917 FXUS62 KCHS 040827 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 427 AM EDT Sun Aug 4 2019 .SYNOPSIS... A stationary front spread across the region will gradually dissipate through tonight. A weak surface trough will then prevail inland with high pressure in the Atlantic during the middle and latter part of the week. && .NEAR TERM /THROUGH TODAY/... The mid-level circulation near Mobile Bay that has been embedded within a broad trough across the lower Mississippi Valley and portions of the Deep South over the past several days will finally kick out today in response to a shortwave moving southeast across the Central Plains. Guidance is similar in showing the vorticity maximum associated with this feature moving across south-central Georgia and into the South Carolina Midlands by early evening with a modest band of DPVA traversing Southeast South Carolina and Southeast Georgia during peak heating while simultaneously becoming aligned beneath a strengthening corridor of 250 hPa divergence. Model vertical cross sections show the influences of this favorable alignment with a pronounced deep-layered omega signature noted propagating from west-east during the 18-00z timeframe. Favorable kinematic forcing coupled with modest instability is expected to support the development of scattered to numerous showers/tstms, first developing across interior Southeast Georgia then spreading east to the coast into the late afternoon/early evening hours. Convection is expected to weaken some as it approaches the coast and crosses over the inland moving sea breeze. Pops have been raised to 70% for all areas this afternoon. Severe weather parameters are not overly impressive given the tropical airmass in place and lack of DCAPE, but a few strong tstms with gusty winds, heavy rainfall and frequent cloud-to-ground lightning can not be ruled out. Highs will warm into lower 90s prior to the onset of convection and hourly temperatures have been adjusted to try and take the influences of convection into account. && .SHORT TERM /TONIGHT THROUGH WEDNESDAY/... Tonight: The initial line of showers/tstms is expected to exit off the coast by early evening while steadily weakening. There are signals that a secondary area of convection could develop into the Walterboro-Shulerville corridor by mid-evening as a secondary area of upper forcing pushes through. However, it is unclear how much instability will be left for this activity to work with. Did opt to hang onto some higher end pops in the Charleston Tri- County to account for this possibility. Convection should be mostly over by midnight with dry conditions expected to dominate early Monday. The boundary layer is likely to decouple early once again with calm/light winds prevailing. Although there will likely be high clouds/debris clouds in place for much of the night, some patchy fog and stratus could still form. Lows will range from the lower 70s inland to the mid- upper 70s at the coast. Monday through Wednesday: The mid-levels will consist of a trough prevailing over the Southeast. At the surface, a weak trough will prevail inland with high pressure in the Atlantic. Plenty of moisture will remain in place across our area. PWATs should hover around 2.0". Though, there will be minor fluctuations higher and lower each day and night. All of the models remain in good agreement showing good coverage of showers along with some thunderstorms on Monday. The coverage/intensity will decrease Monday night. Far inland locations may end up being dry. Though, showers/thunderstorms should persist over the coastal waters and possibly bring occasional impacts to areas closer to the coast. Another round of showers/thunderstorms is expected on Tuesday. Though, the daytime coverage/intensity should be less than what we experience on Monday. The highest POPs are across our coastal GA counties and areas near the Altamaha River. The coverage/intensity of showers/thunderstorms over land will decrease again Tuesday night, but increase over the coastal waters. Occasional impacts to the coastal counties are possible. Even far inland areas could see an isolated light showers. Wednesday we appear to be looking at a more typical summer afternoon of showers/thunderstorms with chance POPs. The severe risk is relatively low Monday and Tuesday. But the risk starts to creep upwards for a few pulse severe thunderstorms on Wednesday due to increasing instability and hints of drier air starting to work into our area. Additionally, we could experience periods of locally heavy rain with localized flooding of low-lying and poorly drained areas if there is training or slow movement of cells. Temperatures will be near normal during the day and slightly above normal at night. && .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... Mid-level troughing will start out over the East Coast Wednesday night. Though, models are in good agreement showing it slowly moving offshore and weakening into Saturday. At the surface, weak troughing will prevail inland while high pressure is over the Atlantic. Models hint at a front approaching from the north Wednesday night into Thursday, but it doesn't appear to make it to our area. However, another front approaching from the north may make it to our area late Saturday. Expect the typical afternoon/evening showers/thunderstorms with temperatures within a few degrees of normal. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/... Risk for fog/stratus looks limited, although some shallow ground fog could still develop around both KCHS and KSAV. Main concern for today is likely tstm impacts this afternoon ahead of potent shortwave energy coming in from the southwest during peak heating. High resolution guidance tends to favor KSAV for greater impacts, but mesoscale processes will eventually drive the convective picture. This is hard to pin down this far out. Will therefore limit conditions to VFR-MVFR at both terminals in tstms, roughly 21-00z. Will likely be able to pin down specifics a bit more with the 12z TAF cycle, thus TEMPO groups for lower conditions may be introduced at that time. Extended Aviation Outlook: Expect brief flight restrictions due to afternoon/evening showers/thunderstorms. && .MARINE... Through Tonight: Light northwest winds will develop in the wake of a weak surface trough pushing offshore this morning. Winds will turn back onshore this afternoon as the sea breeze develops. Speeds will remain less than 10 kt. Winds will veer to the southwest and west overnight and increase to 10-15 kt within a weak nocturnal surging regime. Seas will average 2-3 ft through the period. Sunday through Thursday: No Small Craft Advisories are expected for the extended forecast. The synoptic pattern will consist of a weak surface trough inland and high pressure in the Atlantic during the middle and latter part of the week. Waterspouts: Light wind fields, a tropical airmass with PWATS in excess of 2 inches, and convergence lines of cumulus setting after daybreak will support a risk for waterspouts this morning. Waterspout indices on the latest coastal RAP soundings are in the upper 50s to near 60 which is borderline for a moderate to high risk day. Suspect the best chances will initially be confined to the Georgia and far southern South Carolina marine zones where the latest RAP output shows favorable non-supercell tornado parameters of greater than 1 unit along/ahead of weak surface trough moving offshore. The risk will then shift back closer to all nearshore marine zones once the trough pushes to the east until a weak sea breeze circulation develops by late morning. && .TIDES/COASTAL FLOODING... Tide levels could get close to 7.0 ft MLLW (minor flood) in Charleston Harbor later this morning. Current data suggest levels will fall just short and even if it touches briefly exceeds 7.0 ft the short duration suggests a Coastal Flood Advisory will not be needed. Trends will be monitored, however. Astronomical influences and tidal anamolies will continue to diminish today. Still expect levels to exceed coastal flooding thresholds at Charleston Harbor and possibly Fort Pulaski, so one more Coastal Flood Advisory will likely be needed for at least the South Carolina coastal zones for late evening high tide cycle. The Georgia coastal zones look a bit more marginal. For Monday, despite lowering astronomical tides and winds parallel to the coast, there remains a small risk of minor coastal flooding, mainly for south coastal South Carolina. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$