textproduct: Columbia

WHAT HAS CHANGED

Only minor forecast updates with this package. Aviation discussion updated for 06z 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 104-108F possible.

- 2. 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 104-108F possible.

An intense period of heat is forecast to continue as we head into the July 4th holiday weekend. A strong ridge of high pressure, analyzed at ~597dm 500 hPa heights, is centered to our north across the Mid-Atlantic. Northerly and northeasterly flow on the eastern side of this ridge is forecast to continue advecting a very warm airmass atop the Carolinas over the next several days, with 850 hPa temps forecast to warm to 21C-23C Fri-Sun of this period. While the ridge is forecast to begin breaking down by Sunday, this anomalously warm low-level airmass is forecast to yield hot temps centered on the July 4th holiday. Guidance shows near to below normal 0-3km mixing ratios for early July, which is expected to yield deeply mixed boundary layers each afternoon this weekend. This should result in highs near 100F each afternoon, particularly across the central/northern Midlands and Pee Dee region of our forecast area. NBM/LREF probabilities of maxT >100F peaks Sat/Sun with forecast soundings supporting this possibility.

While temps will be hot, the deep mixing is promoted by a slightly "drier" than normal airmass that is in place across the area. Dewpoints are forecast to dip into the mid to upper 60s each afternoon before they begin rebounding early next week. This creates an odd circumstance where we are in July and don't quite have the humidity to get exactly to our Heat Advisory criteria. However, given this is a prolonged period of heat overlapping with the Independence Day holiday, Heat Advisories look likely on Saturday and Sunday as well. Heat Index values will likely peak today in the 104F-108F range, with actual high temps in the 99F-102F range Sat/Sun. The combination of these two, plus overnight lows only recovering into the mid/upper 70s, should yield a prolonged period of heat stress that the Heat Index really doesn't capture. So with this expected, exercise caution this holiday weekend when spending an extended period of time outdoors. Make sure to review and 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. Heat index values may actually rise again early next week while remaining below advisory criteria as dewpoints increase through the middle of next week. Daily shower and storm chances make confidence in the overall heat risk a bit hazy beyond Sunday.

Key Message 2: Increasing chances for showers and thunderstorms late this weekend into next week.

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 are expected each afternoon within this regime as high CAPE and low shear should lead to downbursts across the area. Details of this will become clearer over the next few days.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

VFR conditions generally expected through the TAF period.

Upper ridging will continue to limit potential for any showers or thunderstorms today. Overall, expect VFR conditions, although brief MVFR visibilities are possible around sunrise at fog prone AGS/OGB, but confidence too low for TAF inclusion. Light winds this morning becoming east to southeast once again, 5-10 kts under clear or mostly clear skies. Skies then clear tonight with light south winds.

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