textproduct: Columbia

WHAT HAS CHANGED

Potential for stronger non-thunderstorm winds late tonight/early Friday is on the increase, but plenty of uncertainty remains. The potential for a swath of locally heavy rain in or near the area has also increased.

KEY MESSAGES

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

- 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.

Overview: Active weather is likely starting this afternoon, continuing overnight and into Friday as the remnants of Arthur move into the region. Arthur and its coupled upper wave can be seen along/near the LA coastline early this morning, and this should continue to gradually move northeastward, moving inland (and weakening) into the lower Mississippi Valley and into the Gulf States. The remnant low level circulation associated with Arthur and upper wave are expected to drift northeastward through the day, 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, mainly in the CSRA, 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 remains rather low at the moment. By late this evening, 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 continues to 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 this 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 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 remains to be the heavy rainfall potential as very efficient rain rates are likely due in part to the high PWAT values, especially late this evening and into Friday morning. Speaking of high PWAT values, they are expected to increase toward 2" by early this 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 this evening and overnight. Some guidance indicates this LLJ could be even stronger. For example, the ECMWF and NAM 3k models show a 70 kt LLJ. 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 continue to support a widespread 0.75-1.25" across the FA. When taking a look at the LPMM product from each suite and individual 00z 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 than previously discussed. The location of this potential pocket of heavy rain is too difficult to pin down, but recent guidance indicates a pocket in or very near the forecast area is very much a possibility. With the heavier amounts being rather isolated, don't think a Flood Watch is warranted, but a few instances of nuisance or flash flooding are possible in the locations that receive the heaviest rain.

Severe Weather: The whole FA is outlined in SPC's Marginal Risk (level l out of 5) for severe weather today. 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. There remains 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 125-200 m^2/s^2 and this is further supported in the 00z mean HREF solution. This risk continues to be 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 today. Overnight gets a little more interesting in regards to the potential for stronger wind gusts. As mentioned earlier, a strong LLJ is expected to move over the area as the remnants of Arthur move into the region. The 18z and 00z suite of model solutions shoe that the core of this system stays intact or potentially redevelops over the area. As such deterministic solutions indicate 30-45 mph gusts, with some higher gusts possible, could occur in the eastern portions of the forecast area. The CAMs are generally on the lower end of that spectrum. HREF and REFS probabilities of over 40 mph have increased to generally 30-40% in areas south and east of I-20. The timing of these gusts look to be early Friday morning, so there is a big question mark whether the strong LLJ winds are able to mix down. Regardless, it is worth paying attention to this potential and we'll continue to evaluate this threat today and 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, more zonal flow is expected to settle over the region. Multiple disturbances are forecast to move through the general flow, bringing a more summer-like pattern of diurnal thunderstorm chances as we head into the week ahead.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Ceiling restrictions this morning and expected again tonight. Increasing rain and thunderstorms chances this afternoon through tonight.

An area of MVFR/IFR stratus has spread into the forecast area early this morning keeping restrictions in the TAFs through mid-morning. Gusts will pick up out of the SW around 20 kts at or shortly after 14Z.

A surface low will lift into the Southeast today and tonight leading to strong moisture advection and strong low level winds. Surface winds will gust from 20 to 25 kts this afternoon. A 40 to 50 kt LLJ will move into the area tonight. Winds will stay up around 10 kts through the night with gusts from 20 to 30 kts kts possible from 06 to 12Z.

Showers and thunderstorms become increasingly likely after 21Z. Rain should move from west to east with AGS, DNL and AIK most likely to see rainfall and reduced visibilities before 00Z. The heavier rain is more likely after 00Z when the LLJ and strong moisture advection moves over the region. Periods of heavy rain and restrictions will be possible through 12Z Friday.

EXTENDED AVIATION OUTLOOK...A front moves through during the day Friday, keeping showers and storms in the forecast. Diurnally driven convection can be anticipated heading into the weekend.

CAE WATCHES/WARNINGS/ADVISORIES

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