Severe Thunderstorm Event from Priest Lake, ID to Newport, WA - July 28, 2009

On the afternoon of July 28th, 2009, a large thunderstorm developed over the north end of Priest Lake around 3:00 pm. This storm moved south to south-southwest around 15 MPH and attained severe intensity once it reached the west-central shore of the lake. The storm continued on its south to southwest path through 4:20 PM producing a impressive damage path as it tracked through the thick forests of north Idaho before weakening as it crossed the Pend Oreille River and moved through Newport, WA.

The interactive map below traces the path of this storm as well as some of the damage it produced along the way. 

View Priest Lake, Idaho to Newport, WA storm damage. 7/28/09 in a larger map. Click on the cloud icons for details.

While sporadic damage occurred along much of the storm path, it was apparent that the tree blowdowns were caused by strong thunderstorm outflows or straight line winds. All trees ended up pointing toward a south to southwest orientation. If a tornado was the cause of the event, debris from the downed trees would be pointing in variable directions, rather than one distinct one. The following images support the notion of straight line wind damage.

The top image shows all trees which fell on the house were pointing in one direction (south).

Meanwhile the bottom picture shows numerous leaning and downed trees. Again these all pointed in the same direction (south). According to the property owners, all these trees in this picture stood straight before the storm rolled through.

The radar images show some of the data that forecasters were viewing as they issued Severe Thunderstorm and Tornado warnings.

The image above shows the radar reflectivity as the storm moved across Priest Lake and onto the west shore of the lake.

Nine minutes later the radar reflectivity showed indications of a rotation ahead of the storm. This prompted the issuance of a Tornado Warning.

The storm relative velocity image above shows the rotation. The green colors indicate winds that are moving towards the radar, which would be a wind blowing from northeast to southwest. The red colors show winds moving away (from southwest to northeast) from the radar. This circulation was west of Highway 57, over mainly for.

As the storm continued to track to the south-southwest, it moved along Highway 57, dropping large hail.

By 400pm, the thunderstorm had produced a gust front ahead of it. The gust front is formed by rain-cooled air in the thunderstorm descending, hitting the ground, and then spreading out.

The last reflectivity image shows the storm as it hit the Diamond Heights area, knocking down large trees.

By Jonathan Fox and Ron Miller