textproduct: Columbia

WHAT HAS CHANGED

Increased confidence in anomalously strong upper ridge and hot and humid conditions returning next week. Aviation discussion updated for 18z TAF issuance.

KEY MESSAGES

- 1. Diurnal thunderstorms expected through this weekend with the main threat of locally heavy rainfall.

- 2. Well above normal temperatures with dangerous heat index values possible next week.

DISCUSSION

KEY MESSAGE 1: Diurnal thunderstorms expected through this weekend with the main threat of locally heavy rainfall.

A lingering outflow boundary was a focus of earlier showers and likely will be again a focus for diurnally induced convection this afternoon and evening. Mesoanalysis reveals a strong gradient in PWATs from around 1.6 inches in the Piedmont to around 2.1 inches in the Coastal Plain. Expect scattered convection to develop with continued daytime heating in a moderately unstable environment but weak shear. Convection likely to favor the eastern Midlands and Coastal Plain where deeper moisture exists and aided by convergence along the sea breeze. DCAPE values are poor as are lapse rates so not expecting much of a severe threat, but with elevated PWATs and weak storm motion, heavy rain remains the primary threat with high rain rates possible. WPC continues to outlook the area in a marginal risk of excessive rainfall. Little change Sunday in the overall pattern, although the upper trough axis will be weakening as the upper ridge to the west begins to expand eastward. Isolated diurnal convection remains possible given abundant atmospheric moisture in place.

KEY MESSAGE 2: Well above normal temperatures expected with dangerous heat index values possible next week.

Increasing confidence in a hot and humid start to the month of September. Ensembles are in good agreement showing an anomalous upper ridge gradually migrating eastward from the MS Valley early in the week to the Appalachians and Carolinas by mid-late week. ESAT values of 500mb heights are above the 97th percentile Monday and above the 99th percentile through mid week and remaining above the 90th percentile through the end of the week. The strong upper ridging will create strong subsidence over the region which should really limit convection much of the week. The result should be well above normal temperatures and this is supported by ensemble forecast temperatures at 700/850/1000mb at or above the 99th percentile with EC EFI values for both max and min temperatures in the 0.7-0.8 range with non-zero shift of tails. Highs expected to be in the mid to upper 90s with lows in the lower to mid 70s with little spread among the NBM IQR. The combination of above normal temperatures and increased low level moisture will yield high probabilities of heat indices in the 100-105 range and possibly higher.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Convection is possible this afternoon and evening, with restrictions expected to return again tonight.

A mix of sun and clouds is present at all TAF sites early this afternoon, with some cumulus beginning to bubble up just west of the sites. The setup is very similar to yesterday, with higher moisture closer to the coastal plain but more sunshine west of AGS/CAE/AIK. So we play the waiting game and hedge towards the possibility for thunderstorms this afternoon. Surface dewpoints are into the 70s, so it shouldn't be difficult to get thunderstorms to develop this afternoon. As a result, have opted for a PROB30 at all sites to account for this possibility. Heading into tonight, guidance is uniformly showing the redevelopment of IFR restrictions. Winds in lower 1kft of the atmosphere look light and we'll have plenty of low-level moisture. IFR cigs look likely as temperatures cool overnight, with visibility restrictions possible everywhere as well. Favoring stratus more at this point but it could be more fog than stratus. Regardless, the restrictions should develop everywhere between 06z and 09z, lasting through mid to late morning Sunday.

EXTENDED AVIATION OUTLOOK...Morning ceilings and visibility restrictions possible into Sunday, along with afternoon restrictions from showers and thunderstorms. Drier conditions expected early next week.

CAE WATCHES/WARNINGS/ADVISORIES

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