textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Heavy rainfall and flash flooding is expected to impact southeast Georgia and southeast South Carolina through this evening.

- 2) An isolated tornado or two will be possible across southeast Georgia and southeast South Carolina today.

- 3) Several hazards are expected to impact the southeast Georgia and southeast South Carolina coast. Moderate to potentially major flooding is possible for Charleston and coastal Colleton Counties this morning.

- 4) A Lake Wind Advisory has been issued for Lake Moultrie through this evening.

DISCUSSION

KEY MESSAGE 1: Heavy rainfall and flash flooding is expected to impact southeast Georgia and southeast South Carolina through this evening.

The forecast area will sit within the feed of deep tropical moisture on the east side of the circulation of Isaias today. All the necessary ingredients will be present including precipitable water values into the 2.2-2.4" range bands of deep convection likely tracking repeatedly over the same areas. Unfortunately, there remains uncertainty regarding exactly where the stripes of the highest rainfall totals will setup. The initial round of rainfall will likely be the most widespread and driven by convergence within the advancing warm front that should clear the area by mid to late morning. Then into the afternoon, we should transition into a regime driven by bands of showers and thunderstorms streaming northward. It is within these bands where the highest event totals will likely occur. Though confidence isn't high in exactly where the highest bands/stripes of rainfall will be, guidance such as the 10/00z HREF is instructive regarding the rainfall potential. The probability- matched mean suggests widespread amounts of 1-3" across all of southeast GA and southeast SC. There continues to be signals in the localized probability-matched mean for pockets of considerably higher amounts into the 4-6" range and potentially even 6-8" depending on training bands eventually setup and evolve.

The potential for drier air to wrap around the remnant circulation of Isaias is the main driver for uncertainty regarding where the heaviest rainfall occurs and what the highest amounts end up being. But, the potential for flash flooding continues to be high enough to warrant carrying the ongoing Flash Flood Watch. The greatest threat for excessive rainfall and flash flooding will run from the early morning pre- dawn hours into the evening. Of greatest concern is the morning along the coast around the time of the ~8:30 am high tide. Coastal flooding along will produce flooding and inundation along the coast and if pockets of heavy rain are also occurring, the flash flood threshold will be even lower.

KEY MESSAGE 2: An isolated tornado or two will be possible across southeast Georgia and southeast South Carolina today.

Surface analysis shows a well entrenched, albeit shallow, wedge of high pressure across the Carolinas and into GA related to the parent high sprawling across eastern Canada and into the Mid Atlantic. A well defined warm front-like feature extends west to east from the center of Isaias off the northeast FL coast along a notable ThetaE gradient. SPC Mesoanalysis shows that south of this boundary is where the reservoir of surface-based instability is and this feature is forecast to steadily advance northward through the overnight and should clear the forecast area by mid morning.

There remains two primary areas and time periods of concern for thunderstorms capable of producing severe weather and perhaps an isolated tornado or two. The first area and time period will come through the overnight and into mid morning along and near the boundary itself, primarily along the coast. It is in this corridor where we will likely see the duo of the best period of low-level veering and SRH, with values peaking in the 200-300 m2/s2 coincident with MLCAPE peaking around 500 J/kg with pockets of 500-1,000 J/kg. Such an environment will support low- topped mini-supercells that will develop across the adjacent coastal waters and attempt to push onshore. We are already seeing this type of convection on the KJAX radar displaying well defined supercell elements and clearly producing tornadic waterspouts. This first threat will likely run from the early morning pre-dawn hours through mid-morning, primarily following the passage of the warm front.

The second area and time period of concern comes in the afternoon and into the evening with diurnal heating and more spatially broad destabilization. During this round we will likely see a combination of less impressive low-level veering and SRH coincident with a potentially more unstable environment. Being south of the boundary, profiles will be a bit more uni- directional and should yield SRH values more into the 100-200 m2/s2 range. However, instability should be higher than during the first round as we start to see some cloud breaks and temperatures warming into the low to mid 80s paired with dewpoints into the upper 70s. MLCAPE values of 1,000-1,500 J/kg appear likely, with pockets of higher values possible. Model guidance depicts the primary zone of highest instability focused across southeast GA especially up to around the I-16 corridor though it should be stressed that the threat will exist across the entire forecast area. During this time period, we should see bands of more discrete convection with supercell structure, mostly low-topped as well. To a lesser degree, this threat will extend through Saturday night and into Sunday morning until the cold front sweeps through from the west. As we move forward in time the amount of instability available should wane through the overnight and profiles will become even more uni-directional.

KEY MESSAGE 3: Several hazards are expected to impact the southeast Georgia and southeast South Carolina coast. Moderate to potentially major flooding is possible for Charleston and coastal Colleton Counties this morning.

