textproduct: Charleston

WHAT HAS CHANGED

Temperatures are off to a warm start this morning. Highs were nudged up slightly from the previous forecast for some areas and near record to record highs are still expected at both KCHS and KSAV. Near term guidance is consistent in showing scattered to numerous showers with a few tstms working in the from the west this afternoon. Hourly near term pops were adjusted slightly to account for these trends but net 12-hour pops were mostly unchanged. Various degrees of sea fog will continue to impact the beaches through the morning and into the afternoon. Conditions on the lake look a bit marginal for a Lake Wind Advisory, especially given lake water temperatures are still in the mid 50s. The Lake Wind Advisory was maintained, but adjustments may be needed if winds to not pick up soon.

KEY MESSAGES

- 1) Near record high temperatures today. Additionally, gusty southwest winds will be highlighted with a Lake Wind Advisory.

- 2) Strong to severe storms possible on Saturday afternoon and evening.

- 3) Sub-freezing temperatures return on Sunday night and persist into the mid-week.

DISCUSSION

KEY MESSAGE 1: Near record high temperatures today. Additionally, gusty southwest winds will be highlighted with a Lake Wind Advisory.

Today, gusty southwest winds will advect very mild llvl thicknesses across the forecast area today. The day should begin with temperatures in the low to mid 60s. After sunrise, temperatures should steadily warm through early this afternoon. Temperatures are forecast to peak in the low 80s across inland GA with values around 80 degrees across most of the SC Lowcountry. The forecast high temperatures are expected to remain within 3 degrees of record values, primarily at KCHS and KSAV (see Climate Section). As mixing heights deepen to around 3 kft, wind gusts may strengthen to 25 to 30 mph.

Recent runs of the HRRR and HREF, indicate that a band of scattered to numerous showers will outpace the cold front, reaching the forecast area this afternoon. Coverage of the showers should remain the greatest inland, especially over the Savannah River. These showers will be supported by CAPE between 300-800 J/kg and 35 kts of 0-1km bulk shear.

Lake Wind: Southwest winds will strengthen to 15 to 20 kts with gusts 25 kts today. A Lake Wind Advisory has been issued for Lake Moultrie until 4 PM this afternoon.

Sea fog could advect onshore and impact the beaches and nearby coastal locations, with visibilities dropping to 1 mile or less at times today. We will monitor the need for Dense Fog Advisories over land.

KEY MESSAGE 2: Strong to severe storms possible on Saturday afternoon and evening.

A stalled front will linger across parts of the Southeast on Saturday as a shortwave trough approaches the region. Aided by additional troughing located over the central and northern Plains, the mean-lvl flow aloft will strengthen and become increasingly parallel by mid-day Saturday. Simultaneously, this will enhance cyclogenesis along the stalled front in the north-central Gulf and then translate across South Carolina by the afternoon. As this cyclogenesis develops, this stationary front will be pushed northward throughout the evening and will allow for the region to be positioned in the warm sector. Expect low-lvl moisture to increase as dewpoints climb into the mid to upper 60s by mid-day as the shortwave trough inches closer. Additionally, 0-6km bulk shear on the order of 50 to 70 kt will be prevalent by late Saturday afternoon. The 20/01z NBM depicts a risk for SBCAPE values >500 J/kg by late Sat. afternoon (~4-5pm) with the greatest potential across portions of southeast Georgia. These factors will support a modestly unstable environment and yield scattered to numerous showers and thunderstorms, with the primary window for severe thunderstorms in the early afternoon through the evening. The main hazard will be damaging straight-line winds with gusts potentially reaching 60 mph within the stronger thunderstorms. However, a brief weak tornado remains a non-zero chance (mainly across southeast Georgia) as modest low-lvl helicity and 0-6km bulk shear will be present. To reflect this severity, SPC has highlighted the region in marginal risk for severe thunderstorms on Saturday. The main uncertainty will be centered around how much instability can develop in the warm sector, as this will determine if the severe risk will need to be heightened.

