textproduct: Columbia

WHAT HAS CHANGED

An isolated tornado risk has become a bit more evident Thursday evening into the overnight hours. Confidence in heavy rainfall potential continues to increase. Aviation discussion updated for 18z TAFs.

KEY MESSAGES

- 1. Active weather increasingly likely Thursday afternoon into Friday as a system from the Gulf moves across the region. Gusty winds, heavy rainfall, and severe weather are possible.

- 2. Drier conditions return this evening into early Thursday ahead of the active weather from Key Message 1.

DISCUSSION

Key Message 1: Active weather increasingly likely Thursday afternoon into Friday as a system from the Gulf moves across the region. Gusty winds, heavy rainfall, and severe weather are possible.

Overview: Active weather is likely starting Thursday afternoon, continuing overnight and into Friday as the remnants of what is currently TS Arthur move into the region. Arthur and its coupled upper wave can be seen near the far southeast TX coast at the moment and both features should continue to gradually move northeastward, moving inland (and weakening) into the lower Mississippi Valley tonight. Through the day tomorrow the remnant low level circulation of Arthur and upper wave should continue drifting northeastward, increasing deep moisture across the FA with strengthening wind fields. A very moist airmass by the afternoon hours should aid in bringing scattered shower and thunderstorm development where a couple strong to severe storms cannot be ruled out. This threat is expected to continue into the evening where a low tornado risk could develop mainly toward the CSRA, but confidence in this risk materializing is rather low at the moment. By the late evening hours the bulk of moisture is expected to overspread the CWA, with scattered to widespread showers and storms moving in, continuing into Friday morning before coverage may decrease a bit. The main risk overnight will be the heavy rain potential where portions of the CWA are in a Slight Risk (level 2 out of 4) for excessive rainfall. Strengthening low level wind fields Thursday afternoon and a tightening surface pressure gradient should also aid in bringing 25-30 mph wind gusts, where isolated gusts toward 35-45 mph cannot be ruled out mainly late Thursday into the overnight period. Conditions are expected to gradually improve Friday morning before a cold front pushes through during the afternoon/evening with additional shower and storm chances that end by the late evening.

Heavy Rainfall: The primary threat from this system will be the heavy rainfall potential as very efficient rain rates are likely, especially Thursday evening and overnight into Friday. As the remnants of Arthur near, PWAT's are expected to increase toward 2" by the early afternoon, before surging toward 2.20-2.40" during the evening and overnight period. As this moisture increases with the approaching upper wave and remnant low level circulation, IVT values reach toward the NAEFS 99th to 99.5th percentile. This is mostly thanks to a slight shift in a few high-res and global models keeping the low to mid level core of the system a bit more intact, leading to a 35-55 kt LLJ overspreading the area Thursday evening and overnight. This increasing LLJ coupled with the aforementioned PWAT's, warm cloud layer depths exceeding 14,000 ft, and elevated instability sets the stage for efficient rain rates. In terms of totals, the latest mean HREF and REFS solutions support a widespread 0.75-1.25" across the FA, highest in the CSRA. When taking a look at the LPMM product from each suite and individual 12z CAM's spots of 2-3.5" are seen where training occurs with the expected rates. This seems reasonable considering the LLJ may enhance the efficiency of this rainfall more that previously discussed. Decided to hold off on a Flood Watch at the moment as confidence in these higher amounts becoming widespread remains low with model to model variance, but areas south of the Pee Dee are outlined in a Slight Risk (level 2 out of 4) for excessive rainfall, meaning isolated flash flooding could be possible, mainly Thursday evening into early Friday morning.

Severe Weather: The whole FA is outlined in SPC's Marginal Risk (level l out of 5) for severe weather Thursday. At least modest heating in an increasingly moist airmass is expected to yield MLCAPE values near 1200-1700 J/kg with increasing deep layer shear as the remnant surface circulation and upper wave near. Increased mixing in the afternoon also is expected to yield DCAPE values near 700-1000 J/kg with TEI values nearing 25. Isolated to scattered storms in the afternoon are expected to bring the main risk of strong to damaging wind gusts from precip loaded downdrafts, but also from possible cold pool consolidation in any cluster. The main change to the severe threat is a low end tornado risk that may develop mainly near the CSRA during the evening into the early overnight hours. Near surface winds are expected to back a bit as the remnant surface circulation nears and the aforementioned LLJ moves in. Modeled soundings indicate 0-1km bulk shear should increase toward 25-30 kts during the evening with 0-1km SRH nearing 125-180 m^2/s^2 and this is further supported in the 12z mean HREF solution. This risk is conditional however on the amount of low level instability that can remain in place during the evening. The severe weather risk is expected to gradually diminish overnight with the probability for convection to become elevated increasing.

Wind Gusts: As mentioned in the overview, increasing wind profiles with a tightening surface pressure gradient are expected to yield wind gusts to 25-30 mph through the afternoon and into the evening hours Thursday. The main talking point here comes late Thursday evening and overnight into early Friday. With the the slight shift in model guidance in keeping the core of the system a bit more intact, there have been more deterministic runs showing the potential for isolated 35-45 mph gusts as the robust LLJ shifts overhead. 12Z HREF probabilities for gusts greater than 40 mph are now near 20-40%, mainly toward the CSRA and into the parts of central/eastern Midlands. Confidence in these stronger gusts is pretty low as the overnight/early morning period favors less mixing occurring as seen in BUFKIT soundings. There is also a generous spread across high-res models on this potential still. Overall, the potential for these stronger gusts is there but confidence remains low, will need to continue to evaluate this threat overnight tonight and early tomorrow.

Key Message 2: Drier conditions return this evening into early Thursday ahead of the active weather from Key Message 1.

The shortwave that brought showers across the area this morning is pushing off the coast, leading to decreasing coverage across the FA. A couple isolated to scattered showers and storms will remain possible toward the CSRA and southeastern Midlands into the early evening, but dry conditions are then expected to take hold tonight into early Thursday before active weather moves in later Thursday (See Key Message 1 for these details). As cloud cover breaks up a bit this afternoon temperatures should reach into the mid to upper 80s before warming a bit Thursday, toward the upper 80s to low 90s.

AVIATION /19Z WEDNESDAY THROUGH MONDAY/

VFR conditions for the rest of the day with restrictions possible near daybreak tomorrow.

Drier air shifted into the area this afternoon which led to restrictions diminishing, although isolated MVFR ceilings possible at the southern terminals for the next hour or so. Moisture will return tomorrow with strengthening southwesterly flow with restrictions possible near daybreak with highest probability in the Augusta/Aiken/Orangeburg terminals. While there remains a brief chance of IFR ceilings, have left at the mention with limited confidence at this point. Convection initiation tomorrow appears to generally be after 18z so have left out any tsra as well. Winds will remain breezy generally out of the southwest with gusts up to 20 knots possible, generally during the daytime hours.

EXTENDED AVIATION OUTLOOK...Diurnally favored convection possible each afternoon, favoring the southern Midlands and CSRA. Breezy conditions expected Thu/Fri with increased chances of rain as a Gulf coast system approaches the area.

CAE WATCHES/WARNINGS/ADVISORIES

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