394 FXUS62 KCHS 232205 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 605 PM EDT Sat Jul 23 2022 .SYNOPSIS... An upper level low over the Atlantic will drift west Sunday and will push onshore and weaken over the Southeast early this week. High pressure will then expand over the region and will prevail into next weekend. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... After the "excitement" of the heavy rains and wind gusts estimated to 80 mph in the North Charleston area near the Cooper River, things have quieted down somewhat. However, numerous outflow boundaries persist, so we still look for scattered convection through mid evening. The best chances will be inland in closer proximity to these boundaries and the sea breeze. The early evening update accounted for these trends. Previous discussion... Tonight: The large scale pattern aloft will feature extensive ridging across much of the southern tier of states, while the sub-tropical Atlantic ridge remains further offshore than is typical. In between is a prominent TUTT Low that is about 500 miles east of Savannah and moving slowly west- northwest. Lobes of vorticity rotating around this system is actually helping to enhance convection over the Gulf Stream. At the surface the main feature is the Bermuda-Azores High, with weak troughiness further inland. Meanwhile, the MCV from the convection last night is spinning about 30-35 miles offshore of McClellanville as of late afternoon. Convection will continue to be driven along the meso-scale boundaries, especially the sea breeze which is near or inland from the US-17 corridor. But outflows and differential heating boundaries will also come into play. The HREF solution has been the best so far, and our forecast will rely heavily on that scenario, which implies that scattered to numerous showers and t-storms will occur. Weak shear and DCAPE values near or greater than 1000 J/kg will support isolated wet microbursts. Elevated PWat about 100-110% of normal, limited storm motion and intense rainfall rates will support localized flooding of low-lying and poor drainage areas. The HREF has low rainfall probabilities of 1 inch or greater in an hour and 3 inches in 3 hours, similar to what we've already seen from near daniel island north to near Huger and Shulerville. Since high tide is late this afternoon and some of the stronger storms are building toward the downtown Charleston area, there will be an elevated flooding risk. In addition, should the heavier rains get into the West Ashley area, Church Creek is just 1.25 ft below Minor Flood Stage. So this is another area that requires keeping a close watch out for. Given the strength of the updrafts and MUCAPE values in excess of 3000 J/kg, frequent cloud to ground lightning will occur in some of the storms. Most convection will fade quicker than last night, with latest indication that they'll generally come to an before midnight. Guidance has not been handling the convection over the Gulf Stream all that good so far, which makes me feel that as the TUTT Low gets a bit closer to the coast, we will see an increase in coverage offshore through the night. Combined with weak coastal convergence and isentropic ascent and more vorticity lobes rotating in from the northeast, we could see some convection brush onshore of Charleston County before daybreak. Wherever we have 50-60% PoPs we dropped hourly temps into the mid and upper 70s. But overnight min temps will average in the lower and middle 70s. If there is enough decrease in cloud cover, there could be some fog where rainfall occurred. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/... Sunday: An Atlantic upper low will approach the Southeast coast. Initially, this feature could support isolated showers/thunderstorms over coastal counties and adjacent coastal waters. Then, with diurnal warming/destabilization, additional thunderstorms are expected, initially where vorticity on the western periphery of the low interacts with the sea breeze. The favored regime for thunderstorms should then spread inland with time along the advancing sea breeze/other mesoscale boundaries. Ongoing maximum PoPs capped at 40-50 percent could prove conservative in some areas. Also, weak steering flow will translate to a chance for locally heavy rainfall, while the threat for brief damaging wind gusts will remain low. Outside thunderstorms, highs in the upper 80s/lower 90s and heat index values from the middle 90s to lower 100s will be common. Showers/thunderstorms should then dissipate Sunday evening. However, as the upper low continues to press closer to the coast, a few additional showers/thunderstorms could develop over or push into coastal counties and adjacent coastal waters, especially overnight/early Monday. Monday and Tuesday: The Atlantic upper low will gradually push onshore and weaken over the Southeast. This feature should continue to combine with typical instability to support diurnal/pulse thunderstorms along the sea breeze and other mesoscale boundaries. Due in part to the weakening upper low, maximum PoPs gradually decrease Monday and Tuesday. While this downward PoP trend is reasonable, our chance PoPs for Monday could be conservative. While Monday heat index values should top out in the 95-100F range, lesser thunderstorm coverage could translate to higher temperatures and expanding maximum heat index values in the lower 100s. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... Building high pressure at the surface and aloft should support above normal temperatures and a below-normal chance for thunderstorms mid week into next weekend. Many locations will experience maximum heat index values exceeding 100F, and coverage of heat index values 105- 109F will expand as the week progresses. Heat Advisories could eventually be required, but as of Saturday afternoon the probability for heat index values exceeding 110F remain low late week into next weekend. && .AVIATION /22Z SATURDAY THROUGH THURSDAY/... Other than nearby SHRA/TSRA early this evening, convection is done with for the night. Any potential for SHRA/TSRA Sunday will occur too late in the 18Z TAF cycle to include at this time. thus we have a VFR forecast. Extended Aviation Outlook: Brief flight restrictions are possible during afternoon/evening showers and thunderstorms, with the highest probabilities for thunderstorms Monday and Tuesday. Also, typically for this time of year, the developing diurnal cumulus field could briefly produce MVFR ceilings any morning through this period. && .MARINE... This evening and tonight: The MCV was located about 30-35 miles east of McClellanville and moving little, before it should dissipate early on. The sub-tropical Atlantic ridge will be the main feature, creating mainly S and SW winds as high as around 15 kt. Of course these winds are outside of t-storms, which will produce winds of 30 or 40 kt. The best coverage will be increasing from the Gulf Stream westward through the night. Seas will average 2-3 ft, except higher in t-storms. Mariners should also be alert for reduced visibilities near zero in very heavy rains, along with frequent cloud to water lightning strikes. Sunday through Thursday: An upper level low pressure over the western Atlantic will advance toward the coastal waters Sunday and Monday. This feature could at times enhanced coverage of thunderstorms and associated hazards over marine zones. Otherwise, offshore high pressure will dominate the environment through this period, with south/southwest winds mainly 15 knots or less enhanced by the daily sea breeze and periodically stronger with nocturnal surging. However, the probability for any SCA winds will remain low. Combined seas 2-3 feet Sunday through Monday will build to 3-4 feet for the remainder of the week. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM... SHORT TERM...SPR LONG TERM...SPR AVIATION... MARINE...