305 FXUS62 KCHS 221803 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 203 PM EDT Tue Jun 22 2021 .SYNOPSIS... A cold front will move through the area tonight, then stall just off the Georgia coast Wednesday. A coastal trough will persist through early next week. && .NEAR TERM /THROUGH TONIGHT/... A cold front that is associated with a broad and deep layered trough that covers much of the eastern half of the country is currently located over southwest Alabama into central Georgia. A rich plume of sub-tropical moisture, with PWat of 2 inches or more within the warm conveyor belt region will exist across the forecast counties throughout the day. The highest coverage of rainfall has passed as earlier convection helped stabilize the atmosphere. Current radar shows only stratiform rain remaining over our SC and coastal GA counties, while some weak convection widely covers the offshore waters. Clearing could allow the atmosphere to destabilize somewhat and as the front becomes closer in proximity from the west this evening, more support from the right entrance region of the upper jet over the Southeast, could support another round of scattered showers and possibly a few rumbles of thunder. Coverage should remain inland and diminish as the front gradually moves east. The grounds are saturated, more so now from the rainfall this morning. The 0-10 cm Soil Moisture analysis from NASA is as great as the 80th or 90th percentile across the local area. So it won't take much to generate at least minor flooding problems. But since the instability is limited, any threat looks to be highly localized. Tonight: The final short wave pulls offshore early on, and the cold front at the surface limps through the area almost parallel to the flow aloft. There is a slight decrease in PWat, and with further stabilizing influences within the nocturnal environment, we do anticipate less activity after dark. We'll still go with scattered coverage in association with the nearby cold front. The saturated atmosphere and winds diminishing, plus low condensation pressure deficits will lead to the formation of stratus build down. Latest projections are that it won't be low enough to produce any significant reduction in visibilities, but we will reevaluate through the day to see if fog should be included in the forecast. Lows tonight will get down close to actual dew points, which will be in the upper 60s and lower 70s. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY/... The cold front will stall off the coast Wednesday morning, then transition into a coastal trough. Moderate moisture will reside over the area Wednesday into Thursday while the trough maintains weak low-level convergence. A late afternoon sea breeze is also expected both days which could trigger additional showers and tstms. The trough is expected to move inland on Friday, accompanied by a weak upper low. We expect to see an increase in moisture along with the trough, pushing PWATs close to 2". Additionally, an early afternoon sea breeze could provide an additional trigger for convection. Conditions favor pretty good coverage of showers and tstms on Friday. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... Atlantic high pressure gradually strengthens over the weekend into early next week, resulting in a slow warming trend with convective coverage more scattered in nature. && .AVIATION /17Z TUESDAY THROUGH SATURDAY/... A disturbance has moved offshore allowing some clearing to occur over the terminals; however, some showers are moving northward toward KCHS/KJZI where mention of -RA/SHRA is included through 20Z. Shower activity could return by the evening ahead of an approaching cold front from the west, hence mention of VCSH, yet guidance shows any returning convection to stay west of the terminals. Since most of the greater rainfall coverage has passed, gusty winds have nearly diminished and models indicate build down of low stratus/stratocumulus which will result in MVFR/IFR ceilings through the night. There could be periods of VFR if enough clearing occurs in the late afternoon, but chances are that it will be brief. Models show a likely chance for flight restrictions to last through the night though they seem to struggle with the timing/possibility of returning to VFR. The current TAF reflects lifting of stratus around 14Z; this will hopefully be refined in the 18Z TAF issuance. Extended Aviation Outlook: Occasional, brief flight restrictions are possible in showers and thunderstorms, especially through Wednesday. && .MARINE... Today: The gradient remains rather tight between the sub- tropical Atlantic ridge extending across Florida and an upstream cold front that is situated in the Southeast. There is also a little mixing of the 30-35 kt geostrophic winds throughout the day. Together this supports a continuation of the Small Craft Advisories for all waters outside of the Georgia waters 0-20 nm offshore. Conditions there will need to be closely watched through the day, but latest indications are for any 25 kt gusts to be too infrequent and too limited in areal coverage to raise the advisory. Also of note, the Charleston Harbor has occasionally been struggling to achieve winds of 20 kt sustained and/or 25 kt gusts. So we do have some reservations about the ongoing advisory. We'll leave it in effect and see what happens when we get enough daytime warming to mix more winds down. As an aside, while most of the wave energy today will be short period and wind driven, there has been an occasional 16-18 second swell at both the Grays Reef Buoy and the St. Marys Buoy. Swell periods of that length are rather unusual in this part of the Atlantic, but appear to be backswell from the former TC Claudette. Tonight: The advisories will remain in effect early on, but come down during the late evening and overnight as the gradient slackens with the approach of the cold front. Due to already low cloud decks, the nocturnal environment will result in clouds lowering further through the night. There is a concern that at least a minor reduction in visibilities will transpire. Wednesday through Sunday: A northeast flow will develop later on Wednesday behind a cold front, then persist into Thursday before shifting to the southeast. Persistent southeast flow then expected through early next week. The current forecast keeps winds/seas below advisory thresholds. Rip Currents: At least some swell energy will impact the beaches today. That along with moderate winds that are roughly parallel to the shoreline, astronomical influences and reports of rip currents of late, supports a "Moderate Risk" of rip currents at all beaches today. Additionally, a strong south- southwest to north-northeast longshore current will also occur. A 1-2 ft swell combined with onshore winds in the afternoon will result in a moderate risk for rip currents along the SC coast on Wednesday. && .TIDES/COASTAL FLOODING... A northeast wind will develop on Wednesday and continue through Thursday evening before shifting to the southeast. Tidal anomalies should gradually increase during this period. Given the approaching full moon and the associated increase in astronomical tide levels, when adding another 0.5-0.8 ft departure, we expect to reach minor coastal flooding in Charleston Harbor with the evening high tides Wednesday through Friday. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for AMZ330. Small Craft Advisory until 10 PM EDT this evening for AMZ352- 374. Small Craft Advisory until 1 AM EDT Wednesday for AMZ350. && $$ NEAR TERM... SHORT TERM...JRL LONG TERM...JRL AVIATION... MARINE... TIDES/COASTAL FLOODING...