textproduct: Columbia

WHAT HAS CHANGED

Minimal changes to Key Messages. Aviation discussion updated for 06z TAF issuance.

KEY MESSAGES

- 1. Diurnal thunderstorms expected through this weekend.

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

DISCUSSION

KEY MESSAGE 1: Diurnal thunderstorms expected through this weekend.

Little change is expected in the overall pattern today as the trough gradually weakens and shifts east, with heights rising ahead of the ridge building into our area. Before the ridge becomes firmly established, another round of isolated showers and thunderstorms will be possible. Abundant moisture will remain in place, supporting diurnally driven convection during the afternoon. Activity should be fairly minimal; however, heavy rainfall will remain the primary threat from any pop-up storms that develop, with the overall severe threat remaining low.

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

Ridging will continue to build as the subtropical ridge becomes established over the southeast this week. Confidence is increasing in a return to hot and humid conditions, keeping a strong taste of summer in place into September. EC ensemble 500-mb height anomalies are forecast to exceed the 97th percentile Monday, rising above the 99th percentile through midweek. The strong upper-level ridge will promote significant subsidence across the region, which should generally suppress convection. The result will be a prolonged period of well-aove- normal temperatures, supported by ensemble temps at the 700/850/1000-mb levels reaching or exceeding the 99th percentile. Temperatures will climb into the mid to upper 90s by midweek and remain there each day into next weekend. The combination of anomalous heat and increasing low-level moisture will result in heat indices reaching 100-105 degrees, with some locations potentially higher.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

Restrictions once again likely to form overnight and through the morning hours.

Majority of guidance is once again showing redevelopment of IFR restrictions overnight and through the morning hours. There is also a small set of guidance showing more vfr conditions instead. Based on persistence from previous few mornings, have leaned more towards the development of ifr ceiling restrictions, along with some periods of lower visibilities. There are already some lower ceilings across the Midlands associated with a decaying outflow boundary that moved through. Even if those scatter out, we should see ceilings lower around 09Z at all sites. Crossover temperatures in the low 70s to upper 60s will favor at least brief, patchy fog but mid and high level cloud cover through the night should hinder widespread fog. Regardless, restrictions are expected from IFR to MVFR around 15Z then improving to VFR after 17Z. Afternoon rainfall coverage is expected to be lower than previous days, with the highest probability possibly over the far eastern Midlands and the coastal plain. Have only placed Prob30 in OGB for Sunday afternoon, with all other sites expected to remain dry.

EXTENDED AVIATION OUTLOOK...Drier conditions expected early next week, but brief early morning restrictions still possible.

CAE WATCHES/WARNINGS/ADVISORIES

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