week, but the upper ridge may begin to restrengthen across the Carolinas. This is shown particularly in the 12z GFS and 00z ECMWF, but not the 00z Canadian which keeps the ridge farther west across the lower Mississippi Valley. A ridge returning would tend to suppress convective coverage which is what our forecast is based on for next week.

KEY MESSAGE 2...Heat and humidity will build and heat indices could exceed 100 degrees yet again.

Despite falling sun angles and shortening day lengths, the airmass across the Carolinas should return to unseasonably hot summertime levels this weekend into early next week.

850 mb temps in the +18C to +19C range would be slightly above normal in mid July, but now that we're approaching mid September it's above the 90th percentile and only about 1C away from records at the Morehead City upper air site. This should translate to inland high temps 89-93 degrees, which when combined with dewpoints well into the 70s should yield heat indices above 100 degrees.

As of now we won't have any record high temperatures forecast, but this just piles onto the excessive amount of heat and humidity we've dealt with across the Carolinas this year.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

VFR conditions will continue through the afternoon beneath a scattered 5000 foot cumulus cloud deck and light south to southeast surface winds. As winds diminish tonight, there is a moderate chance for MVFR fog to develop at KLBT and KFLO airports mainly after 10z. Both NBM and GFS hourly statistical guidance indicates some potential for very low IFR stratus ceilings to develop at KFLO as well. These early morning impediments should burn off 13-14z.

With richer moisture in place Thursday look for a more expansive and slightly cumulus cloud deck to develop during the mid to late morning hours. Scattered convective showers could begin to develop before 18z across South Carolina.

Extended Outlook...Although VFR conditions will prevail, there is increasing potential for pop-up showers and thunderstorms bringing brief MVFR/IFR conditions, especially Saturday and Sunday. There is also a low potential for late night/early morning stratus or ground fog, particularly affecting KLBT and KFLO Saturday and Sunday.

MARINE

Tonight through Monday...High pressure just off the East Coast should retreat farther east over the next couple of days as a cold front, currently extending from Pennsylvania to southern Illinois, approaches the area. At this time the front is expected to stall across inland North Carolina Friday into Friday night, then return north on Saturday. This should lead to primarily southwest winds across the area, interrupted at times by variable thunderstorm outflow winds as storms become more numerous across the area Saturday.

For early next week, what's left of the old front could get pushed south into the area as a new area of low pressure passes eastward across southeastern Canada. While there won't be an airmass change associated with this feature, converging/shifting winds could keep a chance for showers and thunderstorms in the forecast. Wind speeds should remain light outside of storms.

ILM WATCHES/WARNINGS/ADVISORIES

NC...Coastal Flood Advisory from 8 PM to 11 PM EDT this evening for NCZ107. SC...Coastal Flood Advisory from 6 PM to 9 PM EDT this evening for SCZ054-056. MARINE...None.