textproduct: Charleston

WHAT HAS CHANGED

Aviation section updated for the 12z TAFs. Marine section updated to reflect a risk for waterspouts this morning.

KEY MESSAGES

- 1) Continued unsettled through Tuesday with a risk for heavy rainfall, localized flooding and possibly an isolated strong to marginally severe tstm.

DISCUSSION

KEY MESSAGE 1: Continued unsettled through Tuesday with a risk for heavy rainfall, localized flooding and possibly an isolated strong to marginally severe tstm.

Through Tonight: Early morning water vapor imagery shows a deep trough extending through the Deep South into the Lower Mississippi Valley with a continuous plume of tropical moisture streaming out of the eastern Gulf and western Caribbean and across the Southeast States. At the surface, the region remains in the warm sector ahead of an approaching cold front with Atlantic high pressure centered well offshore. The upper trough to the west is forecast to propagates slowly east today while Southeast South Carolina and Southeast Georgia remains within a very moist and unstable environment. Conditions appear ripe for anther round of showers/tstms today with PWATs near 2.50" interacting with bands of vorticity maxima passing through in the southwest flow aloft.

Convective temperatures in the mid 80s suggest convection may get going a bit earlier than the past several days which will likely have a limiting impact on the amount of available instability given the increased cloud cover, less insolation and cooler afternoon highs in the mid-upper 80s. While a considerable amount of convection appears likely again today, its intensity may be a bit more muted compared to the past few afternoons given lower net mixed-layer instability will be realized. Still, there will be a continued risk for heavy rainfall and localized flooding with PWATs forecast to hold 2.20-2.45", especially along/east of the I-95 corridor. The 850-250 hPa mean steering flow looks to remain fairly robust (20-25 kt) ahead of the upper trough so tstms should remain fairly progressive which should limit rainfall at any one spot, despite intense hourly rainfall rates nearing 2-3"/hr. This is reflected in both the HREF and REFS which only show spotty 3-hour maximum QPF values peaking 2-3". Unless deep convection stalls along a mesoscale boundary within either the Charleston or Savannah Metro Areas, the risk for flooding looks fairly low-end and likely remaining confined to low-lying and poor drainage areas (i.e. Flood Advisory levels). An isolated severe tstm can not be completely ruled out this afternoon with damaging wind gusts and cloud-to- ground lightning being the primary hazards in addition to the broader risk for heavy rainfall.

The greatest coverage of shower/tstms should end by early evening. However, as low-level convergence shifts back into the Atlantic closer to the west wall of the Gulf Stream overnight, another round of showers/tstms is expected to develop over the coastal waters. Some of this activity could brush the coast, mainly from Hunting Island north into parts of the Charleston Tri-County (especially the Charleston County).

Tuesday: A similar pattern is expected to persist into Tuesday as a stationary front remains inland while a cutoff low stays anchored over the Southeast. This setup will continue to support periodic rounds of ascent and scattered showers and thunderstorms. Although the axis of deepest moisture is forecast to shift offshore, guidance still indicates pockets where PWAT values could reach 2 inches or greater, remaining well above climatological normals. Combined with modestly higher instability (SBCAPE ~1500 J/kg), the environment will remain favorable for additional rounds of showers and thunderstorms capable of producing locally heavy rainfall and isolated flooding. Similar to Monday, the highest QPF totals are expected across the South Carolina coast, including the Charleston metro area. The flood threat will be highly dependent on antecedent conditions, as locations receiving heavy rainfall Sunday and/or Monday will have limited time to drain before another round of potentially heavy rain develops Tuesday. Meanwhile, pockets of increasing mid-level dry air are expected to promote DCAPE values approaching or exceeding 1000 J/kg. Therefore, an isolated strong to severe storm is possible with damaging wind gusts the primary hazard.

AVIATION /09Z MONDAY THROUGH FRIDAY/

03/12z TAF Discussion: KCHS/KJZI/KSAV: An area of IFR cigs are still trying to get into KSAV as daybreak approaches. Opted for a brief TEMPO through 13z to try and capture this. Another convective active day is expected for the terminals. Best chance for tstm impacts today will be at KCHS and KJZI although KSAV still has modest chances as well. TEMPO groups were included 19-22z at both Charleston terminals where confidence is greatest and a PROB30 at KSAV for the same time. Conditions were limited to MVFR for now, but IFR can not be ruled out at times.

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

MARINE

Through Tonight: Convection yesterday evening really disrupted the wind field across the coastal water with winds observed early this morning still holding below Small Craft Advisory levels. Guidance still suggests winds will pick up over the next few hours, assuming convection this morning does not become too widespread. Small Craft Advisories were maintained for all waters through this afternoon and the South Santee- Edisto Beach nearshore leg until early this evening. The advisory for the Charleston Harbor was allowed to expire. Seas 4-6 ft will subside 3-5 ft in the advisory area this afternoon, but hold near 6 ft in the offshore South Carolina waters where Small Craft Advisories are not currently issued. Seas will further subside to 2-4 ft overnight.

Tuesday through Saturday: Atlantic high pressure will remain the dominant feature through the week resulting in generally benign wind and sea conditions. Winds could briefly increase along the coast during the afternoon and evening each day in association with the sea breeze.

Rip Currents: A moderate rip current risk is in place for all beaches through this evening for gusty winds and lingering elevated seas. A moderate risk of rip currents is expected to continue through Tuesday for the Charleston County beaches due to lingering 2 foot swells at 8 seconds, combined with elevated onshore flow. All other beaches from Edisto southward to Sapelo Island are expected to have a low risk of rip currents for Tuesday.

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. Satellite and radar already suggest a few cloud lines are trying to develop over the Atlantic which could become the focus for waterspouts through late morning. A Marine Weather Statement was issued to alert mariners to this possibility.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for AMZ360. Small Craft Advisory until 5 PM EDT this afternoon for AMZ362- 364-384.