Today: Breezy onshore flow combined with widespread rainfall will yield tidal levels well-above the astronomical tide this morning, with tidal departures increasing between 1.3 to 1.5 ft within the Charleston Harbor. A forecast of 8.0 ft MLLW has been predicted for this morning's high tide (~8AM), therefore a Coastal Flood Warning is in effect for this morning for Charleston and coastal Colleton Counties from 7AM to 11AM. Ongoing heavy rainfall combined with strong onshore flow should keep departures elevated through Saturday evening, and a Coastal Flood Advisory will likely need to be issued for Charleston Harbor with this evening's high tide (~9PM). These conditions will also yield tidal levels into the minor flood stage at Fort Pulaski for this morning's high tide (~8AM) as a 9.8 ft MLLW has been noted in the forecast. A Coastal Flood Advisory has been issued for 7AM to 11AM for Coastal Bryan, Coastal Chatham, Coastal Liberty, and Coastal McIntosh Counties. Thereafter, Fort Pulaski will remain below minor flood stage for the evening's high tide (~9PM).

Sunday: Minor coastal flooding remains possible for the Charleston Harbor on Sunday morning as a 7.2 ft MLLW has been forecasted for the morning high tide (~9AM) and a Coastal Flood Advisory might be warranted. As the cold front passes through on Sunday, flow should turn more offshore and the coastal flood threat should end.

Rip Currents/High Surf/Beach Erosion: Strong east-northeasterly flow combined with east-southeasterly swell will yield a High Risk of rip currents for southeast Georgia and southeast South Carolina beaches through this evening (esp. during the the high tide cycles). It's possible to see large breakers as high as 6 ft along the coastline and this may lead to areas of significant beach/dune erosion.

Sunday: Lingering east-southeasterly swell and strong southwesterly flow will yield a High Risk of rip currents for the Charleston County beaches with a Moderate risk for the Beaufort County beaches and southeast Georgia beaches.

KEY MESSAGE 4: A Lake Wind Advisory has been issued for Lake Moultrie through this evening.

As strong onshore flow persists through the evening, wind speeds will reach into 15 to 20 kts with gusts up to 25-26 kts possible across Lake Moultrie. Therefore, a Lake Wind Advisory has been issued from 5AM to 8PM this evening. Afterwards, expect the wind flow to switch to more offshore and wind speeds to steadily decrease overnight.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

The 06z TAF period begins with showers increasing in coverage across the region as well as MVFR or lower conditions. Starting first at KSAV, steady rain and showers will arrive within the first hour of the TAF period followed by the potential for thunderstorms at least through the 09-13z time period. MVFR conditions should prevail, with periods of IFR expected from both lowering ceiling heights and reduced visibility within the heaviest pockets of rainfall. Winds will start to turn from northeasterly to easterly and eventually southeasterly as a warm front lifts through this morning. The first round of rainfall will likely stick around through about mid morning, followed by a potential lull into the afternoon. Then another round of showers and thunderstorms will be possible.

For KCHS and KJZI, the overall thinking is similar to KSAV through the timeline is shifted a bit later. Showers and MVFR conditions will pass through periodically over the next several hours. The arrival of steadier rainfall and potentially thunder will come after sunrise and continue through the morning. IFR conditions will be possible for periods of time due to lower ceiling heights and reduced visibility that could accompany the heaviest pockets of rainfall. This round should come to an around midday, followed by a lull in shower coverage. Then by the late afternoon another round of showers and storms will be possible that could run into the early evening.

Extended Aviation Outlook: Prevailing VFR conditions will return Sunday and continue into early next week.

MARINE

Today: See NHC forecast for the latest on Isaias. As the pressure gradient continues to strengthen, expect onshore flow to remain 20 to 25 kts with frequent gale-force gusts of 34 kts possible. A Gale Warning has been issued for the nearshore and offshore southeast South Carolina waters, while a Small Craft Advisory has been issued for the Charleston Harbor and the nearshore and offshore southeast Georgia waters. As the warm front lifts northward out of the coastal waters, moderate south- southeasterly flow at 10 to 15 kts with gusts up to 25 kts will develop through the overnight. As east-southeasterly swell surges onward, seas will range from 5 to 7 ft through the early morning, before dropping down into the 4 to 6 ft range this evening.

Sunday into Tuesday: Remnants of Isaias combined with a surface high pressure offshore should keep winds breezy on Sunday and Small Craft Advisory will likely persist for portions of the coastal waters into Sunday night, with the potential for continuing into Monday. Thereafter, expect these marine conditions to become almost tranquil as we head into early next week.

CLIMATE

Record Precipitation:

October 10: KCHS: 1.82/1990 KCXM: 2.13/1990

CHS WATCHES/WARNINGS/ADVISORIES

GA...Flood Watch through late tonight for GAZ087-088-099>101-114- 115-137-216>219-238>241. High Risk for Rip Currents through this evening for GAZ217-219-239- 241. Coastal Flood Advisory from 7 AM to 11 AM EDT this morning for GAZ217-219-239-241. SC...Flood Watch through late tonight for SCZ040-042>045-147>152. High Risk for Rip Currents through this evening for SCZ148>151. Coastal Flood Advisory from 7 AM to 11 AM EDT this morning for SCZ148-151-152. Lake Wind Advisory until 8 PM EDT this evening for SCZ045. Coastal Flood Warning from 7 AM to 11 AM EDT this morning for SCZ149-150. MARINE...Small Craft Advisory until 8 PM EDT this evening for AMZ340- 364. Gale Warning until 5 PM EDT this afternoon for AMZ360-362-380- 382. Small Craft Advisory until 8 PM EDT Sunday for AMZ384.