... The remnants of Tropical Storm Claudette will track northeast across the Southeast through the weekend, increasing shower and thunderstorm chances across our area through early Monday. A cold front pushes into the area Tuesday, keeping shower and thunderstorm chances around before drier weather returns for the middle of next week.
NEAR TERM /THROUGH TONIGHT/... As of 645 AM: We now have Tropical Storm Claudette, which organized as it was making landfall in southeastern LA. The large majority of the moisture and precip is to the east of the center, spreading across MS/AL. Still just a thin cirrus shield over our area attm, and that will continue thru the morning hours, until some thicker clouds start to spread in from the SW this aftn. The clouds may be enough to limit much instability, as most guidance has backed off max temps a tad. The NBM is the warmest of all guidance. So trimmed temps back a deg or two. Still expecting above normal highs in the NE and near to slightly below normal in the SW. It will be breezy across the Piedmont today, as a sfc ridge briefly strengthens between the approaching Claudette and pressure falls to the northeast where there's stronger heating. PoPs weren't changed much from previous forecast, ranging from less than 15% east of I-77 to 40% in northeast GA. Mainly showers, with a few sub-severe tstms.
Tonight, Claudette will likely weaken to a tropical depression or become post-tropical, as it drifts east-northeast across central MS to central AL, spreading in deep tropical moisture into the forecast area from SW to NE overnight. There is some signal that mesoscale forcing will produce some locally heavy rainfall somewhere over northern GA or into Upstate, probably due in part to some 850-700 mb frontogenesis ahead of Claudette's circulation. But there is still a lot of disagreement among the different models and the CAMs on the magnitude and placement of the heaviest QPF. There is also a minimum of MUCAPE with this precip, which may limit the heavy rain potential. Given the uncertainty, will hold off on any Flash Flood Watch issuance with the morning package. But perhaps things may come into better agreement for an FFA later today. QPF amounts where heavy rain sets up could be 2-3" in a short period of time. Temps will remain above normal with the tropical moisture.
SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 330 am Saturday: Still plenty of model discrepancy regarding the evolution of potential TC 3 over the next 24 hours, with guidance sources very much sticking to their various camps: the GFS quickly spinning the cyclone down into a weak wave over the Deep South; the ECMWF maintaining a well-organized/evolving extra-tropical feature through GA and the Carolinas, and the Canadian acting as somewhat of a reasonable-looking compromise between those two extremes. The ECMWF therefore features the highest QPF, although a perfect prog of its surface low track would have the swath of heaviest precip falling just south of our CWA. Nevertheless, even in this scenario, any extra-tropical transition would result in a deformation zone developing to the left of the cyclone track, leaving our forecast area very much under a threat of heavy rainfall, especially across roughly the southern half of the area. Indeed, many of the CAMs develop some healthy-looking bands within this deformation zone area and drag them across the Upstate and northeast GA during Sunday morning. The other operational guidance...in particular the GFS...is considerably less concerning. With confidence still relatively low, WPC guidance...which has incorporated a healthy amount of CAMs output...has been followed closely for our official forecast, with the current best guess having 2 to 3.5 inches, with locally higher amounts from the southern tier of our NC zones south across the GA/SC zones. While certainly healthy, high flash flood guidance (4 to 6 inches in 6 hours across much of the forecast area) and lingering uncertainty leads us to kick the can down the road in terms of Flood Watch consideration.
The other concern will be the potential for embedded mini-supercells, especially Sunday afternoon/evening, when guidance depicts surprisingly robust levels of sbCAPE (2000 J/kg give or take) along with adequate effective helicity for rotating updrafts. Again, confidence in shear/helicity forecasts are low in light of doubts about the strength of the system, but a non-zero tornado threat is evident, especially south and east of I-85.
Heavy rainfall and marginal tornado threat should push east of the forecast area Sunday evening, with token small PoPs generally advertised after midnight. However, PoPs ramp up again into the solid chance range by the end of Monday, as strong height falls and an associated cold front infiltrate the East. While the front will likely remain west of the area through Mon evening, at least moderate instability is forecast to develop within a moist warm sector over our area, which should support development of diurnal convection. PoPs remain elevated Mon night, even ramping up a bit in western areas as the front draws closer.
LONG TERM /TUESDAY THROUGH FRIDAY/... As of 350 am Saturday: Likely to categorical PoPs are advertised Tuesday as a cold front sweeps through the forecast area. Shear and instability are forecast to be adequate to support at least a low-end severe storm threat during this time, while locally excessive rainfall will also be possible, especially in areas that receive heavy rainfall this weekend. Otherwise, cooling/drying conditions are expected during mid-week as anomalously low upper heights become established across the East. Small PoPs return for Day 7/Friday when the next cold front may make a run for the area.