textproduct: Charleston

WHAT HAS CHANGED

All Key Messages and sections were updated.

KEY MESSAGES

- 1) Locally dense fog possible this morning.

- 2) Rain chances to increase late this weekend into Labor day.

- 3) Hot and humid conditions expected into the upcoming weekend.

DISCUSSION

KEY MESSAGE 1: Locally dense fog possible this morning.

Conditions still appear favorable for some degree of low clouds and fog early this morning. The lowest 1000 hPa condensation pressure deficits are still centered across the interior and this is likely where the greatest chances for fog will occur over the next few hours. The various 03/00z statistical guidance packages are not overly aggressive on the fog potential, but the HREF still paints ~20-40% chance for vsbys dropping to less than 1/2 mile. Visibilities will be closely monitored heading into and throughout the morning rush hour and a Special Weather Statement and/or Dense Fog Advisory could be needed at some point pending trends.

KEY MESSAGE 2: Rain chances to increase late this weekend into Labor Day.

Today: The expansive upper-level anticyclone that has been influencing the area for the past several days will nudge ever so slowly east and southeast today which keep the dry and broadly subsident pattern in place. Early morning water imagery shows a weak mid-level low (850-700 hPa) and associated shear axis extending from offshore of Cape Canaveral across Florida and into the eastern Gulf. This feature is forecast to meander north with pronounced surge of low-level moisture poised to push into northeast Florida and the extreme southern Georgia coast later today. The moisture plume looks to quickly encounter subsidence and dry mid-level airmass associated with the upper high as it approaches the middle/upper Georgia coast where low 850 hPa theta-e values and unfavorable K-indices are noted. The end result will likely be an area of isolated to perhaps scattered showers and maybe even a tstm or two brushing parts of Long, Liberty and McIntosh counties later in the day. Significant impacts from this activity are unlikely given the unfavorable moisture profiles in place.

Friday: The bimodal discrepancies between the GFS and European ensembles yesterday morning appear to have improved. The 03/00z GFS and most of its ensembles have trended a bit wetter and better matches trends noted in the European. The main driving force for convection will be a weakening and retrogression of the strong upper high to the north and a mid-level shear axis to the south. This shear axis and its associated band of vorticity is forecast to extend from far southwest Georgia into northeast Florida by Friday afternoon with its associated low/mid-level moisture plume gradually expanding north into much of Southeast Georgia and the South Carolina Lowcountry. This will likely yield a region of isolated to scattered showers/tstms with the greatest concentration occurring over interior Southeast Georgia. It is not exactly clear right now how far north convection will reach as much of the Charleston Tri-County and possibly Colleton County could still be influenced by the weakening upper high.

Saturday into Labor Day: Saturday may end up being the least active of the upcoming holiday weekend with rain chances poised to increase Saturday night and Sunday as a cold front pushes into the Southeast States. There are signals that the front could briefly stall over the area Sunday afternoon as it interacts with the sea breeze, but should then quickly push south into northern/central Florida Sunday night into Labor Day as high pressure begins to wedge south into the Carolinas and Georgia. Despite the front being displaced well to the south, unsettled conditions will likely persist into Labor Day. In addition to the unsettled conditions, it could be breezy at the coast given the possibility of a pinched gradient associated with the inland wedge.

KEY MESSAGE 3: Hot and humid conditions expected into the upcoming weekend.

As the center of a high pressure ridge remains just north of the region, hot and humid conditions will persist into the beginning of the weekend. Similar to Wednesday, the subsidence provided by the ridge along with h850 temperatures near 19 degrees C will result in afternoon highs in the upper 80s along the coast and up into the mid 90s for inland areas. Heat index values of 100-105 degrees F are expected across the region. For Friday, isolated morning showers and thunderstorms will bring increased cloud coverage, thus limiting heating into the mid 80s along the coast to lower 90s. However, due to an increase in dewpoints into the upper 70s, heat index values will remain in the 100-105 degree F range.

While the ridge breaks down on Saturday as a trough develops along the east coast, Saturday continues to look like the hottest day. Ahead of an approaching cold front, compressional heating combined with h850 temperatures near 20 deg C are expected to push afternoon temperatures into the upper 80s/lower 90s along the coast and up into the mid 90s for inland areas. Will need to watch to see where the highest dewpoints end up, as areas that see dew points rise into the upper 70s will likely see heat index values rise into the 108- 112 degree F range, which would require a Heat Advisory. Currently, highest dewpoints are expected across portions of southeast South Carolina, with lower values across southeast Georgia where heat index values of 103-108 are still expected. A cold front looks to push through on Sunday, bringing an end to this stretch of hot and humid conditions, though portions of southeast Georgia will likely still reach up into the mid 90s before the front pushes through. While there will likely be rain, Monday's highs looks to stay in the 80s.

AVIATION /06Z THURSDAY THROUGH MONDAY/

03/06z TAFs: KCHS/KJZI/KSAV: There is still a possibility for some fog/stratus impacts at all three terminals just before daybreak. Near term guidance is not overly aggressive in lower vsbys or even IFR or LIFR cigs, but atmospheric conditions are favorable for some degree of fog and stratus formation. VFR conditions were mostly maintained for now with vsbys limited to 5-6SM with scattered cloud bases around 400 ft. A TEMPO group for very brief period of LIFR cigs were introduced at KSAV 10-12z given there is a bit more agreement in a thicker band of stratus impacting that terminal. Once low clouds/fog mix out, VFR will prevail.

Extended Aviation Outlook: Isolated showers and thunderstorms may bring brief flight restrictions during the morning and afternoon hours on Friday. Regional chances for scattered showers and thunderstorms return Saturday afternoon and evening.

MARINE

Through Tonight: There are no concerns. Onshore winds 15 kt or less with seas 2-4 ft will prevail.

Friday through Tuesday: In general, onshore flow should peak between 10-15 kt daily through Saturday with greatest enhancement along coastal areas during afternoon sea breeze circulations. Friday morning and afternoon will likely see scattered showers and thunderstorms, with another round possible Saturday morning. A cold front should then arrive Sunday morning and push southwards by the afternoon hours, resulting in a slight uptick of easterly winds (15- 20 kt) and building seas (3-4 ft) into early next week. The pressure gradient looks to increase on Monday post fropa, bringing the risk for Small Craft Advisories.

Rip Currents: There is a moderate risk for rip currents from Beaufort county extending southwards into southeast Georgia, with 3-4 ft seas every 10-11 seconds.

Coastal Flooding: Elevated high tides return Sunday afternoon during the lunar Perigee (9/6), and linger throughout the week until after the new moon (9/11). With astronomical high tides ranging from 6.3 to 6.55 ft MLLW in the Charleston harbor throughout the week, tidal departures of 0.45 to 0.7 ft would result in minor coastal flooding. A high pressure wedge may set up along the east coast next week, which could be capable of producing the tidal departures leading to coastal flooding, especially along coastal Charleston and Colleton counties. For the rest of the southeast South Carolina and southeast Georgia coastline, tidal departures would need to be in excess of 1 foot, so coastal flooding appears unlikely at this time.

CHS WATCHES/WARNINGS/ADVISORIES

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