textproduct: NWS Wilmington

WHAT HAS CHANGED

No significant changes have been made to the forecast over the weekend into early next week.

KEY MESSAGES

1) Shower and thunderstorm chances will increase this weekend.

2) Heat and humidity will build and heat indices should exceed 100 degrees through Monday.

3) Grand Strand minor coastal flooding is possible during multiple high tides over the coming days.

DISCUSSION

KEY MESSAGE 1...Shower and thunderstorm chances will increase this weekend.

Models look broadly similar to yesterday showing the persistent mid and upper level ridge across the Southeast U.S. breaking down this weekend as a shortwave digs into the mid-Mississippi Valley region. This will remove the subsidence inversion that's been capping us for days, and establish a deeper flow of Gulf moisture that should bring precipitable water value up toward 2.2 inches by Saturday.

Today's isolated showers are being vertically inhibited by warm, dry conditions aloft within the upper ridge. Once that factor is removed this weekend, forecast rain chances blossom: 20-40 percent Friday, 60-80 percent on Saturday, and 50-60 percent on Sunday.

None of these days appear to be washouts and model storm-total QPF through Sunday appears to have decreased in spots from yesterday. A few lucky locations should still pick up 2-3 inch totals over the coming days, with half inch totals likely more common. Storm mode should remain single-cell with 0-6 km bulk shear 10 knots or less. Loose storm organization along outflow boundaries and the seabreeze front should occur.

Storm coverage should begin to decline Monday as the 700-500 mb ridge redevelops just to our west, intercepting the flow of Gulf moisture. The subsidence inversion reappears in forecast soundings by Tuesday which should lead to much drier conditions Tuesday and Wednesday.

KEY MESSAGE 2...Heat and humidity will build and heat indices should exceed 100 degrees through Monday.

Light south to southwest low level winds should advect plenty of Atlantic and Gulf moisture across the Carolinas through the weekend. Dewpoints remaining in the 70s coupled with well-above normal high temperatures in the lower to middle 90s should equal heat indices rising above 100 degrees each day through Monday.

Tomorrow (Friday) appears to be the hottest day of the period. Forecast 850 mb temps near +19C are 4C above normal and 1.5C above the 90th climatological percentile at the KMHX upper air site. Our current forecast high for Friday is still 4-5 degrees F below daily records.

This unseasonably hot airmass should remain across the area through Monday, but will be tempered by additional clouds and precipitation. By Tuesday, Canadian high pressure moving across New England should introduce a northeasterly low level flow which should bring a slightly cooler marine airmass onshore with forecast highs generally in the 85-90 degree range Tuesday and Wednesday.

KEY MESSAGE 3...Minor Grand Strand coastal flooding is possible during multiple high tides over the coming days.

September's New Moon occurs today which is leading to a period of larger astronomical tides. Tidal anomalies along the Grand Strand beaches as measured at Myrtle Beach's Springmaid Pier should average around +1.0 feet today through the weekend. Half to two-thirds of this anomaly is likely due to sea level rise leaving onshore winds responsible for the remainder.

Forecast water levels should exceed flood stage at Springmaid Pier by about 0.3 feet this evening, and may reach or slightly exceed flood stage during high tides Friday morning through Sunday morning. See the recently-issued Coastal Flood Advisory for details.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

An area of showers offshore is spreading broken low stratocumulus back toward the Grand Strand coastline. There is a low probability of ceiling impacts at KMYR and KCRE between 18-20z before the offshore showers weaken or dissipate. New convective showers are developing inland along and west of the South Carolina seabreeze front this afternoon, but the probability of impacts on airports is too low for mention in the TAFs at this time.

Isolated showers should dissipate this evening and VFR conditions are expected overnight. New convective showers may begin to develop again Friday along the seabreeze front prior to 18z.

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 Tuesday...High pressure off the Southeast coast is weakening as a cold front moves southward. This front is still expected to stall across interior North Carolina Friday night before lifting back to the north Saturday. South to southwesterly winds should continue with speeds less than 15 knots.

Increasing coverage of showers and thunderstorms is expected beginning late Friday night as Gulf moisture increases across the area, and as high pressure aloft dissipates and is replaced by cyclonic flow ahead of an upper level trough to our west. Rather high chances for showers and storms should continue until Monday when high pressure should begin to redevelop aloft.

ILM WATCHES/WARNINGS/ADVISORIES

NC...Coastal Flood Advisory from 9 PM this evening to midnight EDT tonight for NCZ107. SC...Coastal Flood Advisory from 7 PM to 10 PM EDT this evening for SCZ054-056. MARINE...None.