textproduct: Columbia

WHAT HAS CHANGED

Updates for Sunday's system and Aviation discussion updated for 18z TAF.

KEY MESSAGES

- 1) Widespread rain expected Sunday with some thunderstorms possible in the Southern Midlands.

- 2) Well above normal temperatures mid to late next week.

DISCUSSION

KEY MESSAGE #1: Widespread rain expected Sunday with some thunderstorms possible in the Southern Midlands.

The long discussed, fast moving low pressure system will slide across the southern US throughout Saturday and into Sunday morning. A fairly strong cool season MCS is expected to develop and propagate along the gulf coast into early Sunday morning before weakening a bit thanks to the mediocre moisture and instability reservoir. By late Sunday morning, sufficient WAA and isentropic lift will overspread our area, while this residual MCS remains to our west. This initial surge of PWAT's and WAA should produce a widespread shield of moderate rain for much of the forecast area, with good agreement across the HREF members for the most part. QPF totals from this initial isentropically forced rain should be around 0.5" before the WAA lifts northward after 18z. PWAT's and theta-e will continue surge northward and allow for some weak instability to develop as the cold front forcing arrives in the afternoon. There are some more question marks as to how this potential convection will develop in the afternoon based on the MCS progression to the west and how much instability can actually develop. South of the warm front in the Midlands, the hodographs and kinematics will be very impressive so the instability and moisture availability are the limited and uncertain factors. Given those impressive hodographs, the 12z HREF and REFS suites is notably more aggressive with theta-e advection and therefore sufficient instability for a low end conditional tornado threat across the southern Midlands; parameter space wise the overlap of 50-100 0-3km CAPE and 30-40 knots of 0-3 km shear is plausible during the afternoon. A Marginal Risk driven by the low end tornado threat extends into the CSRA and southern Midlands as a result.

KEY MESSAGE #2: Well above normal temperatures mid to late next week.

No significant changes in the thinking for well above temps later this week. Global ensembles favor a strong upper level ridge building over the central CONUS mid- week, extending into the Southeast US. NAEFS climatological percentiles are near the maxima for heights in these regions for the latter half of the week. This favors well above normal temperatures from mid- week through the end of the work week. Highs will likely be in the 70s on multiple days. NBM QMD probabilities for highs above 70 degrees are above 80 percent from Wednesday through Friday. Thursday may be the warmest day with 80 degree probabilities from 30 to 60 percent for the southern FA.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

VFR conditions are expected into Sunday morning before steadily deteriorating throughout the rest of the day.

Dry and VFR conditions are expected into Sunday morning with southerly winds generally between 6-10 knot. Winds will weaken this evening, shifting out of the southeast. Cigs begin to drop starting early Sunday morning, but should remain VFR through 13z or so. Widespread persistent rain is then expected after 15z for all TAF sites, with cigs and vsby falling to IFR by the afternoon. Some thunderstorms are possible at the very end of the TAF period along with some heavy rain and LIFR conditions.

EXTENDED AVIATION OUTLOOK...Restrictions will remain Sunday evening eventually improving into early Monday before drier air moves in and VFR conditions return.

CAE WATCHES/WARNINGS/ADVISORIES

SC...None. GA...None.