textproduct: Charleston

WHAT HAS CHANGED

All Key Messages and sections were changed.

KEY MESSAGES

- 1) Rounds of showers and thunderstorms will bring a risk for heavy rainfall and localized flooding through this evening. A few strong to marginally severe thunderstorms could also occur.

- 2) Wet conditions will bring the potential for heavy rainfall and localized flooding Monday, with the threat possibly lingering into Tuesday.

- 3) There is a High Risk for rip currents for the Charleston County beaches today through Monday with a Moderate Risk elsewhere.

DISCUSSION

KEY MESSAGE 1: Rounds of showers and thunderstorms will bring a risk for heavy rainfall and localized flooding through this evening. A few strong to marginally severe thunderstorms could also occur.

The region will remain embedded within the warm sector of a maturing storm system centered over the Ohio Valley through tonight. Two distinct rounds of showers/tstms are expected to impact the area today.

Round 1: The first round will be associated with shortwave energy exiting out of the northeast Gulf. This feature is already producing deep convection over Apalachee Bay early this morning and is poised to spark additional convection off the Georgia and lower South Carolina coast through daybreak as forcing for ascent propagates in from the south and southwest. Guidance is generally similar in showing the bulk of this activity remaining just offshore with its western periphery impacting coastal areas of Colleton, Charleston into eastern Berkeley Counties through mid-morning. Elevated PWATS 2.35-2.50" coupled with MLCAPE 1000-1500 J/kg sourced from off the Atlantic will support a risk for locally heavy rainfall, which could poise a risk for some urban flooding for parts of the Charleston Metro Area, especially Downtown Charleston. Both the 02/00z HREF and REFS suggest a quick 1.5-2.5" could fall in spots over a very short period with neighborhood probabilities for >1"/3-hr running about 50-70% and for >3"/3-hr running 20-30%. Thankfully, the mean 850-250 hPa steering flow looks to remain fairly progressive which should limit the flood potential somewhat, but few Flood Advisories or even an isolated base-level Flash Flood Warning may be needed later this morning.

Round 2: The second round should hold off until later this afternoon as the first batch of convection exits the area and instability steadily builds in its wake. Guidance suggests another shortwave rounding the sharpening/elongating base of the weakening upper low over the Mid-Mississippi Valley will approach the region later this afternoon and pushing offshore prior to midnight. Similar to yesterday, building mixed-layer instability coupled with near record PWATS of 2.35-2.50" will support several large clusters of showers/tstms moving in from northern Florida as well as southwest/central Georgia. This activity will pose a risk for heavy rainfall and localized flooding, but as the 850-250 hPa mean flow strengthens through the day, storms should become increasingly more progressive, thus curtailing the flood potential somewhat. Still, some flooding of low-lying and poor drainage areas appears likely and Flood Advisories could be needed, maybe even an isolated base-level Flash Flood Warning where training and/or back- building occurs along mesoscale boundaries. In addition to heavy rain, increasing wind fields could support a risk for strong to marginally severe tstms with damaging wind gusts and cloud-to- ground lightning possible.

KEY MESSAGE 2: Wet conditions will bring the potential for heavy rainfall and localized flooding Monday, with the threat possibly lingering into Tuesday.

Surface low pressure will lift northeast into eastern Canada Monday, with its trailing cold front extending southward along the western slopes of the Appalachians. Meanwhile, the upper trough digging across the Ohio Valley is forecast to evolve into a cutoff low over the Southeast, allowing a series of embedded shortwaves to continue lifting northward across the forecast area. Ahead of the front, an anomalously moist airmass will remain in place with PWATs around 2.3 inches during the first half of the day, or roughly two standard deviations above climatology, before gradually decreasing into the 2.0-2.2 inch range by the evening. Although instability is expected to diminish to generally less than 1000 J/kg, forecast soundings continue to display long, narrow CAPE profiles, favoring efficient warm-rain processes and the potential for high rainfall rates despite the weaker instability.

