... 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 11am EDT Saturday: Several updates this morning. Cloud shield is keeping temperatures down across SW zones so temperatures have been adjusted accordingly. Still expect quite a warm day across the NC Piedmont/I-77 corridor but across the middle of the area some bust potential depending on how quickly the cirrus overspreads the area. Of course, the rest of the forecast updates have to do with Claudette. 11am advisory update (and 8am before that) continue to trend farther south with the track, so any wind impacts with the actual center of the storm are minimal, but for today as the pressure gradient increases ahead of the storm and with the weak surface ridging from the Atlantic high, likely winds could be stronger today with low-end gusts than tomorrow (unless the stronger ECMWF ends up correct, but still with the trend farther south with the storm wind impacts are minimal).
Main concern remains the rain, with the deformation zone setting up on the NW side of the storm and of course typical topographic effects with the NE winds resulting in the orographic upglide on the north side of the storm. 06z and now 12z NAM have trended farther north with the heavy QPF axis, but the 12z NAM is a faster shot so some of the totals are down a bit, but the trend away from the Piedmont/Lakelands and toward the higher elevations where anytime guidance is pointing to 3-4+" in less than 12h is a concern. In any case, there is enough indications that SOME place will receive heavy rain to justify a Flash Flood Watch, but not enough confidence just yet to know where. Will monitor the rest of the 12z guidance as it trickles in and coordinate with surrounding offices/WPC and plan to issue with the afternoon package.
Will also have to reevaluate any severe/tornado potential with the afternoon package.
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.