textproduct: Columbia

WHAT HAS CHANGED

Updated forecast for showers and thunderstorms today and tomorrow with marginal threat of severe weather. Aviation updated for 18Z TAFs.

KEY MESSAGES

- 1. Showers and thunderstorms expected through next week

DISCUSSION

Key Message 1: Showers and thunderstorms expected through next week

The synoptic pattern is persistent through next week with upper level ridging over the western Atlantic and upper disturbances lifting to the west and north. Global ensemble mean precipitable water values are in the 1.5-2.0 inch range through most of next week. This pattern supports at least scattered showers and thunderstorms enhanced by the shortwave activity throughout the period. The most active timeframe may be tonight and Saturday. The axis of the ridge will be a little further east at this time allowing the deeper moisture and better upper support into the forecast area. The blended guidance shows likely to categorical rain chances through the holiday weekend. While most areas will likely see rainfall (especially northern and western portions of the forecast area), this may be a bit deceiving as the convection may be more periodic as opposed to a complete washout with persistent rain. Showers and thunderstorms will likely lead to localized heavy rainfall each day, particularly if training cells develop. This raises concern for localized flash flooding mainly in urban and flood prone areas.

Severe potential through Saturday: Isolated strong thunderstorms will be possible this afternoon with SPC Mesoanalysis showing mlCAPE around 1500 J/kg in the CSRA and locally enhanced helicity along the wedge boundary in the Upstate and western Midlands. A low level jet will lift out of the Deep South and into the Southern Appalachians tonight and Saturday advecting moisture and increasing shear over the forecast area. An outflow boundary from storms to our west should trigger additional convection this evening as it moves east. An upper level shortwave will help sustain the storms as they move into the western FA possibly leading to elevated thunderstorms this evening and tonight. With poor low level lapse rates the threat of severe weather is low but can't be ruled out. The higher threat of severe weather is Saturday with 20-30 kts of 0-6 km shear over the region which could allow multicell clusters or bowing segments to develop in the afternoon. Confidence is low though given the expected widespread cloud cover and rainfall over the area which could hinder lapse rates.

The pattern remains active in the extended with a persistent ridge to our east and troughing to the west. PWAT values remain above 1.5 inches through the period with SW flow over the Southeast. This pattern will support daily shower and thunderstorm chances through next week. In general the convective activity should follow a diurnal trend but shortwave troughs may sustain storms outside of the afternoon timeframe.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Mix of restrictions likely throughout the TAF period with scattered showers and storms.

Fairly moisture rich summer pattern continues across the region with scattered showers and isolated thunderstorms this afternoon. Coverage will become more widespread later this evening with thunderstorm potential also increasing. TAFs reflect the increasing coverage throughout the afternoon and into the evening with a transition from prob30 to tempo. Winds this afternoon and evening will remain generally out of the south, 6-10 knots before subsiding overnight. Showers and storms should then subside later tonight leaving widespread again for Saturday morning, likely IFR cigs. Additional rounds of showers and storms likely Saturday afternoon towards the end of the TAF period.

EXTENDED AVIATION OUTLOOK...Abundant low level moisture will bring more rounds of morning restrictions from fog/stratus through the weekend along with scattered afternoon showers and storms possible each day into the middle of next week.

CAE WATCHES/WARNINGS/ADVISORIES

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