textproduct: Columbia

WHAT HAS CHANGED

Confidence in convective coverage today is low given chance of persistent cloud cover. Aviation discussion updated for 12z TAF issuance.

KEY MESSAGES

- 1. Diurnal thunderstorms expected through this weekend with the main threat of locally heavy rainfall.

- 2. Well above normal temperatures with dangerous heat index values possible next week.

DISCUSSION

KEY MESSAGE 1: Diurnal thunderstorms expected through this weekend with the main threat of locally heavy rainfall.

Overall, there has not been a significant shift in thinking this weekend, but confidence in convective coverage is low today. Some shower activity could develop in the early morning hours but the main window would still come during the afternoon into the evening. A similar set up to yesterday is expected with the upper trough remaining in place and PWAT's near 2" as deep/moist southerly to southwesterly flow persists over the region. Looking at forecast soundings, this brings a nearly saturated column with much of guidance including the NBM and mean HREF solution, having cloud cover hang on much of the day. In RAP model soundings, this is limiting surface temperatures toward the lower 80s, but dewpoints in the low to mid 70s still allow around 1500-2000 J/kg of MLCAPE to develop. This cloud cover will likely modulate storm coverage and timing this afternoon, where at least isolated chances are possible, but cannot rule out more scattered to numerous storms if more robust heating is realized. With saturated profiles, the severe risk is low, but efficient rain rates and slow storm motions under 15-20 kts would bring an isolated flash flood risk. Sunday is expected to be fairly similar environment wise, but with troughing now weakening and lifting northward, convective coverage has trended a bit more isolated as little in way of forcing remains in the region.

KEY MESSAGE 2: Well above normal temperatures with dangerous heat index values possible next week.

The outlook for the end of August and start of September remains on track with hot/humid conditions expected, especially Tuesday through the end of the forecast period. The well advertised upper ridge is expected to slowly build overhead into the mid to late week with 500 mb heights in both the EC ensemble and mean GEFS solutions becoming nearly 2 standard deviations above normal. As this occurs, 850mb temperatures are expected to reach near the NAEFS 99.5th percentile. When these fields are coupled with seasonable moisture in place, hot and humid conditions become likely. There is increasing confidence temperatures in the mid to upper 90s will be seen during the period as seen in decreasing NBM IQR ranges for maximum temperatures. Probabilities for heat indices above 100F Tuesday through Friday have also increased from 24 hours ago with chances now reaching 60- 80% in the latest LREF run. This all goes to say, be prepared for another shot of heat and humidity to start September with limited rain chances as the ridge should act to largely suppress diurnal convection.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Although some restrictions expected this morning, it is a lower confidence forecast due to higher cloudiness and its impact.

Guidance remains rather inconsistent regarding development of ifr/lifr conditions this morning. GFS keeps mainly vfr condition, nam is mvfr, while the lamp guidance has now trended from ifr back into vfr Much uncertainty with the inconsistency in the models, along with current conditions fluctuating at times between mvfr and back to vfr at all locations. Have gone mostly with predominant vfr, and a tempo for lower restrictions through 14z. Outlier may be kogb and areas further east where deeper moisture and potential rain could keep lower conditions in for a good portion of the day, but even that isn't a given.

Showers along the coastal plain this morning, with isolated showers developing in the Midlands could impact several airports. Have included vcsh at least to start with, and can amend if any activity looks to impact taf sites. Then for this afternoon, isolated to scattered showers and storms are expected to develop again. Will continue with mention of Prob30 in all terminals, although cloud cover may cause some additional uncertainty in development. A return of at least ceiling restrictions looks more likely again after 03z tonight.

EXTENDED AVIATION OUTLOOK...Morning ceilings and visibility restrictions possible into Sunday, along with afternoon restrictions from showers and thunderstorms. Drier conditions expected early next week.

CAE WATCHES/WARNINGS/ADVISORIES

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