textproduct: Charleston

WHAT HAS CHANGED

The Aviation section was updated for the 12z TAF cycle.

KEY MESSAGES

- 1) There is risk for locally heavy rainfall and isolated flash flooding across Southeast Georgia into the western South Carolina Lowcountry this afternoon and evening. A few strong to marginally severe thunderstorms are possible.

- 2) Scattered to numerous showers and thunderstorms will bring the potential for heavy rainfall, localized flooding, and/or gusty winds Sunday through early next week.

DISCUSSION

KEY MESSAGE 1: There is risk for locally heavy rainfall and isolated flash flooding across Southeast Georgia into the western South Carolina Lowcountry this afternoon and evening. A few strong to marginally severe thunderstorms are possible.

The upper flow will become increasingly cyclonic through the day as a powerful upper low digging across Mid-Mississippi Valley propagates into the western Ohio Valley by tonight. At the surface, a weak warm front extending across parts of the Lowcountry into the Midlands will meander north through the day placing all of Southeast South Carolina and Southeast Georgia solidly within the warm sector of the maturing cyclone associated with the upper low. A few morning showers could brush parts of the lower South Carolina coast through daybreak, but the better chances or showers/tstms will hold off until later this afternoon and evening as the sea breeze moves inland and potent shortwave energy approaches from the northwest. Forcing with this feature coupled with modest instability will support scattered to numerous showers/tstms by mid-late afternoon with activity likely concentrating across much of Southeast Georgia into the far western South Carolina Lowcountry by early evening.

PWATs are forecast to surge to 2.35-2.50" which will be near daily maximums for 1 August per SPC sounding climatology. While 850-200 hPa flow will remain out of the southwest at around 10 kt suggests storm motions should remain somewhat progressive, mesoscale boundary collisions will likely augment the steering environment somewhat with the potential for training and/or back-building increasing with time as CAPE profiles elongate and narrow (a classic heavy rain signature). Given the near record net moisture values, impressive hourly rainfall rates will likely occur with the 01/00z HREF and REFS both suggesting localized amounts in excess of 3-4"/3-hr are possible (neighborhood probabilities for >3" running 30-50%), mainly across interior portions of Southeast Georgia into Allendale and Hampton Counties in Southeast South Carolina. This could lead to pockets of flash flooding even with fairly high 1-hr and 3-hr flash flood guidance values in place. The need for a Flood Watch for the above mentioned areas was considered, but the decision was made to hold off given the degree of flash flooding should remain fairly isolated and likely limited to the base- level category. Coordinated with WPC to introduce a slight risk across the far interior areas through tonight for the Day 1 WFO excessive rainfall guidance. Right now, the corridor of heaviest rains look to remain west of both the Charleston and Savannah Metro areas. In addition to the risk for heavy rainfall and isolated flash flooding, a strong to marginally severe tstm could occur just about anywhere this afternoon and evening where updrafts can become enhanced, most likely near mesoscale boundary collisions.

Convection should wane during the evening hours, but there are signals that a secondary wave could spread north through the region early Sunday morning and more shortwave energy approaches and interacts with a continued very moist and unstable atmosphere.

KEY MESSAGE 2: Scattered to numerous showers and thunderstorms will bring the potential for heavy rainfall, localized flooding, and/or gusty winds Sunday through early next week.

As a mid-level trough settles over the eastern CONUS Sunday, a cold front will inch toward the forecast area Sunday into Monday before stalling. This pattern, combined with a very moist airmass, will support scattered to numerous showers and thunderstorms each day through early next week. Forecast soundings indicate PWAT values increasing to around 2.2 inches by Sunday morning and remaining between 2.2 and 2.4 inches through Monday and possibly Tuesday. These conditions will favor efficient rainfall production and high rainfall rates, increasing the potential for localized flooding. While most flooding impacts should remain minor, repeated heavy downpours or training convection over urban areas such as the Charleston and Savannah metro areas could warrant Flood Advisories or Flash Flood Warnings.

