textproduct: Columbia

WHAT HAS CHANGED

Updated Key Message. Aviation Discussion updated for 00z TAFs.

KEY MESSAGES

- 1. Gusty winds, heavy rainfall, and severe weather are possible this afternoon into Friday morning.

- 2. A more typical summer-like pattern returns for the weekend into next week.

DISCUSSION

Key Message 1: Gusty winds, heavy rainfall, and severe weather are possible this evening into Friday morning.

Overview: The remnants of Arthur continue to move northeastward toward the FA this evening. Deep moisture remains in place with surface dewpoints in the low to mid 70s and PWAT's now over 2" across the CWA. The first wave of storms has by and large cleared the area, and we are now turning our attention to central GA. This next line of storms will carry with it a slightly elevated tornado threat, particularly due to a strengthening wind field. As has been advertised, the potential heavy rainfall threat also continues this evening into early Friday morning, when very efficient rain rates are expected and isolated flash flooding will be possible. Wind gusts overnight could near 35-45 mph, but some of this appears to be convectively enhanced rather than purely synoptic winds. Overall, with the timing of the main hazards being this evening and overnight, be sure to have multiple ways to receive warnings.

Heavy Rainfall: There has not been a major change in the outlook for rainfall outside of the 12z suite of CAM's continuing to drift the axis of heaviest rain closer to the Upstate/northern Midlands. Majority of the CWA remains in WPC's Slight Risk (level 2 out of 4) for excessive rainfall today into Friday morning. As mentioned in the overview, PWAT's are now surging toward 2.20-2.40" this evening and overnight. As this moisture increases with the approaching upper wave and remnant low level circulation moving near the northern FA, IVT values are now forecast to reach near climatological maximum values as a robust 50-65 kt LLJ moves into the FA. Some guidance continues to indicate this LLJ could be a bit stronger (near 70 kts) like the 12z NAM and NAM 3k. This impressive LLJ coupled with the aforementioned PWAT's, warm cloud layer depths nearing 14,000 to 15,000 ft, and lingering instability sets the stage for the advertised efficient rain rates. As mentioned above, the 12z suite of guidance generally has shifted the axis of highest totals a bit further north toward the Upstate and northern Midlands, closer to where the remnant low level circulation is forecast to move. Due to this, widespread amounts near 0.50-1.50" still seems reasonable, but pockets of 2-4" could be in the cards, especially along and north of the I-20 corridor. These higher amounts could be realized mainly where training of convection occurs tonight. This axis appears heavily dependent on the track of the low level circulation so shifts may occur still. Due to this, a Flood Watch still does not seem warranted considering the potential shifts in the axis of greater amounts and that 2"+ amounts appear fairly scattered.

Severe Weather: This evening and overnight is when kinematics improve as the remnant core of Arthur moves in with its low level circulation. These improved wind fields can already been seen from upstream VAD profiles, which should bring in 1km shear values up to near 30 kts with SRH in this layer exceeding 150-200 m^2/s^2. This area of enhanced low level helicity along the track of the low level circulation is expected to move in late this evening and overnight, bringing the increased risk for a few tornados, mainly along/north of I-20. With the increased shear profiles, convection could organize as clusters or potential linear segments as depicted in recent HRRR runs. Either mode will bring the main hazard of damaging wind gusts tonight, but any linear segments could bring the increased tornado risk as southwesterly 40-45 kt 3km shear vectors would be favorably oriented with north-south linear segments for the development of mesovortices.

Wind Gusts: The increasing wind field aloft and aforementioned LLJ may aid in bring gusts of 35-45 mph overnight and into early Friday morning. A couple higher gusts in eastern spots cannot be ruled out. The forecast situation around these gusts is a bit complex as these could be partially attributed to the mostly intact core of the remnants from Arthur, but with likely on-going convection overnight, some of these winds could be convectively enhanced rather than purely synoptic. Also, BUFKIT soundings are not particularly aggressive in terms of mixing down some of the 50kt+ LLJ with fairly neutral conditions near the surface in terms of mixing. Despite these complications, the signal remains across 12z high-res guidance and in the latest HREF where probabilities for gusts greater than 40 mph has increased slightly toward 40-60%, highest along and south of the I-20 corridor. Overall, expect breezy winds the remainder of today before stronger winds will be possible tonight into Friday morning, associated both with convection and synoptic winds. One other note worth mentioning is saturated soils from the discussed heavy rainfall may make some trees more susceptible to falling in stronger winds tonight.

Key Message 2: A more typical summer-like pattern returns for the weekend into next week.

After the remnants of Arthur move out of the area, a front is expected to slide through the FA Friday afternoon/evening, bringing isolated to scattered shower/storm chances. Slightly drier conditions are expected behind the front this weekend, but with warming temperatures into the low 90s. Multiple disturbances are forecast to move through upper zonal flow, bringing a more summer- like pattern of diurnal thunderstorm chances as we head into next week.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

Scattered convection this afternoon wrapping up. Widespread rain tonight with restrictions developing.

The first round of showers and thunderstorms have all but pushed out of the terminals this afternoon. A few vicinity showers remain in areas to our north as the activity begins to exit and move into NC. Winds remain gusty into tonight with gusts around 20 knots continuing consistently and enhanced gusts possible at times tonight as the remnants of TS Arthur move into the area. This will lead to strengthening winds aloft, although surface winds should remain elevated enough to prevent llws criteria to be reached. Widespread moderate to heavy rain moves over the terminals later tonight, although generally guidance keeps visibility above 3sm. There could be a few brief periods of IFR visibilities possible, especially at the Augusta terminals. Similarly ceilings will likely be predominantly MVFR but may drop to IFR at times, lingering into Friday morning. Breezy winds continue Friday with gusts close to 15-20 kt, although drier air will start to move into the area.

EXTENDED AVIATION OUTLOOK...Diurnally driven convection can be anticipated heading into the weekend, although coverage will be isolated with higher coverage next week.

CAE WATCHES/WARNINGS/ADVISORIES

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