Forecast Discussion
Summary
SYNOPSIS
... Weak high pressure remains over the area through Friday. A weak cold front drops south across the area Saturday night which brings our next chance for rain, and then a brief cooldown Sunday. Another cold front will arrive from the north Tuesday or Wednesday.
NEAR TERM /THROUGH TONIGHT/... As of 6:25 AM EDT Friday: Things remain quiet early this morning with winds having gone light to calm at most sites. Current IR satellite imagery is showing another round of mtn valley fog and low stratus over the southwest mtn and portions of the French Broad river valleys. Most of this should burn off pretty quickly once daytime heating commences. Temps appear to have already bottomed- out at most locations and have even warmed a bit at some Upstate and southern Piedmont sites over the past few hours.
Otherwise, the "messy" upper-level pattern will persist thru the near-term period with broad upper ridging centered to our north and a southern stream upper trof well to our south over the eastern Gulf of Mexico. By the end of the period early Sat, another upper trof will dive down over New England creating more of an Omega-type blocking pattern across the CONUS. Overall, today should be similar to yesterday, albeit a few degrees warmer this afternoon. The latest model profiles have been trending downward wrt sfc-based instability across much of our fcst area, although they still generate 500+ J/kg over the SW mtns and NE Georgia. This will allow for a slight chance of ridgetop showers and maybe a few weak tstorms this aftn/evening with dry conditions elsewhere. It is notable that the last few runs of the CAMs have been generating fewer showers across any of our zones thru the near-term period. If any convection does develop, the NELY flow should push the activity to the southwest and away from our CWA fairly quickly. Otherwise, high temps will be near- normal, if not slightly above, for early June.
SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 100 AM Friday: The large-scale omega block will remain in place across much of the North American continent through the weekend. Northeasterly flow will continue across the CWA. A sfc high will migrate southward on the east flank of the blocking upper anticyclone, bringing a backdoor cold front south along the Eastern Seaboard and a reinforcing shot of dry air. The core of the upper low will remain well east of our area, but a vort lobe will extend inland. The effects of the front should come too late to affect max temps Saturday, so temps will rise a category or so warmer than Friday.
Convection may struggle to get going at peak heating Saturday afternoon; profiles are quite dry with unimpressive lapse rates, and notably they feature an exceptionally deep mixed layer. Precip thus will have to overcome a lot of dry air to reach the sfc. Midlevels moisten with the front, which probably will reach the I-40 corridor during the evening. The GFS/NAM/GDPS depict most of their QPF after 00z. Shear is pretty weak no matter how you slice it, and MUCAPE appears likely to top out less than 1000 J/kg. Given that, despite the inverted-V-like soundings the damaging wind threat would probably end up pretty low. PWATs rebound to around 1.3 to 1.5 inches with the front, and locally heavy rain appears a possibility, with weak steering flow.
Showers may continue to bubble along the front into early Sunday morning. Subsidence in its wake most likely will completely suppress any deep diurnal convection in the Piedmont Sunday afternoon. Models seem to vary in how far they push the subsident airmass, with the ECMWF for one showing convection developing in the Savannah Valley. Several sources still depict storms firing over the western mountains, so will carry chance range PoPs there, tapering off east and south. Temps will be refreshingly cooler, in many spots falling back around 10 degrees from the previous day.
LONG TERM /MONDAY THROUGH THURSDAY/... As of 200 AM Friday: Most likely, we will continue to see convective coverage limited by subsidence through the day Monday, with mentionable PoPs only near the mountains, and temps moderating back to around normal. The omega block doesn't appear to truly break down over the course of the medium range period, but it effectively retrogrades a bit. One of the early factors in this evolution is a shortwave diving past the blocking ridge/anticyclone, which helps to deepen the upper low over the Northeast, and bring much stronger height falls to our region than have occurred since the block set up in mid-May.
Attendant to the shortwave, a relatively strong cold front will push south thru the Ohio Valley and perhaps even into the Deep South between Tuesday and early Wednesday. The deterministic GFS and EC, as well as most members of the global ensembles, respond with some QPF in our area along the front. We won't really have had an opportunity for moisture return prior to its arrival, and similar to the weekend fropa, relatively dry profiles and high LCLs look to be a limiting factor. Instability looks too limited for a severe threat. These models do force the front far enough south that we would appear likely to see a period of settled weather in its wake; we should get at least a couple of days of below-normal temps as a result (most likely Wed and Thu). The 02/00z Canadian GDPS offers an alternative solution in which the upper low fragments, leaving behind an almost cut off low to our west, a less progressive front, and generally wetter weather for the whole of the South. Only a handful of the NAEFS and EC ensemble members do this, so this solution presently appears an outlier. Temps should moderate heading toward next weekend, barring the Canadian type solution.