textproduct: Charleston

WHAT HAS CHANGED

All Key Messages and sections were updated.

KEY MESSAGES

- 1) Showers and thunderstorms will impact parts of the Charleston Tri-County this morning bringing a risk for heavy rainfall and localized flooding through mid-morning.

- 2) Another round of showers and thunderstorms will impact the region this afternoon ahead of the afternoon sea breeze.

- 3) A typical summertime pattern will remain in place through the upcoming weekend. Isolated strong to severe thunderstorms and the threat for localized heavy rainfall will be possible each day.

DISCUSSION

KEY MESSAGE 1: Showers and thunderstorms will impact parts of the Charleston Tri-County this morning bringing a risk for heavy rainfall and localized flooding through mid-morning.

A combination of modest to strong surface moisture convergence across the Atlantic is promoting the development of showers/tstms offshore early this morning. As the mean steering flow steadily backs from the southwest to south the risk for convection moving inland from off the Atlantic will increase across parts of the lower South Carolina coast through daybreak with the greatest coverage expected to occur across the eastern portions of the Charleston Tri-County region, including Downtown Charleston. Satellite derived PWAT data from GOES-E show a large plume of PWATS >2.25" lurking just offshore which matches output from the latest RAP. The RAP shows this area of PWATS 2.25-2.50" advecting quickly onshore across the Charleston Tri- County over the next few hours as bands of showers/tstms propagate inland. Intense hourly rainfall rates will likely occur within pockets of deeper convection as PWATS surge, especially where a ribbon of MLCAPE 1500-2000 J/kg is forecast to nudge inland from off the Atlantic. This will bring a risk for heavy rainfall and possibly some localized flooding across parts of Charleston, Berkeley, Dorchester and Colleton Counties, especially in the urban areas across the Charleston Metro Area as well as low-lying and poor drainage areas in the more rural locations. Downtown Charleston will need to be watched carefully as a quick 1-2" in an hour could easily yield significant issues should convection begin to align across the peninsula. The 05/00z HREF neighborhood probabilities for >1"/3hrs are running about 40-70% and 10-20% for >3"/3hrs, highest roughly along and east of an Edisto Island-North Charleston-Jamestown line.

KEY MESSAGE 2: Another round of showers and thunderstorms will impact the region this afternoon ahead of the afternoon sea breeze.

This Afternoon and Tonight: The risk for isolated to scattered showers/tstms will refocus inland this afternoon/evening as a modest sea breeze moves inland. The upper low to the west will begin to weaken today and is forecast to gradually open up into a wave by tonight as a strong shortwave pivots into the Mid- Mississippi Valley. Similar to yesterday, there are no discernible spokes of vorticity that are progged to cross the area during the diurnal maxim. There is no meaningful forcing is noted in model cross sections as a result. Convection will therefore once again be airmass driven and its evolution will be mainly driven by mesoscale processes. An isolated strong to severe tstm could occur just about anywhere along/west of the sea breeze. Activity will quickly wane this evening, but there are increasing signals that another round of noctural convection could develop over the Atlantic overnight as low-level convergence refocuses closer to the west wall of the Gulf Stream. The 850-250 hPa steering flow looks to remain south to south/southeast through the night, so this activity will likely push onshore early Thursday morning and impact areas roughly along/east of the I-95 corridor.

KEY MESSAGE 3: A typical summertime pattern will remain in place through the upcoming weekend. Isolated strong to severe thunderstorms and the threat for localized heavy rainfall will be possible each day.

Thursday through the weekend: The pattern aloft will be dominated by deep high pressure over the western Atlantic and the western CONUS. This will support daily scattered convection through the weekend, with the sea breeze and any remnant outflow boundaries serving as the primary focus for afternoon and evening thunderstorm development. Persistent southerly flow will continue to advect deep moisture into the region. Moderate instability looks to develop each afternoon, with SBCAPE values rising to 2500-3000 J/kg. PWAT values near 2 inches nearly each day will support efficient rainfall rates, with some storms capable of dropping a quick 1-2 inches of rain in a short period. In addition, slow storm motions of around 10 kt, combined with training or repeated rounds of convection, could increase the potential for isolated instances of minor flooding, especially in low-lying and poor drainage areas. Friday could be the exception, as somewhat drier air with pockets of PWATs around 1.5 to 1.7 inches should limit the flooding threat. Lastly, with little shear in the environment, storms will remain pulse-like. Although widespread severe weather is not expected, a few stronger storms could produce damaging wind gusts.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

05/12z TAFs: KCHS/KJZI: Showers/tstms are beginning to fire along the coast early this morning which will bring impacts to KJZI and possibly KCHS through mid-morning before weakening. TEMPO groups for TSRA were maintained at both terminals. At KJZI, conditions were limited to MVFR for now, but if convection consolidates then prevailing with occasional IFR or lower conditions may need to be considered. Activity should exit the terminals by mid- morning (14-15z). For the afternoon, showers/tstms should remain west of the terminals. Another round of nocturnal showers/tstms are expected to move onshore from off the Atlantic early Thursday morning. A PROB30 group for SHRA was included at KCHS and TSRA at KJZI from roughly 09-12z.

KSAV: VFR for much of the period. The risk for showers/tstms looks to consolidate west of the terminal this afternoon, so the previous mention of TSRA was removed for the 06z TAFs.

Extended Aviation Outlook: Periodic flight restrictions are expected in showers and thunderstorms this week.

MARINE

Through Tonight: Widespread convection offshore of lower south Carolina coast this morning will pose hazards to mariners, including gusty winds, cloud-to-waters lightning, waterspouts and locally heavy rainfall reducing vsbys to 1 NM or less. A southerly flow regime will persist through tonight and winds should average 15 kt or less (outside of the nocturnal surge in progress this morning). Winds and seas will be higher in and near tstms. Another round of nocturnal convection will impact the waters overnight into early Thursday morning. Seas will average 2-4 ft.

Thursday through Sunday: Atlantic high pressure will be the dominant synoptic feature through the weekend. This will generate a typical summertime wind field with increased winds along the land/sea interface each afternoon associated with the sea breeze. Marine conditions are expected to remain below Small Craft Advisory criteria through the weekend.

Waterspouts: Forecast sounding output and elevated non-supercell tornado parameters values over the nearshore and offshore waters look to support a risk for waterspouts this morning. A Marine Weather Statement was issued to alert mariners to this possibility.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...None.