textproduct: Charleston

WHAT HAS CHANGED

KEY MESSAGE 1 has been updated for recent trends and isolated severe weather potential today. The Aviation Section has been updated for the 12Z TAF issuance.

KEY MESSAGES

- 1) Isolated severe thunderstorms are possible today.

- 2) Unsettled weather returns Friday night and lingers through early next week.

DISCUSSION

KEY MESSAGE 1: Isolated severe thunderstorms are possible today.

A moist south/southwest flow will prevail across the Southeast United States in advance of cold front approaching the region late day, driven by a longwave trough advancing across the Great Lakes region toward the Mid-Atlantic/Northeast Coast. Prior to daybreak, instability will remain quite low, which remains evident on recent radar imagery with showers and thunderstorms in a weaker state ahead of enhanced forcing arriving mid-late morning. Guidance suggests this activity to be the first of two convective waves entering Southeast South Carolina and Southeast Georgia today, likely the weaker of the two while through early daylight hours.

The key to isolated severe weather today will continue to focus on the ability of the atmosphere to destabilize prior to a second wave of convection arriving mid-late morning into early afternoon. The main issue revolves around limited instability during timing of enhanced deep-layer shear and forcing associated with low-lvl jetting while a strong h25 jet core passes well inland and north of the local area. The longer morning convection occurs, the less likely the atmosphere recovers for an isolated severe weather scenario late morning and afternoon. However, hires guidance maintains a second wave of convection prior to cold fropa, mainly in the form of small thunderstorm clusters that arrive at a time when instability peaks across the local area. Some guidance places an axis of modest instability (SBCAPE ~1500 J/kg) across Southeast Georgia and perhaps into coastal parts of southern Southeast South Carolina this afternoon where a few breaks in cloud cover and warm air advection allow for sfc temps to reach the mid-upper 80s. Sfc temps are likely to become warmest near the I-95 corridor in Southeast Georgia, even possibly reaching 90 degrees before the onset of additional shower and thunderstorm activity later today.

The combination of deep moisture (PWATs 2.0 inches/sfc dewpts lower 70s) and 0-6 km Bulk Shear (~50 kt) supports an isolated severe weather threat, conditional on sfc heating/instability realized prior to the second wave of convection arriving late morning into early afternoon. Mostly unidirectional wind profiles depicted on soundings with modest directional change in h85-h5 crossover winds along with low-lvl lapse rates around 6.5-7 C/km support primarily a damaging wind threat by a few thunderstorms that become strong/severe within clusters and/or perhaps small linear segments. However, there remains a non-zero risk for a brief tornado across Southeast Georgia (south of I-16 and closer to the Altamaha River) should ample instability develop prior to the onset of afternoon convection ahead of a sea breeze. The bulk of thunderstorm activity should remain sub-severe today, but still capable of producing brief moderate-heavy rains and perhaps a minor flooding episode within a deep moisture environment, especially if the progression of the arriving cold front slows while passing across the area starting late day/early evening. The Storm Prediction Center continues to place a Marginal Risk (1/5) for severe weather across the entire area today.

KEY MESSAGE 2: Unsettled weather returns Friday night and lingers through early next week.

Dry high pressure will briefly rebuild on Friday, then the front will lift back north and stall over the area Friday night through Sunday. A series of upper level shortwaves will ripple through the Southeast during this period. Meanwhile, PWATs approaching 1.8" expected to advect into the area Saturday through Sunday, providing ample moisture for shower development. Pockets of surface-based instability will support isolated thunderstorms, particularly during the afternoon hours across coastal Southeast GA. Total precipitation Friday night through Sunday expected to range from 0.75"-1.25", though a few locations could see amounts in excess of 1.5". Rainfall amount and intensity not currently expected to result in a significant flooding threat.

A strong cold front will sweep through Monday afternoon, bringing numerous showers and a few thunderstorms. Guidance indicates the potential for moderate shear and instability preceding the front, so an isolated severe thunderstorm with damaging winds is possible.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Light to moderate showers are expected at all terminals early this morning, with MVFR/IFR conditions possible between the 14-18Z timeframe at CHS/JZI and 12-16Z at SAV due to thunderstorms. Gusty winds up to 20-25 kt are also possible with shower/thunderstorm activity late morning and afternoon. Periods of MVFR cigs are likely at all terminals mid-late afternoon into possibly early evening, prior to a cold front shifting through the region with dry high pressure building in its wake. VFR conditions are then expected by late evening and are likely to persist through 12Z Friday.

Extended Aviation Outlook: Occasional flight restrictions Saturday through Monday as a stalled front brings showers and thunderstorms.

MARINE

Today and Tonight: A somewhat enhanced pressure gradient across local waters will slowly weaken in advance of pre-frontal convection late morning and afternoon. Southerly winds continue to gust up to 20-25 kt across nearshore South Carolina waters and across offshore waters with seas as large as 4-6 ft (largest beyond 20 nm from the coast across South Carolina waters). Small Craft Advisories remain in effect for nearshore South Carolina waters and offshore Georgia waters through 10 AM. A cold front arrives across the area this evening, likely passing offshore across all waters overnight. Although winds could briefly surge post fropa, cold air advection does not appear strong enough to support winds higher than 20 kt across local waters.

A relatively strong northerly flow will develop Monday night into Tuesday behind another cold front as high pressure builds in. Small Craft Advisories may be needed for most waters outside Charleston Harbor due to frequent 25 kt wind gusts.

Rip Currents: Enhanced winds along the lower South Carolina coast coupled with building 2-4 ft swell supports a Moderate Risk for rip currents along the Charleston County beaches for today.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory until 10 AM EDT this morning for AMZ360- 362-384.