textproduct: Columbia

WHAT HAS CHANGED

Some lingering convection may impact upper CSRA this evening. Aviation discussion updated for 00z TAF issuance.

KEY MESSAGES

- 1. Lingering convection just west of CWA may impact upper CSRA this evening.

- 2. Uncertainty in temperatures Friday and this weekend.

- 3. Rain chances increase late week onward.

DISCUSSION

Key Message 1: Lingering convection just west of CWA may impact upper CSRA this evening.

Late afternoon convection across central and northeast GA has gradually moved eastward toward the upper CSRA this evening. Cannot rule out a few showers or isolated storm impacting Lincoln and McCormick counties this evening before convection wanes as the atmosphere stabilizes. Will include a few hours of pops to account for this. Otherwise, skies should again be mostly clear overnight with another round of possible predawn fog/stratus.

Key Message 2: Uncertainty in temperatures Friday and this weekend.

Highs today and Thursday will remain above normal but still under any Heat Advisory thresholds.

As high pressure moves into the Northeast late this week, cold air damming develops over the Mid-Atlantic States pushing a surface boundary down into the Carolinas. This would lead to a large temperature gradient over the forecast area. However blended guidance has nearly the entire forecast area in the 80s or low 90s for Friday and Saturday. This deterministic guidance may not be properly showing the uncertainty of where that boundary may set up. A portion of the forecast area could struggle to rise out of the mid 70s Friday and possibly Saturday given that the wedge boundary typically remains in place longer than models show. Confidence in the wedge holding is increased by the expectation of rain falling through the cool lower levels.

Key Message 3: Rain chances increase late week onward.

Upper ridging will shift slightly eastward Thursday and Friday allowing deeper moisture and shortwave energy to move over the forecast area. Increased moisture and better upper support should allow convection to develop each afternoon as the air mass becomes conditionally unstable. Friday and Friday night may be particularly active as the wedge boundary discussed in KM1 pushes into the area from the north and a series of shortwaves moves over the region. This should help sustain convection past the diurnally favored timeframe and into the overnight period. REFS mean sbCAPE values rise to 750 to 1250 J/kg each afternoon. With weak instability and in a weakly sheared environment we should see thunderstorms develop but we expect them to be sub- severe.

The deepest moisture appears to move into the region on Saturday as moisture transport strengthens over the wedge boundary. This should lead to widespread showers and thunderstorms. Deep layer shear will be a little stronger Saturday, with some helicity enhancement along the front. There is a question as to how unstable the environment will be given the expected widespread rainfall and cloud cover, but there will be some potential for organized convection and severe weather Saturday afternoon.

The deepest moisture moves out of the region on Sunday with upper ridging building back in over the Southeast. PWAT values remain above normal allowing showers and thunderstorms to develop each afternoon coinciding with daytime heating. With stronger ridging over the area and the wedge boundary shifting back to the north there should be lower convective coverage Sunday and early next week compared to Saturday.

AVIATION /00Z THURSDAY THROUGH MONDAY/

VFR conditions continue with brief restrictions possible again early Thursday morning.

FEW to SCT cumuli will dissipate this evening with light winds around 5-8 kts gradually becoming southeasterly to south- southeasterly over the next few hours. Winds will turn light/variable after sunset beneath passing high cirrus. A few isolated showers are popping up west of our CWA but are expected to remain away from all terminals this evening. Abundant low- level moisture will support another round of patchy fog and stratus development late tonight into early Thursday morning, primarily after 08-10Z. Latest HREF and SREF guidance continues to indicate a 40-60% probability of cigs below 1,000 ft with the highest risk focused across the southeastern Midlands and southern CSRA. Climatologically favored sites of AGS and OGB remain the most likely terminals to experience IFR/LIFR restrictions overnight. Any restrictions should improve after 12-14Z Thursday as light southerly to southwesterly winds around 3-5 kts develop.

SCT cumuli is expected to redevelop Thursday afternoon ahead of an approaching front, with increasing coverage of showers and thunderstorms expected during the afternoon and evening hours. The better chances for scattered convection appear near the end of the current TAF period, mainly after 20-22Z. Confidence in direct terminal impacts remains uncertain at this time, so only included a VCSH mention.

EXTENDED AVIATION OUTLOOK...High low level moisture may bring more rounds of morning restrictions from fog/stratus into the weekend. Scattered afternoon showers and storms become possible tomorrow with more chances this weekend.

CAE WATCHES/WARNINGS/ADVISORIES

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