A Convective Wind Event over Southeastern Alberta

on 15 July 2008

Stephen Knott and Chris WielkiPrairie and Arctic Storm Prediction Centre - Edmonton

Outline• Synoptic Setup

• Radar Evolution

• Mesoscale Convective Vortex?

• Post Storm Investigation

• Summary

00z 16 July 2008250 MB 500 MB

700 MB 850 MB

Surface Conditions

Adjusted 12z Olds SoundingUNSTABLE 2008 (25/12 0C)

Radar Evolution (1 Hr)

SUDDS 21:00z

SUDDS 22:00z

SUDDS 23:00z

SUDDS 00:00z

SUDDS 01:00z

SUDDS 02:00z

SUDDS 03:00z

SUDDS 04:00z

Vulcan Storm2240z 15 July 0820 minutes prior to report of tornado

Vulcan TornadoRated F1 (Sills)

• Path Length 100m; Path Width 30m, motion 330• Lat/Long50.4303/ -113.189• Two empty grain silos (approx. weight 5000lbs) were torn from their

foundations (bolted about 8 cm into cement pads) and thrown through the air approx. 70m. No sign of being rolled or dragged. Adjacent silos had some damage but did not leave their foundation (even ones with wood foundations). A few trees in the path were damaged or destroyed. Straw bales (approx. weight 1000lbs) were tossed around. Evidence of convergence in the flattened grass along the path.

Mesoscale Convective Vortex

• At 0120z strong persistent rotation found at all three levels.

• Vortex persists 90 minutes, diameter of 15-20 km, tracks 80km from 260 and moves north of Cypress Hills.

• Radar echoes initially show hook but disintegrate into no definable pattern and yet wind damage to homes reported at Irvine.

Meso-Vortex

Meso-Vortex

Meso-Vortex

Meso-Vortex

Meso-Vortex

Meso-Vortex

Meso-Vortex

7:31 pm

7:32 pm

RFD Dry Slot?

Flanking Line associated with RFD gust front

7:33pm

7:48pm

Medicine Hat

Funnel cloud picture is from Seven Persons looking towards 290 degrees ~ 0120 UTC

0120 UTC 3.5 km CAPPI

0120 UTC

0120 UTC

Damage Survey Route

Main damage swath was from anarea just south of Seven PersonsEastwards.

0120 UTC 0130 UTC 0140 UTC 0150 UTC

215 kmh-1 measured 80 m above ground

Peak Winds and damage swath

Estimated damage path (F1 or greater) was approx 250 km long and up to 31 km wide

2100 UTC

0200 UTC

Speeds in kmh-1

Summary• During the event operations believed the mesoscale vortex had a

high likelihood of generating a tornado• EPWS triggered 3 times during storm lifetime, first as a dangerous

supercell then along with a tornado warning• Damage survey results suggest this large-scale persistent rotation

may not have equated to a tornado– apparent predominant straight line wind damage on the order of F1 and

low F2 categories in the RFD region of the supercell• More work required with respect to origin of large-scale meso-vortex

but authors speculate a low-level jet from near Alberta US border interacted with storm enhancing the RFD and spinning up vortex.

• Was some speculation from damage path that the event may reach derecho criteria though this not the case

• Weakening radar echoes did not correlate with diminishing of severe winds

Thank You