406 FXUS62 KCHS 202112 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 412 PM EST Thu Dec 20 2018 .SYNOPSIS... A strong storm system will impact the area tonight into Friday. High pressure will then return this weekend, persisting through early next week. More significant rain chances are expected to return late next week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... The heaviest rainfall appears to be winding down across the area as a result of the axis of deep moisture now shifting to the east. Low-level water vapor imagery shows quite a bit of dry air moving into the area from the west. The 40-50 kt 925 mb winds on the southeast periphery of a surface low over central SC/GA have shown themselves this afternoon in the form of occasional 30-40 kt wind gusts at a few sites. As the low-level jet shifts to the east through early this evening, the strongest winds will also shift east. Moderate southerly flow will continue through tonight but the strongest gusts should drop off by early evening. A bit of a lull in the precipitation is possible through this evening. Later tonight we expect an increase in isentropic ascent, especially across inland areas, as a result of wrap around moisture on the back side of the surface low. We thus show increasing PoPs later tonight with mainly light to moderate rain expected. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/... Friday: The mid-levels will consist of a strong trough stretching from the Great Lakes region into the Southeast during the morning hours. The trough will move eastward with time and become negatively tilted. At the surface, the strongest portion of the storm system will be far to our northeast. However, the second low initially to our west early in the morning will quickly move to the northeast with time. Gulf Coast high pressure will begin building into the Southeast overnight. Wrap around precipitation associated with the departing low should gradually wind down during the daytime. QPF shouldn't add up to much. Dry conditions should prevail overnight. Temperatures will be near normal. Lake Winds...The winds across Lake Moultrie will increase to 15-20 kt with gusts up to 30 kt on Friday. Hence, a Lake Wind Advisory has been issued with this package. Wind Advisory...Southwest winds are expected to increase to 20-30 mph across Charleston County on Friday. Wind gusts up to 45 mph are expected at the immediate coastline. Hence, a Wind Advisory has been issued. If winds along other portions of the coastline rise a few mph, then it's possible the advisory may need to be expanded. Saturday and Sunday: The mid-levels will consist of a negatively tilted trough moving off the coast early Saturday, followed by zonal flow developing over the Southeast into the afternoon and prevailing into Sunday. At the surface, broad high pressure initially over the northeastern Gulf of Mexico will move towards the Southeastern U.S. through Sunday. Dry conditions will prevail. Temperatures may be a few degrees below normal Saturday and near normal on Sunday. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... Mainly dry conditions expected until Wed night or Thu when deeper moisture returns to the area. Cooler and dry conditions expected to prevail this period with high pressure generally dominating. Temperatures should be near to above normal. && .AVIATION /21Z THURSDAY THROUGH TUESDAY/... Mainly IFR conditions at both sites through late afternoon or early evening due to a combination of low ceilings and vsbys in rain. Gusty winds also anticipated this afternoon as low pressure moves through the area. Overnight, mainly MVFR ceilings expected behind a cold front with decreasing coverage of rain showers. VFR conditions expected to return by daybreak Friday though winds will become quite gusty heading into the afternoon. Extended Aviation Outlook: A departing storm system will bring flight restrictions into Friday night along with gusty winds. VFR is expected Saturday through Tuesday. && .MARINE... Winds have been quite gusty this afternoon as a strong low-level jet moves through the waters. As this jet shifts east this evening, marine winds will subside to some degree. However, winds will ramp back up later tonight as the inland surface low strengthens and the gradient tightens. Small Craft Advisories continue for all waters outside Charleston Harbor through the night. The SCA for Charleston Harbor drops off at 11 pm after which the gusts should be just below 25 kt. A storm system will be departing the area on Friday. However, the pressure gradient will remain steep behind the departing system, at least through the afternoon hours. With wind gusts approaching 40 kt, we have Gale Warnings in effect for all of the waters and the Charleston Harbor through Friday evening. Steep, wind-driven seas are expected to peak above 10 ft across the SC waters, above 8 ft for the nearshore GA waters, and possibly as high as 15 ft across the outermost portion of the offshore GA waters. The gradient will lessen into Friday evening and overnight, allowing winds and seas to lower. We did adjust the end time of the Gales for the Charleston Harbor and the nearshore GA waters to end several hours sooner, based on model trends and increasing certainty. The remaining warning end times were left unchanged. Regardless, Small Craft Advisories will be needed after the Gales end and these advisories will possibly last into Saturday afternoon, depending on how long seas take to subside. High pressure is expected to bring low winds and seas for the rest of the forecast period. High Surf: NWPS breaking wave guidance indicates high likelihood of at least 5 foot breakers along the coast through midday Friday in coastal GA and Friday evening on the SC coast. Thus we maintained the High Surf Advisory for all areas. && .TIDES/COASTAL FLOODING... Tides will likely be running above astronomical values through tonight though anomalies should start to drop as wind directions shift more SW. We are currently showing Fri morning high tide just below 7 ft MLLW. Friday night winds are forecasted to turn offshore, which will cause tides to drop to about -2 to -3 ft MLLW. Winds stay either offshore or parallel to the coast Saturday through the middle of next week, which should keep tides below advisory levels. && .CLIMATE... Daily precipitation records for Dec 20: KCHS...1.16 inches in 1973. KCXM...2.18 inches in 1922. KSAV...2.14 inches in 1911. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...High Surf Advisory until noon EST Friday for GAZ117-119-139- 141. SC...Lake Wind Advisory from 8 AM to 9 PM EST Friday for SCZ045. Flash Flood Watch until 8 PM EST this evening for SCZ045-050- 052. High Surf Advisory until midnight EST Friday night for SCZ048>051. Wind Advisory from 10 AM to 6 PM EST Friday for SCZ050. MARINE...Small Craft Advisory until 7 AM EST Friday for AMZ350-352-354. Gale Warning from 7 AM Friday to 7 AM EST Saturday for AMZ350- 352. Gale Warning from 7 AM Friday to midnight EST Friday night for AMZ330-354. Gale Warning until 7 AM EST Saturday for AMZ374. Small Craft Advisory until 11 PM EST this evening for AMZ330. && $$ NEAR TERM...JRL SHORT TERM...MS LONG TERM...RJB AVIATION...JRL/MS MARINE...JRL/MS TIDES/COASTAL FLOODING... CLIMATE...