textproduct: Charleston

WHAT HAS CHANGED

Aviation section updated for the 12Z TAFs.

KEY MESSAGES

- 1) Hot and humid conditions with periods of showers and thunderstorms across SE GA/SC today and Wednesday. Locally heavy downpours are possible.

- 2) An unsettled wet pattern is expected to develop Friday into the weekend, increasing the potential for heavy rainfall and localized flooding.

DISCUSSION

KEY MESSAGE 1: Hot and humid conditions with periods of showers and thunderstorms across SE GA/SC today and Wednesday. Locally heavy downpours are possible.

Early this morning, satellite water vapor and RAP40 indicated a mid- level vort max near KJAX, drifting to the north. This feature was aiding in the formation of a band of showers and thunderstorms across inland GA this morning. Based on radar trends and recent runs of the HRRR, the showers will develop to the east on an old outflow boundary. MLCAPE is forecast to remain around 1000 J/kg, the forecast will mention thunderstorms.

NAM indicates the H5 heights across the forecast area will gradually decrease through today as a trough ripples over the lower Mississippi Valley. Mild llvl thicknesses and partly sunny conditions should allow temperatures to warm steadily through early this afternoon. Using a blend of MOS, high temperatures are forecast to range from around 90 along the coast with upper 80s inland. In addition, sfc dewpoints are forecast to range in the low to mid 70s this afternoon. The combination of the above normal temperatures and warm dewpoints may yield heat index values above 100 degrees across portions of the coastal counties of SE GA/SC this afternoon.

As temperatures warm, a sea breeze will push inland this afternoon and evening. Recent runs of the HRRR has trended lower with thunderstorm coverage this afternoon, however, it seems a bit overactive with convection this morning. Forecast soundings indicate a weaker subsidence inversion across the coastal counties this afternoon. Hot and humid conditions should support a broad field of 1500-2000 J/kg across the region this afternoon. Following the 0Z HREF and REFS, scattered to numerous showers and thunderstorms should develop along the sea breeze during the mid to late afternoon hours. PW values are forecast to range between 1.9" to 2" with WBZ around 12 kft. This environment should support efficient rainfall producing thunderstorms, primarily along and inland of the sea breeze. HREF probs indicate a 50-70% for 1"/3hr and 10% for 3"/3hr rainfall rates. Thunderstorm activity may remain a bit closer to the coastal counties, compared to the past couple of days. It is possible that a Flood Advisory or two could be issued late this afternoon, potentially over the inland I-16 and I-26 corridors. Deep convection should wane during the evening hours.

Thursday, conditions appear very comparable to today. Humidity may range a bit higher as the upper ridge continues to weaken and area of rain soaked ground increases. Given high temperatures in upper 80s to around 90 and dewpoints in the low to mid 70s, conditions will remain hot and humid. Coastal counties may see heat index values between 100 to 103 degrees. Storm coverage could be a little less on Thursday. However, the weakening ridge will allow storm motions to approach the coast, resulting in an increase in coastal PoPs.

KEY MESSAGE 2: An unsettled wet pattern is expected to develop Friday into the weekend, increasing the potential for heavy rainfall and localized flooding.

As the aforementioned front stalls across the region, chances for rainfall will remain elevated throughout the weekend. Models continue to show a weak surface low pressure or two may form along the front before moving out into the Atlantic, which would bring a period of heavier rainfall to the region. With precipitable water values remaining near the 2 inch mark throughout the weekend and slow storm motions of 10-15 knots, thunderstorms will be capable of producing locally heavy rainfall.

Guidance continues to suggest widespread totals of 2 to 4 inches over the 72 hour period from Friday through Sunday, with NBM probabilities for 3 inches during the same period remaining in the 15 to 30% range. Breaking it down into 24 hour periods, NBM probabilities for 2 inches remains below 20% throughout the weekend, further reinforced by the parent ensembles themselves which show the same. Ensemble clustering analysis reveals that the most favorable solutions (probabilistically speaking) keep the heaviest precipitation out of the immediate area, with the lesser likely solution being the ones producing heavier precipitation totals across the region. So, while the risk for higher rainfall totals (4+ inches) remains on the lower end, they are still possible, with the details continuing to be refined as additional model runs come in.

Chances for severe weather appear to be on the lower side given the general lack of shear, but cannot be ruled out. Depending on the time of day, severe weather would most likely be associated with the development of a surface low pressure, but given the lack of agreement on if/when that may occur, confidence is generally low.

As for temperatures, there is good agreement that we'll see a cool- down from the current daily mid 80s to lower 90s dropping a few degrees into the lower to mid/upper 80s Friday and Saturday. A surface high pressure moving into the northeast pushes the front a bit further south on Sunday, further bringing high temperatures down into the upper 70s to lower 80s for Sunday and Monday.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

Prior to the 12Z TAFs, KCLX detected a band of showers between KSAV and Tybee Island. The KSAV TAF will be initialized with a mention of VCSH. An approaching disturbance and deep moisture should support rounds of MVFR ceilings over KJZI until 14Z. This morning, KCHS and KJZI should remain VFR. However, MVFR ceilings and showers may approach from the SW during the pre- dawn hours, we will monitor forecast trends. Recent runs of the HRRR, RRFS, and HREF indicates that a band of showers and thunderstorms are expected to develop along the seabreeze just inland of the terminals this afternoon. The convection will be highlighted in the TAFs with TEMPOs at KCHS and KSAV between 19-23Z, and a PROB30 at KJZI between 20-23Z. Convection should gradually dissipate this evening.

Extended Aviation Outlook: Brief flight restrictions possible within showers and thunderstorms at CHS/JZI/SAV terminals during the afternoon hours into the later half of the week. More periodic flight restrictions are possible Friday into the weekend as the potential for widespread showers/thunderstorms increases. In addition, patchy overnight fog and/or stratus cannot be ruled out after recent rains.

MARINE

Today and tonight: The marine zones will remain under the influence of broad high pressure through the period. South winds should remain between 10 to 15 kts with some gusts into the low 20s. Wave heights will favor values between 3-4 ft.

Thursday through Sunday: Conditions are generally forecast to remain below Small Craft Advisory criteria into the weekend, with daily chances for showers and thunderstorms. A surface front pushes down into the region overnight into Friday, with a surge in winds 15 to 20 knots behind the front Friday afternoon before the front stalls. The surface front lingers across the region throughout the weekend, and a surface low pressure or two may form along the front, possibly pinching the pressure gradient a couple times throughout the weekend. This would bring brief periods periods of Small Craft Advisory level winds and seas, though confidence in timing and location are currently low.

Rip Currents: An elevated rip current risk will remain through the week with a modest easterly swell around 3 to 4 feet every 8 seconds and sea breeze influences. A moderate risk of rip currents is in place for all area beaches through Thursday.

CHS WATCHES/WARNINGS/ADVISORIES

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