Along with the severe weather risk, expect rainfall values from 0.40 to 1.0 inches from Sat. morning through Sun. morning as the scattered to numerous showers and thunderstorms (mainly induced by isentropic ascent and warm-air advection) push across the region. Vertical cross sections show modest deep-layered UVVs induced by DPVA ahead of a shortwave that is forecast to dig across the Tennessee Valley into the southern Appalachians. This coupled PWATs climbing above 1.5 inches and the presence of some surface-based instability should easily support some spots receiving over an 0.50 inches over a 6-hour period. This could lead to brief urban, or poor drainage flooding, particularly in the Charleston and Savannah metro areas.

These showers and thunderstorms should gradually shift offshore by late Saturday evening as the front clears off the coastline, followed by veering winds and drying from west to east overnight. Expect cooler and more stable air to filter into the region by early Sunday morning and effectively end the severe weather threat.

KEY MESSAGE 3: Sub-freezing temperatures return on Sunday night and persist into the mid-week.

As a cold front slides off the coastline on Sunday morning, expect cold surface high pressure to build in behind and allow the return of winter-like temperatures on Monday and Tuesday morning. Some uncertainty still exists with this system as the 00Z.LREF cluster-1 analysis contains roughly 45% of the GEPS members and 35% of the ensemble members, emphasizing the split of solutions. However, the NAEFS ESAT seems to be supportive of below-normal temperatures as it depicts temperature anamolies ranging from -2 to -4 on Monday and Tuesday morning. The forecast reflects these below-average temperatures with minimum temperatures on Monday morning ranging from upper 20s to low 30s and low to mid 20s on Tuesday morning. These temperatures combined with breezy winds will aid in wind chill values in the low to mid 20s each morning. The chance for wind chills to drop below 20 degrees (Cold Weather Advisory criteria) has lessened and seems less likely with the 20/01Z NBM. Both Monday and Tuesday, expect highs to recover to only the upper 40s to low 50s as the cooler airmass sticks around. Afterwards, expect temperatures to warm back up on Wednesday through the end of the week.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

12Z TAFs: Prior to the 12Z, KCLX VAD wind profile observed 40 kts at 2 kft. Wind sfc winds less than 10 kts, LLWS is expected until 13Z over KCHS. All terminals will develop gusts into the low 20 kts by 13Z, which will end the LLWS at KCHS. By 17Z, mixing may deepen to around 3 kft across the terminals, resulting in gusts around 25 kts. High resolution guidance indicates that band of scattered pre-frontal showers may pass over the terminals between 18-22Z, highlighted with a TEMPO or PROB30 at each terminal. Wind gusts should end by 22Z, wind winds remaining from the southwest through the rest of the 6Z TAF period.

Extended Aviation Outlook: Flight restrictions are likely at times through the weekend due to potential advection of marine stratus inland as well as scattered showers/thunderstorms Friday and Saturday.

MARINE

Sea Fog: Webcams up and down the coast show sea fog is rapdily expanding across the local waters this morning. The Marine Dense Fog Advisory has been exanded south to include all remaining waters with the exception of Charleston Harbor through 5 PM.

The pressure gradient pinches today, wind gusts across a portion of the SC waters may gusts to around 25 kts during the daylight hours. Small Craft Advisories have been issued for Charleston County nearshore, CHS Harbor, and outer GA waters. A frontal wave is forecast to develop over the I-20 corridor of the Deep South tonight, tracking east on Saturday. The associated cold front is timed to push over the marine zones Saturday evening, with CAA and strengthening winds Saturday night. On Sunday, Small Craft Advisory conditions should spread across the nearshore SC/GA waters with gales possible across the outer GA waters. Gusty winds and elevated seas may linger into Monday.

CLIMATE

Record High Temperatures:

February 20: KCHS: 82/2018 KSAV: 84/1991

February 21: KSAV: 84/2018

Record High Minimum Temperatures:

February 20: KSAV: 64/1939

February 21: KCHS: 62/2023 KSAV: 63/2023

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...Lake Wind Advisory until 4 PM EST this afternoon for SCZ045. MARINE...Small Craft Advisory until 7 PM EST this evening for AMZ330- 350. Dense Fog Advisory until 5 PM EST this afternoon for AMZ350- 352-354-374. Small Craft Advisory until 3 PM EST this afternoon for AMZ374.