textproduct: Columbia

WHAT HAS CHANGED

Aviation discussion updated for 00z 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 few lingering showers and thunderstorms are hanging around this evening near the eastern Midlands and Coastal Plain where deeper moisture exists aided by convergence along the sea breeze. DCAPE values are poor, as are lapse rates, so not expecting much of a severe threat. With elevated PWATs and weak storm motion, heavy rain remains the primary threat with high rain rates possible. Activity should diminish over the next few hours. 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 /00Z SUNDAY THROUGH THURSDAY/

Restrictions likely again tonight and Sunday morning.

Guidance is once again showing redevelopment of IFR restrictions tonight and early Sunday morning. Winds in the lower levels will be light and abundant low-level moisture remains in place. This adds confidence in the forecast for IFR cigs. We should see ceilings lower around 09Z, with visibility restrictions possible at all sites as well. Crossover temperatures in the low 70s to upper 60s will favor at least brief, patchy fog but mid and high level cloud cover through the night should hinder widespread fog. Regardless, restrictions are expected from IFR to MVFR around 15Z then improving to VFR after 18Z. Convective coverage on Sunday is expected to be lower than previous days so the mention of TS has been omitted for now.

EXTENDED AVIATION OUTLOOK...Drier conditions expected early next week, but brief early morning restrictions still possible.

CAE WATCHES/WARNINGS/ADVISORIES

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