textproduct: Columbia

WHAT HAS CHANGED

New Heat Advisory issued for tomorrow from Noon until 8 PM for counties along and north of I-20. Aviation discussion updated for 18z TAF's.

KEY MESSAGES

- 1. Above normal temperatures continue, peaking this weekend. Another Heat Advisory will be in effect along and north of I-20 from 12pm-8pm Saturday with heat indices up to 105F possible.

- 2. Increasing chances for showers and thunderstorms late this weekend into next week.

DISCUSSION

Key Message 1: Above normal temperatures continue, peaking this weekend. Another Heat Advisory will be in effect along and north of I-20 from 12pm-8pm Saturday with heat indices up to 105F possible.

Temperatures are creeping into the mid 90s early this afternoon with mixed out dew points in the mid to upper 60s and some locations still seeing dew points near 70F. Heat indices are already ranging between 100-106F and could reach up to 108F as we top out in the upper 90s to near 100F later this afternoon. The Heat Advisory currently in effect expires at 8 PM, but recovery overnight is not expected to be all that impressive with lows that only lower to the mid 70s.

Heading into Independence Day weekend, the 594-596dm mid level ridge is expected to slowly shift eastward with its center remaining just north of the FA across the Mid Atlantic. As it does so, 850 mb temperatures are expected to warm toward 20-22C, which is approaching the NAEFS 99th percentile. One difference from previous day's is 925mb dewpoints around 18-19C should drop toward 14-16C on Saturday as slightly drier low to mid-level air works in from the east/northeast. This is expected to promote deep mixing Saturday afternoon with surface dewpoints mixing out into the mid to upper 60s as depicted in latest forecast soundings and CAM guidance. The mixing out of drier dewpoints and drier soil moistures in place with the little rain and prolonged heat seen over the past couple days could help afternoon high temperatures push 98-102F tomorrow and again on Sunday. One caveat is slightly greater moisture should start moving in on Sunday as low level flow becomes increasingly southerly. As discussed previously, heat index values this weekend are generally expected to top out near 105F, with a couple spots pushing 108F possible. While widespread values meeting Heat Advisory criteria are unlikely, the multitude of outdoor events for the holiday combined with prolonged heat will bring an increased risk for heat illness across the area this weekend. Due to this, another Heat Advisory has been issued for counties along and north of I-20 for Independence Day from Noon to 8 PM. The possibility of expanding this further south/east will be evaluated with the next forecast package. With similar conditions possible Sunday, another advisory may be needed. Increasing low level moisture into next week, will likely bring more days with heat indices above 100F (LREF probabilities above 70% through at least Wednesday) and the possibility of more heat products. The main factor that complicates heat headlines next week will be isolated to scattered diurnal convection.

Be diligent to take appropriate heat safety measures this holiday weekend. 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: Increasing chances for showers and thunderstorms late this weekend into next week.

A spotty shower cannot be ruled out Saturday afternoon as shown in the recent HREF solution, but confidence is low in this. As a shortwave trough moves into the Great Lakes region, the upper ridge over the region will become increasingly suppressed with pieces of shortwave energy expected to move through the Mid Atlantic down toward the Carolinas starting Sunday and into next week. Increasing southerly IVT is expected to bring PWAT's back near 2" as shown in over 75% of EC Ensemble members. The signal for seasonable to strong diurnal instability persists across ensemble guidance with both GEFS and EC Ensemble mean solutions having CAPE values over 1000 J/kg for multiple days next week. Some strong to severe storms are expected each afternoon within this regime as high CAPE and low shear should lead to an environment conducive for downbursts across the area.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

VFR conditions expected through the TAF period.

Strong upper ridging overhead will continue to limit convection today with only fair weather cumulus expected. Southeasterly winds around 5 to 10 knots expected through sunset then diminishing winds to light and variable to near calm overnight through Saturday morning. Some river fog possible but did not happen this morning so going with persistence and not including in the forecast but cannot rule it out.

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