textproduct: Charleston

WHAT HAS CHANGED

All sections and Key Messages were updated.

KEY MESSAGES

- 1) Unsettled weather will continue, with increasing chances of showers and thunderstorms this weekend.

DISCUSSION

KEY MESSAGE 1: Unsettled weather will continue, with increasing chances of showers and thunderstorms this weekend.

Today: The region will remain embedded within a large mid-level weakness between a large subtropical ridge centered over New Mexico and West Texas with a secondary ridge over the open western Atlantic. Guidance shows a well pronounced conveyor of mid-level vorticity streaming out of the northeastern Gulf across Southeast Georgia, the South Carolina Lowcountry and into the North Carolina Outer Banks. These impulses coupled with a ribbon of deep moisture characterized by PWATs 2.0-2.3" and some modest mixed-layer instability will support another round of showers/tstms today. Initially, convection is expected to develop along/just offshore of the lower South Carolina coast prior to daybreak within a region of weak/modest convergence associated with what appears to be a land breeze circulation. This activity is poised to spread/redevelop farther north into the Charleston Tri-County then translate ever slow slowly inland through the morning hours with additional activity possibly developing later this afternoon as temperatures rise into the 80s. The various CAMS are having a hard time resolving how much and where convection will concentrate later in the day with some refocusing activity closer to I-95 while others are keeping activity across the Lowcountry redeveloping closer to the beaches and across Downtown Charleston. Suffice to say, there is much uncertainty how much the atmosphere will recover later this afternoon along the lower South Carolina coast with morning convection likely overturning the atmosphere pretty well there. For now, pops were limited to 50-70% across the region with the highest pops clustered across the Charleston Tri-County where confidence in morning convection is the highest. Adjustments will likely be needed later as trends become clearer.

Heavy rainfall with a potential for some localized flash flooding will be the primary hazard today, especially where convective training occurs. There could also be a brief waterspout and/or tornado through late morning/early afternoon across the coast where RAP and H3R soundings show low-level wind fields veering with height atop the low-level east/northeast flow. While the lack of stronger instability will likely be the limiting factor for a higher localized tornado threat, it is certainly something to watch out for, especially as convective outflows and the sea breeze become better defined.

Tonight: Convection is expected wane fairly quickly this evening. As inland areas cool and low-level winds back, a corridor of modest convergence is forecast to develop along mainly the lower South Carolina coast from roughly midnight through daybreak Sunday. With such a warm and moist atmosphere in place, it will not take much to initiate additional showers/tstms during the early morning hours. There is a potential for some convective training to occur within the plume of PWATS >2" and the 29/00z HREF are showing some elevated probabilities for >1"/3-hr in the vicinity of Downtown Charleston. This will need to be monitored as a quick inch in Downtown Charleston is known to cause flash flooding.

Sunday: There may be some lingering showers and thunderstorms Sunday morning, though expect that activity to wane as shortwaves aloft move northwards. Expect ridging to build throughout the day, leading to afternoon highs back up in the upper 80s to lower 90s across the region. Instability values build to near 2000 J/kg, though the lack of flow aloft should keep thunderstorms from getting well organized, thus keeping the risk for strong-severe thunderstorms on the low end as the expected sea-breeze likely sparks off a few showers and thunderstorms. As has been the case, we will be watching for locally heavy rainfall given the lack of flow aloft and precipitable water values in excess of 2 inches.

Monday and Tuesday: Expect much of the same for Monday as ridging aloft continues, and surface high pressure remains the main surface feature. Tuesday into the work-week will likely see lesser convective coverage as the center of the ridge axis moves over the region, leading to slightly increased high temperatures in the upper 80s to mid 90s. Dewpoint temperatures currently look to remain in the mid 70s, keeping heat index values below critical thresholds.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

29/06z TAFs: KCHS/KJZI: Showers/tstms are expected to fire along the lower South Carolina coast prior to daybreak. This activity will likely make a run for both terminals, possibly yielding some impacts. TEMPO groups for MVFR conditions in TSRA were introduced 10-13z at KCHS and 09-12z at KJZI. Later this afternoon, it is not clear how much the atmosphere will recover but a number of models are indicating another round of shower/tstms impacting the terminals. Given this uncertainty, a PROB30 group for TSRA was introduced 19-22z to trend.

KSAV: A large area of low stratus is poised to settle into the terminal through daybreak. Prevailing LIFR cigs are likely with cigs possibly approaching airfield minimums. LIFR cigs will slowly lift by mid-morning with VFR returning by 16z. Risk for showers/tstms will increase by mid-afternoon. PROB30 19-22z for TSRA was maintained to account for this activity.

Extended Aviation Outlook: Showers and thunderstorms will bring brief flight restrictions into early next week, especially during the afternoon hours.

MARINE

Through Tonight: There will be a risk for waterspouts near the coast later this morning. Otherwise, northerly winds with the land breeze along the lower South Carolina coast will gradually veer more onshore through the day as the sea breeze develops. Onshore winds will linger through tonight with directions near the beaches likely backing more east to northeast late. Speeds will remain less than 15 kt through tonight with seas 1-3 ft.

Sunday through Wednesday: No large-scale marine concerns. Flow will become increasingly onshore, though overall fairly weak, strongest during the afternoon hours as the sea-breeze forms and moves ashore.

CHS WATCHES/WARNINGS/ADVISORIES

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