Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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444 FXUS62 KCHS 010232 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 1032 PM EDT Sun Jun 30 2024 .SYNOPSIS... A cold front will impact the area Monday into Tuesday before dissipating offshore. High pressure will return for the latter half of the week. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... The strong ridging aloft has weakened enough with a short wave moving through, and that along with the associated slightly lower heights, and an approaching cold front has allowed for convection to occur. That convection has congealed into cold pool clusters, which continues to ride south and southeast along a moisture axis and also where we find higher Theta-e air, plus the better MLCAPE. We show 50-70% PoPs as a result from northern Colleton, Hampton, and Allendale, into the interior counties of Georgia. Rainfall rates of at least 2-3 inches an hour in some areas will produce some minor flooding concerns, as PWat is 2.4 to 2.6 inches. The DCAPE is less than earlier, down to just 500-600 J/kg. So there is only a small chance of isolated gusts in excess of 40 mph in the stronger storms. Lightning networks are not showing any diminishing trend as of late Sunday, and with echo tops in excess of 45 or 50K feet, and cloud top temperatures as cold as minus 70-75C, it could be awhile before lightning diminishes. The HRRR has been doing an excellent job of depicting the more southward movement to convection, showing it mostly avoiding the Charleston Tri-County district. It also has greater Georgia of the convection making it to the Savannah area, where we have 40% PoPs occurring after midnight. It`ll be another warm and muggy night even with the storms, since dew points are still so high. In general low temperatures will be just in the mid and upper 70s, with even some coastal sites not dropping below 80F degrees. The temperature at KCHS for June 30 could be a record high minimum if it doesn`t rain between now and the end of the day (See CLIMATE section below). && .SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/... Monday: Models continue to advertise a heavy rain/flooding signal along parts of the Georgia and lower/middle South Carolina coast for Monday. Subtropical ridging aloft will quickly give way to broad troughing along much of the Eastern Seaboard during the day as pronounced shortwave energy embedded in the northwest flow aloft approaches the Carolina coast. This will help push a slow moving cold front south into southern South Carolina, propagating south of the Santee River by early afternoon. Modified soundings ahead of the front show solid mixed-layer instability (MLCAPE 1500-2000 J/kg with K-indices pushing 40C) developing by late morning embedded within a weak shear environment (0-6km bulk shear averaging ~15 kt). Moisture is also abundant with surface dewpoints progged to hold in the mid- upper 70s with PWATs nearing 2.50" at times (daily max for KCHS for July 1 is just over 2.30" per SPC sounding climatology). Expect convection to develop near the Santee River in the vicinity of the front by late morning, then slowly spread south through the rest of southern South Carolina into Southeast Georgia into the afternoon/evening hours in tandem with the front. Given the degree of instability and moisture coupled with surface moisture convergence values surpassing 50 g/kg/12hr at times near the front and some forcing aloft with approaching shortwave energy, the setup looks favorable of periods of extremely heavy rainfall with hourly rainfall rates potentially exceeding 3 in/hr. This is reinforced by the 30/12z HREF QPF progs showing an ~90% chance for 3+ inches to fall in 3 hours clustered in the Savannah-Charleston corridor. There are even some signals that a few spots could see localized amounts in excess of 6 inches (noted 12-hr probs of >5" averaging 70-75% over central Charleston County, including Downtown Charleston). Although antecedent conditions are fairly dry with D0 and D1 drought conditions in place, there is an increasing concern that pockets of significant flash flooding could occur given the intense hourly rainfall rates expected with the slow moving convection, especially in urban areas such as Beaufort and both the Charleston and Savannah Metro Areas. Complicating matters is heavy rains could be falling at the afternoon high tide which could enhanced the flood risk, especially in Downtown Charleston and nearby communities. A Flood Watch has been issued from Noon until 9 PM Monday for the South Carolina and Georgia coastal counties, including Effingham, Hampton, Inland