textproduct: Columbia

WHAT HAS CHANGED

The severe thunderstorm risk has increased a bit and could persist overnight into early Friday morning. Northward trends in the axis of heaviest rainfall have continued, but this axis is still near the northern FA. Aviation Discussion updated for 18z 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 afternoon into Friday morning.

Overview: The remnants of Arthur are pushing across the Deep South early this afternoon and are expected to continue pushing northeastward toward the FA the remainder of the day and tonight. A tightening pressure gradient is bringing gusts of 20-30 mph that should continue the remainder of the afternoon and into this evening. Deep moisture is already making its presence known with surface dewpoints in the low to mid 70s and PWAT's nearing 2.0" across the CWA. The first wave of scattered showers and storms are already developing in the region as a uncapped and unstable airmass is in place. A couple strong to severe storms with the main risk of damaging winds gusts will be possible with this activity before the main surge of moisture and widespread rain/thunderstorms moves in this evening and tonight. A strengthening wind field will aid in bringing an increased tornado risk in thunderstorms this evening and overnight, but damaging wind gusts are expected to remain the main severe hazard. As has been advertised, the main threat continues to be the potential heavy rainfall this evening into early Friday morning, where very efficient rain rates are expected and isolated flash flooding will be possible. Wind gusts overnight could near 35-45 mph as is still supported in recent 12z guidance, 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 already nearing 2" and should continue 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: The main change to the forecast comes with the potential severe weather this evening and overnight tonight as the FA is now bisected along the I-20 corridor by a Slight Risk (level 2 out of 5) for severe weather north of I-20 and a Marginal Risk (level 1 out of 5) south of here. Any isolated strong to severe storm this afternoon is in a weakly sheared environment (0-6km shear under 15 kts), but MLCAPE values are now exceeding 1500-2000 J/kg as strong heating occurs in in the very moist airmass. DCAPE values are a bit marginal still (500-800 J/kg), but sufficient for precip loaded downburst potential (TEI values currently near 25) and thus bringing the isolated damaging wind gust threat into this evening. This evening and overnight is when kinematics improve as the remnant core of Arthur moves in with it's low level circulation. These improved wind fields can already been seen from the 17z VAD profile in Mobile, AL where a layer of 45-50 kt winds is seen from ~500m up to ~4km. This is bringing 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: A tightening surface pressure gradient this afternoon and evening is bringing gusts to 25-30 mph before 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 /18Z THURSDAY THROUGH TUESDAY/

VFR with scattered convection this afternoon and evening. Widespread rain tonight with restrictions developing.

Thunderstorms are beginning to move into the CSRA and impact the Augusta terminals early this afternoon with tsra likely continuing into this evening. A bit of uncertainty to the northern terminals as higher resolution guidance indicates not as high of instability owing to warmer temperatures aloft. This may limit tsra threat this afternoon and evening, although have included a prob30 group. 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 tonight, although generally guidance keeps visibility above 3sm, think there will be a brief period 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, 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.