textproduct: Columbia

WHAT HAS CHANGED

A bit more of uncertainty in temperatures Sunday due to convection. Aviation discussion updated for 18z TAFs.

KEY MESSAGES

- 1. Above normal temperatures continue this weekend into early next week. A Heat Advisory is in effect along and north of I-20 until 8 PM this evening with heat indices up to 105 possible.

- 2. Chances for scattered diurnal showers and thunderstorms start Sunday, continuing into next week. Strong storms are possible each afternoon and evening.

DISCUSSION

Key Message 1: Above normal temperatures continue this weekend into early next week. A Heat Advisory is in effect along and north of I-20 until 8 PM this evening with heat indices up to 105 possible.

Temperatures are already approaching the mid to upper 90s early this 4th of July afternoon with dew points being a bit slow to mix out (in the upper 60s to low 70s right now), bringing heat indices already near 100-105F. Signs of deeper mixing beginning are seen in satellite imagery/obs with higher based cumulus developing and a slight lowering of dewpoints into the Pee Dee toward the mid to upper 60s. This trend should continue the remainder of the afternoon with temperatures topping out in the upper 90s to near 100F with heat indices pushing as high as 105F with spots closing in on 108F possible. The Heat Advisory along and north of I-20 will remain in effect until 8 PM this evening but keep in mind heat indices will likely still be in the 90s around the time for the various firework displays across the FA. As has been emphasized the last couple of days, be diligent to take appropriate heat safety measures today as the risk for heat-related illness is elevated due to the amount of outdoor activities and prolonged heat exposure.

Heading into Sunday, the upper ridge is expected to gradually weaken toward 590dm and it's axis is expected to start shifting a bit eastward as a shortwave trough impinges on it across the upper Midwest down into the Tennessee Valley. This will allow low level flow to become more southerly to southwesterly and bring surface dewpoints back into the upper 60s to low 70s with afternoon high temperatures into the mid to upper 90s. Overall, increased moisture should aid in bringing heat indices back toward 105F across the FA during the afternoon. The main caveat to this is convective temperatures could be quickly attained early in the afternoon, bringing scattered convection (See Key Message #2 for more information) a bit earlier than currently depicted in the latest HRRR runs for example. This would not allow temperatures to peak in the upper 90s and would keep heat indices in check a bit more. Due to this, have held off from issuing another Heat Advisory for now, but this will be evaluated for the next forecast package. This trend of diurnal convection complicating temperature/heat indices forecasts is expected to continue into next week as deep moisture moves in place with temperatures near to above normal. Even with this complication, heat indices likely will exceed 100F through at least Wednesday (LREF probabilities are over 70% for this), possibly necessitating additional heat products.

This all goes to say it remains likely that a prolonged period with heat indices above 100F will continue into at least the mid week next week, so heat safety measures should continue to be taken. The duration of this heat will continue to make the elderly, young children, pets, and those without indoor cooling at particular risk Sunday and into next week.

Key Message 2: Chances for scattered diurnal showers and thunderstorms start Sunday, continuing into next week. Strong storms are possible each afternoon and evening.

As temperatures continue into the upper 90s and near convective temps, an isolated shower or weak storm cannot be ruled out this afternoon as shown in the 12z HREF solution and latest HRRR runs. A weak mid level perturbation is also noted in WV imagery across northern AL/GA, thus cannot rule out this forcing initiating a couple showers or storms mainly near the western CSRA.

For Sunday, robust surface heating is expected to allow convective temperatures to be reached fairly early in the day (between 15-18z), bringing scattered showers and storms across the FA. This is further supported in the mean HREF and REFS solutions. With increasing moisture from southerly IVT transport, MUCAPE values are expected to near 2000-2500 J/kg and deep mixing should yield impressive DCAPE values over 1000 J/kg. Lingering subsidence from the weakening ridge could keep storms a bit shallow, but any more persistent updraft could deepen and bring the risk for a strong to severe storm. The main hazard from any strong storm would be damaging winds from localized downbursts. This type of typical strong to severe summer pulse environment is expected to persist into the mid week next week. Ensemble guidance is in good agreement that the suppressed and eastward moving ridge will allow pieces of shortwave energy to traverse the region, bringing a bit more forcing than that on Sunday. Scattered convection is expected each day with the main threat continuing to be downburst winds. Locally heavy rainfall will also be possible as PWAT's raise to near 2.0-2.20" early in the week.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

VFR conditions expected through the TAF period.

Winds remaining light and variable, they will be turning more southerly around 6 knots through the afternoon. After sunset winds should return to more light and variable for the remainder of the period. Only cloud cover for the majority of the sites will be some scattered mid-level afternoon cumulus. Later tonight, with there being 4th of July celebrations and fireworks, there could be some brief visibility restrictions associated with smoke from the fireworks at cae/cub/ags/dnl.

EXTENDED AVIATION OUTLOOK...Daily rain chances and possible restrictions start to increase Sunday afternoon into early next week.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Heat Advisory until 8 PM EDT this evening for SCZ016-018- 020>022-025>030-115-116. GA...Heat Advisory until 8 PM EDT this evening for GAZ040-063>065.