textproduct: Columbia

WHAT HAS CHANGED

Above normal temperatures continue with the highest heat index values expected Saturday. Aviation discussion updated for 18z TAF issuance.

KEY MESSAGES

- 1. Heat Index values from 105 to 110 possible Saturday.

- 2. Severe storms possible Saturday afternoon or evening.

DISCUSSION

KEY MESSAGE 1. Heat Index values from 105 to 110 possible Saturday.

Upper level ridging shifts to the west late this week and into the weekend allowing for NW downslope flow ahead of a cold front approaching from the north. Moisture increases ahead of the front with PWAT values nearing or exceeding the 90th percentile of the NAEFS climatology. The warm, moist air mass and downslope flow will push heat index values to near 110 during the afternoon assuming storms hold off until later in the day as currently expected. If this holds, a heat advisory may be needed for Saturday.

KEY MESSAGE 2. Severe storms possible Saturday afternoon or evening.

As the upper ridging retrogrades to the west a NW flow pattern aloft will allow for shortwave energy to dig into the Mid Atlantic on Saturday. A cold front will trigger convection in the warm, moist air mass upstream of the forecast area. The concern is that as storms push south and east in the afternoon and evening that steep low level lapse rates and dry mid-level air will favor damaging winds. The progression of the storms and the southern extent of the severe weather threat will be something to monitor over the coming days.

For the remainder of the forecast period, with the cold front forecast to be over the Southeast on Sunday we can expect at least a chance of showers and thunderstorms with moisture still in place and some shortwave activity moving through the trough. However, with lapse rates not as strong as Saturday, the threat of severe weather should be low. Next week is expected to yield near normal temperatures with continuing at least isolated chances for diurnal convection. This comes as the front slowly clears the area and troughing persists before the ridge is expected to slowly slide back in mid week.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Mainly VFR conditions through the TAF period with a potential for fog or stratus near daybreak.

Typical fair weather cumulus field has developed across the region. Expect this to continue until sundown with southeast winds in the 5-8 kt range. A rogue shower or storm remains possible, but confidence still too low to include in the TAF. Winds become light and variable to calm overnight before becoming southerly after daybreak. With low level moisture in place, there is potential for fog or stratus at the terminals near daybreak. Confidence remains too low at this time to include and restrictions.

EXTENDED AVIATION OUTLOOK...Chances for restrictions and thunderstorms increase this weekend as a cold front approaches.

CAE WATCHES/WARNINGS/ADVISORIES

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