textproduct: Columbia

WHAT HAS CHANGED

Heat Advisory issued along and south of the I-20 corridor today. Convective coverage will increase by late afternoon. Aviation discussion updated for 18z TAFs.

KEY MESSAGES

- 1. Heat Advisory in effect from 12 PM to 8 PM along and south of the I-20 corridor as heat indices reach between 105-108F. Isolated to scattered convection expected with a couple strong to severe storms possible this afternoon/evening.

- 2. More thunderstorm chances Sunday into Monday with possible severe thunderstorms. Another chance for strong convection possible during the midweek.

DISCUSSION

Key Message 1: Heat Advisory in effect from 12 PM to 8 PM along and south of the I-20 corridor as heat indices reach between 105-108F. Isolated to scattered convection expected with a couple strong to severe storms possible this afternoon/evening.

Not much change overall in the forecast for both heat and convective activity this afternoon. Observations for both temps and dew points are on track for the forecast today with heat indices expected to push between 105-110F along and south of I-20. Despite the high temps and dew points, mid-level capping is still in control in the Midlands with generally flat cu and a lack of any vertical development yet. Convective coverage this afternoon/evening should trend a bit similar to that seen yesterday. This lingering mid-level capping should hold on into the mid-afternoon before a combination of increased surface troughing and the sea breeze initiate deeper convection late in the afternoon and into the evening hours. Upper support remains very meager with effective shear values under 10 kts, but strong instability is still progged to develop (SBCAPE near 3000 J/kg) in this very moist environment. A couple strong to severe storms will be possible as robust DCAPE values between 1000-1300 J/kg, 3km CAPE near 100 J/kg, and TEI values between 25-30 bring an increased risk for strong downbursts and the associated damaging wind risk. Due to this, portions of the FA are in SPC's Marginal Risk (level 1 out of 5) for severe weather today with a northern sliver in a Slight Risk (2 out of 5).

Key Message 2: More thunderstorm chances Sunday into Monday with possible severe thunderstorms. Another chance for strong convection possible during the midweek.

As has been discussed, a deepening shortwave is progged to move into the Mid-Atlantic through the day Sunday, dragging a surface front that could near the Pee Dee Sunday night. Enhanced moisture advection is shown across guidance ahead of these features with PWAT's reaching above 2", partly thanks to Gulf moisture being transported northward on the eastern periphery of a developing weak tropical disturbance in the northeastern Gulf. Overall, the environment is not expected to be all that different from the last couple of days, favoring a damaging wind risk from precip-loaded downdrafts. The main difference is marginal upper support should bring increased coverage and effective shear values near 10-15 kts, especially toward the Pee Dee, which could aid in better organization of a few clusters. This has resulted in most of the FA remaining in a Marginal Risk for severe weather, but a Slight Risk (level 2 out of 5) still scrapes the Pee Dee with the greater shear here. The frontal boundary is expected to remain near the Pee Dee into Monday with weak shortwave energy passing through the region, bringing another day with increased afternoon convective coverage and the chance for a couple strong to marginally severe storms.

NHC now has a 40% outlook area as the EC Ensemble and Google DeepMind have trended more towards the weak tropical disturbance meandering through the northeastern Gulf early in the week before slowly drifting westward into the mid week. This should still allow deep moisture to advect into the region from the south, but both ensemble and deterministic guidance have displayed increased confidence that an even sharper shortwave should dig into the eastern CONUS during the mid-week, possibly dragging another frontal boundary near the FA. Enhanced kinematics coupled with seasonable to strong instability could bring another chance for stronger convection during the midweek as highlighted in CSU/CIPS/NCAR Machine Learning and AI guidance. Timing difference still need to be sorted out across deterministic guidance, but the pattern seems favorable for potential severe weather again.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

VFR conditions are expected to prevail for the forecast period, although brief restrictions are possible associated with afternoon/evening thunderstorms.

Diurnal cumulus field continues to expand across the area this afternoon, and any ceilings should quickly improve to VFR levels. Scattered convection should initiate along a boundary to the north and drop into the area later this afternoon, and along the inland progressing sea breeze. The TAFs include a PROB30 group from 20-01Z to account for the thunderstorm potential. Winds will generally be from the southwest at 6-10 kt into this evening, then subside a bit overnight.

EXTENDED AVIATION OUTLOOK...Potential for restrictions from showers and thunderstorms continues into next week.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Heat Advisory until 8 PM EDT this evening for SCZ016-022- 027>031-035-037-038-041-135>137. GA...Heat Advisory until 8 PM EDT this evening for GAZ063>065-077.