textproduct: Columbia

WHAT HAS CHANGED

Issued a Heat Advisory for counties along and north of the I-20 corridor tomorrow. Aviation discussion updated for 18z TAF's.

KEY MESSAGES

- 1. Above normal temperatures continue, peaking during the Independence Day weekend. A Heat Advisory will be in effect along and north of I-20 from 12pm-8pm Friday with heat indices between 105-108F possible.

- 2. An isolated strong to marginally severe storm could sneak into the northern FA late this afternoon. Increasing chances for showers and thunderstorms late this weekend into next week.

DISCUSSION

Key Message 1: Above normal temperatures continue, peaking during the Independence Day weekend. A Heat Advisory will be in effect along and north of I-20 from 12pm-8pm Friday with heat indices between 105-108F possible.

There has not been a significant change to the forecast the coming days with hot/humid conditions persisting. Temperatures this afternoon are already reaching the low to mid 90s and should top out in the mid to upper 90s with heat indices approaching 105F, though some spots mainly in Lancaster County could reach 108-110F, thus the Heat Advisory remains in effect through 8pm this evening here.

Heading into Friday and the Independence Day weekend, the robust upper ridge (near the NAEFS 99.5th percentile) is progged to move overhead in the Carolinas as 850mb temperatures warm further toward 20-22C, keeping afternoon highs in upper 90s to near 100F each day. Slightly drier air is shown in model guidance shifting in this weekend with the center of the high (PWAT's less than 1.50"), but surface dewpoints should remain in the upper 60s to low 70s, possibly mixing out into the low to mid 60s mainly Saturday when the deepest mixing is expected. Due to these hot/humid conditions, heat indices could near 105-108F Friday afternoon, which is marginal for Heat Advisory criteria, but with extensive outdoor activities for the start of the holiday weekend, an increased number of individuals will likely be exposed to the prolonged heat. Due to the potential impacts from this heat, have decided to issue a Heat Advisory for counties along and north of the I-20 corridor, running from 12pm to 8pm Friday. Decided to hold off further south as the axis of 925-850mb dewpoints greater than 18C is expected to remain near/north of the I-20 corridor with more mixing out of dewpoints expected south of here, leading to slightly lower heat indices.

Forecast soundings indicate deep mixing on Independence Day, which could bring surface dewpoints toward the low to mid 60s, but heat indices should still approach 100-105F. The potential need for another more so impact based Heat Advisory will continue to be evaluated in coming forecast packages, but confidence in needing this is low as heat indices should be even more marginal than Friday. Moisture is expected to uptick slightly Sunday and into the early week as low level flow becomes increasingly southerly, bringing multiple days with heat indices above 100F and the possible need for more heat products.

Exercise caution this holiday weekend when spending an extended period of time outdoors and make sure to take appropriate heat safety measures. The elderly, young children, pets, and those without indoor cooling will be at particular risk this weekend and over the coming week.

Key Message 2: An isolated strong to marginally severe storm could sneak into the northern FA late this afternoon. Increasing chances for showers and thunderstorms late this weekend into next week.

A moisture axis on the eastern side of mid level ridge is seen into the Upstate and toward the far northern Midlands with surface dew points in the low to mid 70s. A thermally induced surface trough axis is seen here as well and this region is where isolated convection is expected to develop this afternoon into the early evening. MLCAPE is already approaching 2000 J/kg north of a line through McCormick to Newberry to Lancaster with DCAPE values exceeding 1000 J/kg thanks to generous mixing. CAM's continue to show isolated convection developing later this afternoon near the Upstate and Charlotte that could clip the northern Midlands. A strong to marginally severe storm cannot be ruled out with the threat of damaging wind gusts thanks to robust downdrafts, but widespread impacts here are not likely.

The signal persists across ensemble and global models that the upper ridge will become increasingly suppressed Sunday into next week as a shortwave trough works into the Great Lakes region. This will allow for some weak height falls and shortwave energy to move into the region early in the week with PWAT's ticking back up near 2". With the continued heat and now increased moisture early next week, seasonable to strong diurnal instability could develop with daily showers and storms possible. Some strong to severe storms cannot be ruled out as highlighted in CSU ML guidance and NCAR AI guidance.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

VFR conditions generally expected through the TAF period.

Dry and hot conditions persist under the influence of a strong upper ridge. Southeasterly winds 5 to 10 knots expected this afternoon which should then diminish to light and variable to calm overnight. Winds will gradually pick up by late morning again from the east southeast. Some brief river fog may be possible at prone AGS/OGB locations 09z-12z time frame but confidence is low so only in a tempo group. Scattered diurnal cumulus clouds developing now and likely to persist until sunset then clearing skies overnight.

EXTENDED AVIATION OUTLOOK...Rainfall unlikely through Saturday as ridging builds overhead. Brief early morning vsby restrictions possible. Rain chances and possible restrictions start to increase Sunday into early next week.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Heat Advisory from noon to 8 PM EDT Friday for SCZ016-018- 020>022-025>028-030-115-116. Heat Advisory until 8 PM EDT this evening for SCZ115-116. GA...Heat Advisory from noon to 8 PM EDT Friday for GAZ040-063>065.