Current guidance continues to focus the greatest QPF along the coastal corridor, especially north of the SC/GA border. Given antecedent rainfall expected through the remainder of the weekend, localized flooding concerns will increase, especially where storms move over the same locations. Urban and poor- drainage areas, including the Charleston metro area, could be particularly susceptible, potentially warranting Flood Advisories or Flash Flood Warnings. The latest NBM and HREF indicate an additional 1 to 2 inches of rainfall along the immediate coast on Monday.

A similar pattern is expected to persist into Tuesday as the cold front stalls inland while the cutoff low remains anchored over the Southeast. Although PWATs should decrease slightly, values near 2 inches will remain above normal. Combined with modestly higher instability than Monday, the environment will continue to support additional rounds of showers and thunderstorms which could be capable of producing heavy rainfall and localized flooding.

KEY MESSAGE 3: There is a High Risk for rip currents for the Charleston County beaches today through Monday with a Moderate Risk elsewhere.

Today: The combination of building seas and increasing winds will support a High Risk for rip currents along the Charleston County beaches with a Moderate Risk for the beaches from Edisto Beach south along the Georgia coast.

Monday: A Moderate Risk of Rip Currents continue for areas from coastal Colleton County/Edisto Island south to the Altamaha River. Additionally, the High Risk will continue for the Charleston County coastline where the strongest winds are expected to occur.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

02/06z TAF Discussion: KCHS/KJZI: KCLX shows showers beginning to develop off the Georgia and far southern South Carolina coast early this morning. This activity is expected to expand in coverage through daybreak with most of it remaining just offshore. Both KJZI and KCHS will likely be along the western fringe of this activity as it passes by with KJZI the most likely to see impacts. A TEMPO group for TSRA was included 11-13z at KJZI with just VCSH at KCHS for now. Amendments will most certainly be needed as trends become more apparent over the coming hours. An area of MVFR cigs should also spread north, mostly likely impacting KJZI. A second round of showers/tstms could impact both terminals by late afternoon/early evening. A PROB30 group for TSRA was included for both terminals 00-03z.

KSAV: MVFR cigs have spread into the terminal and will linger through mid-morning before improving to VFR. There is a non-zero chance that a brief period of IFR cigs could impact the terminal just before daybreak, but confidence in this is not high. A ground of showers/tstms could approach form the southwest late this afternoon and a TEMPO group for MVFR conditions in TSRA was included 20-23z to account for this.

Extended Aviation Outlook: Periodic flight restrictions are expected in showers and thunderstorms through midweek.

MARINE

Through Tonight: The pressure gradient will steadily tighten this afternoon into tonight as high pressure strengthens offshore and the storm system to the west and northwest draws closer. Southwest winds are expected to increase to 20-25 kt this afternoon and linger into tonight with seas building as high as 4-7 ft, highest over all the offshore legs as well as the South Santee-Edisto Beach nearshore legs. For Charleston Harbor, winds as high as 15-20 kt with gusts to 25 kt is expected with waves 1-2 ft. Small Craft Advisories are in place for all waters beginning this afternoon and continuing through tonight.

Monday through Friday: A stationary front inland will weaken through the early week allowing the pressure gradient over the waters to relax. Ongoing Small Craft Advisories are progged to drop off throughout the day Monday as conditions gradually improve. Benign wind and sea conditions are to expected to return by Tuesday morning, prevailing through the week.

High Surf: The latest NWPS breaking wave height guidance suggests a few Charleston County beaches could see a breakers as high as 5 ft at times, mainly from late afternoon into the tonight period. These instances look pretty sparse and with most breakers holding in the 3-4 ft. The need for a High Surf Advisory was deferred to the day shift given the marginal nature of the event.

CLIMATE

Record High Minimum Temperatures:

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...High Risk for Rip Currents from 8 AM EDT this morning through this evening for SCZ150. MARINE...Small Craft Advisory from noon today to 2 AM EDT Monday for AMZ340. Small Craft Advisory from noon today to 2 AM EDT Tuesday for AMZ360-362. Small Craft Advisory from noon today to 8 AM EDT Monday for AMZ364. Small Craft Advisory from noon today to noon EDT Monday for AMZ384.