Colleton, Dorchester and Inland Berkeley. Storm total QPF Monday into Monday night will run 2-4" with localized amounts in excess of 6". Categorical pops 80-100% look reasonable, which is a bit higher than the various 12z statistical guidance packages, but closer to the 30/13z NBM. Highs will warm into the upper 80s/lower 90s, warmest across Southeast Georgia. Convection will slowly wane through night, although elevated pops will hold through daybreak Tuesday as the front moves south and northeast onshore winds will keep moisture in place. This will be especially true for coastal Georgia. Lows will range from the upper 60s/lower 70s well inland to the near 80 along the Georgia and far southern South Carolina beaches. Tuesday and Wednesday: The front will slowly dissipate to the south and offshore through the period as subtropical ridging aloft builds back to the east. The pattern will return to more typical summer time convective regime with onshore winds supporting higher low- level moisture values across Southeast Georgia. Scattered to numerous shower/tstms are expected each afternoon and evening with activity diminishing during the overnight hours. Highs will only warm to the mid-upper 80s Tuesday with upper 80s/lower 90s for Wednesday. Lows Tuesday night will range from the lower 70s well inland to the upper 70s/near 80 at the beaches. && .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... An impressive upper-level anticyclone will setup across the Deep South and Southeast U.S. mid-late week. The risk for showers/tstms will generally average below climatological normals as a result. The heat will also steadily build with highs reaching the upper 90s/near 900 inland Friday into the weekend. Heat indices will push the 108 Heat Advisory criteria during this time as well. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... KCHS/KJZI/KSAV: Upstream convection associated with a short wave and cold front will continue to approach the terminals during the late evening and first part of the post-midnight period. Recent guidance and trends suggest that this activity will fade before getting to KCHS and KJZI. We`ll show VCTS from 04Z to 06Z since there does remain various boundaries from convection to form along. KSAV has a better chance of convection from 04Z to 07Z. We might need to include a period of TSRA with gusty winds and flight restrictions if the upstream activity holds together. The next chance for convection and prolonged flight restrictions will occur Monday afternoon and evening as a cold front stalls nearby. Considerable moisture, low level convergence, and instability will favor several hours of at least MVFR weather with moderate to heavy rains in SHRA and TSRA. Some potential for IFR at times, and we can adjust in later TAF issuances if necessary. Extended Aviation Outlook: Brief flight restrictions are possible in showers and thunderstorms, mainly in the afternoon and evenings through the period. && .MARINE... Tonight: There remains a fairly tight gradient between an inland cold front and Atlantic high pressure across Florida. This will produce S and SW winds as high as 15 or even 20 kt, which in turn allows for seas of at least 3 feet throughout. The inland t-storms will weaken as they approach the coastal waters. But if they hold together enough, a few of them could generate some strong winds and lightning strikes on the waters south of Edisto into the Georgia waters after 1 or 2 am. Monday through Friday: A cold front will drop south through the waters Monday with east/northwest winds prevailing all areas Monday night. Widespread showers/tstms with heavy rain could reduce vsbys to less than 1 NM at times and be accompanied by gusty winds and cloud- to-ground lightning. There will be a bit of a post-frontal surge Monday night into Tuesday with speeds increasing to 15-20 kt over the South Santee to Edisto Beach nearshore leg with 10-15 kt elsewhere. Northeast winds will gradually veer more southerly for Thursday and Friday. Seas will average 2-4 ft through the period, except up to 5 ft over portions of the South Carolina nearshore waters in the northeast flow behind the cold front. && .CLIMATE... Record High Minimum Temperature for June 30: KCHS: 78F set in 1977. (Low so far today 79F) Daily Rainfall Records for July 1: KCHS: 2.66 set in 1948 KCXM: 1.64 set in 1966 KSAV: 2.76 set in 1871 && .CHS WATCHES/WARNINGS/ADVISORIES... GA...Flood Watch from Monday afternoon through Monday evening for GAZ101-116>119-137>141. SC...Flood Watch from Monday afternoon through Monday evening for SCZ042>045-047>052. MARINE...None. && $$ NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE...