textproduct: Columbia

WHAT HAS CHANGED

High probabilities of above normal temperatures and heat indices over 100 degrees this week. Aviation discussion updated for 18z TAF issuance.

KEY MESSAGES

- 1. Generally dry weather and well above normal temperatures expected with dangerous heat index values possible next week.

- 2. Active weather and decreasing temperatures possible next weekend.

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

DISCUSSION

KEY MESSAGE 1. Well above normal temperatures expected with dangerous heat index values possible next week.

Ensemble guidance continues to be consistent in showing a strong 596DM upper ridge currently over the southern Plains and mid MS Valley area building eastward through the week becoming situated over the southern OH Valley and Carolinas by mid week then gradually weakening and beginning to slowly retrograde by the end of the week. This should result in strong subsidence over the region through the week with ensemble forecast 700mb temperatures at the 99th percentile with values around +10C. These high values and the notable subsidence inversion at 700mb increases confidence in the general lack of convection expected. Ensemble temperatures at 850/1000mb are forecast to be above the 90th percentile early in the week and at the 99th percentile mid to late week suggesting a strong warming trend through the week with anomalously high temperatures (about 10 degrees above normal). Minimum temperatures also expected to be anomalously high with EC EFI showing values around 0.7-0.8 and a nonzero shift of tails (similar to the max temperatures) with values about 5 to 10 degrees above normal. Southeast to southwesterly surface flow through the week will keep the low level moisture in place resulting in heat indices rising above 100 degrees all week and approaching heat advisory criteria mid to late week.

KEY MESSAGE 2. Active weather and decreasing temperatures possible next weekend.

Ensembles and global guidance in good agreement showing the strong upper ridge retrograding westward back over the Plains states allowing for upper troughing along the East Coast next weekend. This should support slightly cooling temperatures and also allow shortwave energy to dive down the back side of the retreating ridge, bringing increased chances of rainfall back to the forecast area. Rainfall will be welcomed again after a long hot and generally dry week ahead.

KEY MESSAGE 3. Diurnal thunderstorms expected through this evening with the main threat of locally heavy rainfall.

Stratus clouds have transitioned to a stratocumulus deck as daytime heating continues and temperatures rise through the 80s. Dewpoints are generally in the lower 70s and should support moderate instability as higher clouds are diminishing and more sun is expected. However, wind shear remains weak and organized convection is not anticipated. Hi-res guidance suggests isolated convection through the afternoon into early evening favoring the eastern Midlands closer to the deeper moisture (PWATs around 2 inches) and better upper forcing along the coast (500mb vorticity). Due to slow storm motion and PWATs around 2 inches, heavy rainfall could be a concern for more intense storms providing high rain rates.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

VFR expected through this evening, with vis/cig restrictions expected again towards Monday morning.

MVFR and IFR ceilings from earlier this morning have given way to some relatively flat cumulus this afternoon. Expecting this to remain largely the case despite PWs and CAPE being relatively seasonal for August. Heights are gradually rising as a ridge from the west builds overhead and this is likely to keep convection isolated to scattered at best. The sea breeze does look to be going to our southeast and should bring a threat for showers at OGB later, prompting a PROB30 group there. As we get into tonight, mostly clear skies, moisture rich low-levels, and very light winds in the lower 5kft of the atmosphere are likely to prompt another round of fog/stratus. It looks like it may favor fog tonight, so have trended more towards that in the predominant and TEMPO groups. As with most fog forecasts, this is not an overly confident forecast but the setup is good and guidance is confident enough that its in the TAF from 08z to 14z at all sites. Monday looks nice and VFR once the restrictions break.

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

CAE WATCHES/WARNINGS/ADVISORIES

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