National Weather Service Charleston SC
202am EDT Monday July 1 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 6am This Morning
Tonight: Aloft, h5 shortwave energy will continue to nudge southeast across the local area along the eastern periphery of a weakened ridge extending across the Deep South. At the sfc, a weak trough will persist inland ahead of an approaching cold front. The pattern will continue to support scattered to numerous showers and a few thunderstorms across inland areas, mainly south of I-16 during the next few hours. Here, observed rainfall totals between 2-3 inches have been common, and an additional 1/2 inch of rainfall could occur as an outflow boundary nudges activity south and eventually beyond the Altamaha River during the next 1-3 hours. Outside of a temporary cooling from precip, temps will remain mild, with lows generally ranging in the mid 70s inland to upper 70s/near 80 near the coast.
Short Term - 6am This 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 forecast to hold in the mid- upper 70s with PWATs (Precipitable Waters) nearing 2.50" at times (daily max for KCHS for July 1 is just over 2.30" per Storm Prediction Center 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 Quantitative Precipitation Forecast 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 9pm Monday for the South Carolina and Georgia coastal counties, including Effingham, Hampton, Inland Colleton, Dorchester and Inland Berkeley. Storm total Quantitative Precipitation Forecast Monday into Monday night will run 2-4" with localized amounts in excess of 6". Categorical probability of precipitation 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 probability of precipitation 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/thunderstorms 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/thunderstorms 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.
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 to 20 kt, and perhaps a few gusts up to 25 kt along the coast during the next hour or two, but trends favor a gradually diminishing of winds late. The duration should fall just short of the need for a Small Craft Advisory across local waters overnight. Seas will range between 2-4 ft, largest across offshore Georgia waters.
Monday through Friday: A cold front will drop south through the waters Monday with east/northwest winds prevailing all areas Monday night. Widespread showers/thunderstorms with heavy rain could reduce visibilities 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
Daily Rainfall Records for July 1: KCHS: 2.66 set in 1948 KCXM: 1.64 set in 1966 KSAV: 2.76 set in 1871
NOAA Charleston SC Office: Watches - Warnings - Advisories
GA...Flood Watch from noon EDT today through this evening for GAZ101-116>119-137>141. SC...Flood Watch from noon EDT today through this evening for SCZ042>045-047>052.
Marine
None.
