textproduct: Columbia

WHAT HAS CHANGED

Added second key message to discuss convection expected each day through Monday. Aviation discussion updated for 12z TAFs.

KEY MESSAGES

- 1. Prolonged hot pattern expected with dangerous heat possible this weekend and a signal for a more impactful, intense heat event by the end of next week.

- 2. Scattered afternoon showers and storms each day through Monday. Isolated severe storms possible Saturday and Sunday.

DISCUSSION

Key Message 1: Prolonged hot pattern expected with dangerous heat possible this weekend and a signal for a more impactful, intense heat event by the end of next week.

Upper ridging continues to be favored for this weekend with 500mb heights expected to begin to rise today, although a weakening shortwave moving through the area today will keep temperatures close to seasonal average with scattered showers and storms limiting high temperatures somewhat. Above average temperatures along with triple digit heat indices expected by this weekend as NAEFS mean indicates heights above the 90th percentile. Confidence in the need for a Heat Advisory is low, however, as the area will be situated on the eastern edge of the upper ridge which leaves it more likely for shortwaves passing through the periphery of the ridge to generate showers and storms. Even without any heat advisories, however, much of the area will be in major HeatRisk this weekend and caution should be taken if spending significant time outdoors during the hottest portion of the day. Blended guidance favors a slightly drier air mass moving in Tuesday and Wednesday, although above average temperatures expected to continue.

As the upper ridge shifts eastward, potential for a more substantial heat event for the second half of next week. Not only is there strong agreement in the overall strength of the upper ridge with EC and GEFS ensemble means indicating 500mb heights will be around 2 standard deviations higher than normal along with higher probabilities of 850mb temperatures greater than 20C (70-80%) but also the location of the upper ridge depicted by global models and ensemble would favor subsidence over the area, limiting chances for afternoon showers and storms. Remarkably high blended probabilities this far out for the probability of max temperatures greater than 100F, generally 50-70%, each day Thursday through Saturday. With the 4th of July holiday weekend, heat illnesses would be at a greater risk with a higher amount of people spending time outdoors.

Key Message 2: Scattered afternoon showers and storms each day through Monday. Isolated severe storms possible Saturday and Sunday.

Hot and humid air mass in place will support scattered showers and storms each day, at least through Monday. For today, a shortwave moving through the area will likely lead to the highest coverage of showers and storms with showers already observed in the CSRA. While moderate destabilization is expected into the afternoon, forecast soundings indicate relatively skinny CAPE profiles and not particularly strong low level lapse rates. With rising heights into Saturday, storm coverage will likely be a bit more limited with SPC HREF members favoring highest coverage north of the area. With more robust low level lapse rates and a bit of intrusion of dry air in the mid levels, however, potential for any storms that do develop to produce isolated damaging wind gusts. A marginal risk from the SPC is in place for the northern half of the area for this potential. Weaknesses in the ridge will likely be a bit more pronounced on Sunday and Monday with blended guidance favoring higher pops each day, still generally scattered during the afternoon, favoring the northern and eastern portion of the forecast area. Slightly lower severe potential Sunday, although cannot completely rule out a strong to marginally severe storm.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

Scattered diurnal showers/storms expected this afternoon and evening with associated restrictions possible.

Scattered to broken mid level clouds in place this morning and while there was a very brief period of visibility restrictions at AGS, VFR conditions are expected to continue through the TAF period. Scattered showers and thunderstorms developing this afternoon, although latest guidance has been backing off a bit on the coverage of storms this afternoon. As a result, have kept tsra within prob30 group for this afternoon into evening. Any storms that do form will dissipate after sunset with VFR conditions expected overnight with some mid level debris cloudiness possible over the terminals.

EXTENDED AVIATION OUTLOOK...Increasing heat and humidity Saturday and Sunday should yield diurnal convection and possible associated restrictions. Brief morning vsby/cig restrictions also possible this weekend and early next week.

CAE WATCHES/WARNINGS/ADVISORIES

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