With regards to the severe potential, the primary limiting factor will be modest instability, with forecast soundings suggesting SBCAPE peaking near 1500 J/kg as widespread cloud cover limits diurnal heating. Despite this, the combination of deep moisture and forcing aloft as lobes of vorticity rotate around the upper trough, should support at least scattered thunderstorms, with the strongest cells capable of producing isolated damaging wind gusts. Sunday appears to have the greatest threat for isolated strong to severe storms due to the proximity of the approaching front providing an additional source of forcing, but also increasing shear values to around 15 kt. This could could allow some storms to become somewhat organized, especially inland.

AVIATION /10Z SATURDAY THROUGH WEDNESDAY/

01/12z TAF Discussion: KCHS/KJZI: VFR should prevail for much of the 12z TAF period, trending to MVFR prior to daybreak Sunday The risk for showers/tstms should generally remain west of both terminals this afternoon/evening, but could get close enough to justify a PROB30 at KCHS from roughly 19-21z until the sea breeze can push farther to the west. A secondary around of showers/tstms will move in from the south prior to daybreak Sunday, which could produce MVFR conditions. A PROB30 group was introduced 09-12z at both KCHS and KJZI to address possibility.

KSAV: VFR this morning with an increasing risk for tstm impacts by mid-afternoon. A TEMPO group for MVFR conditions in TSRA were introduced 21-00z. Conditions could certainly end up lower to IFR or even LIFR levels at times, but pinning that down this far out is nearly impossible. Conditions were limited to MVFR for now. In addition, gusty winds could accompany the tstms, but to what degree is uncertain. Further refinements will mostly certainly be made as the day progress and short term trends become more apparent. A secondary around of showers/tstms will move in from the south prior to daybreak Sunday, which could produce MVFR conditions. A PROB30 group was introduced 09-12z at both KCHS and KJZI to address possibility.

Extended Aviation Outlook: Periodic flight restrictions are expected in showers and thunderstorms through early next week. Locally heavy rainfall may produce brief IFR or lower visibility restrictions.

MARINE

Through Tonight: There are no concerns. South to southeast winds this morning will persist today, then become more south to southwest tonight. Winds will remain 15 kt or less today, then increase to 15 kt to 15-20 kt tonight as nocturnal surging takes hold. Seas 1-3 ft today will build to 2-4 ft tonight.

Sunday through Thursday: The pressure gradient will strengthen Sunday as broad low pressure develops inland with an approaching front and high pressure remaining in the Atlantic. Southwesterly winds will increase during the afternoon, with frequent gusts between 25 and 30 kt expected across the local waters. In addition, wave heights are forecast to increase to 4 to 7 feet Sunday evening. Small Craft Advisories have been issued for the nearshore marine zones out to 20 nm and the outer Georgia waters from 20-60 nm, as well as the Charleston Harbor. Conditions should gradually improve Monday night, with benign wind and sea conditions expected for Tuesday through the rest of the period.

Rip Currents: Gusty southwest winds combined with a 7 to 8 second swell will support an elevated rip current risk on Sunday. A Moderate Risk of Rip Currents has been posted for the South Carolina beaches, and a similar risk may eventually be needed along the southeast Georgia coast.

CLIMATE

Record High Minimum Temperatures:

August 1: KCHS: 81/1999 KCXM: 86/1999 KSAV: 79/2020

August 2: KCHS: 79/2022 KCXM: 84/1999 KSAV: 80/1886

August 5: KCHS: 79/1980 KCXM: 82/1991 KSAV: 80/1883

August 6: KCHS: 79/1991 KCXM: 82/2006 KSAV: 80/1899

August 7: KCHS: 79/2007 KCXM: 83/2007 KSAV: 80/1879

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory from noon Sunday to 2 AM EDT Monday for AMZ340. Small Craft Advisory from noon Sunday to 2 AM EDT Tuesday for AMZ360-362. Small Craft Advisory from noon Sunday to 8 AM EDT Monday for AMZ364. Small Craft Advisory from noon Sunday to noon EDT Monday for AMZ384.