National Weather Service releases map of Austin microburst wind damage

AUSTIN (KXAN) -- A microburst associated with a supercell thunderstorm moved through Austin on Wednesday, May 28. The National Weather Service in Austin/San Antonio surveyed the damage from the storm to determine the cause and strength and determined a "long-track microburst" moved through parts of the city.

While they released their survey results last week, they've illustrated the areas of Austin hardest hit with a new map (below).

Harrison Tran, a meteorologist at NWS Austin/San Antonio, illustrated where the strongest winds from the microburst and rear flank downdraft were. The map shows wind gusts up to 85 mph generally hit areas from Mopac to Austin-Bergstrom International Airport, through parts of central, east and southeast Austin.

The rear flank downdraft wind gusts reached up to 75 mph and were situated slightly south of the microburst wind damage, extending through downtown Austin and points southeast.

Microburst and rear flank downdraft explained

While many microbursts are caused by dry air intruding in the upper levels of a thunderstorm -- causing the rain to evaporate, cooling the air and forcing that air to the ground -- this particular microburst started due to hail.

Large amounts of hail lofted high into the thunderstorm melted and cooled the surrounding air. That cool air is more dense than warm air and plunged to the ground, bringing 85 mph straight-line winds to parts of the Austin area.

At the same time, rear flank downdraft winds reached up to 75 mph and hit downtown Austin. A rear flank downdraft is caused by pressure patterns within supercell thunderstorms that force cool mid-level winds quickly down to the ground before getting pulled back into the rotating thunderstorm.

Storm motion through Austin

The supercell thunderstorm that caused so much damage and hail in Austin originated in the northern Hill Country near the San Saba and Lampasas county line around 4:30 p.m. Thursday.

This illustration from NWS shows the timeline of the strengthening supercell that reached peak intensity around 6:30 p.m. in Austin before it weakened in Bastrop County later in the evening.