Prolonged Atmospheric River in the Northwest; Snow in the North-Central US

A prolonged atmospheric river will bring heavy rainfall which may lead to widespread urban and river flooding, heavy mountain snow, along with gusty winds to the Pacific Northwest into the northern Rockies through much of the week. A clipper will bring a period of snow across the upper Midwest and the Great Lakes into Tuesday, before a stronger system bring more wintry impacts. Read More >

Overview

During the early morning of August 28, 2007, a mesoscale convective system (MCS) moved over the southern Twin Cities metro area and into western Wisconsin. This MCS was first associated with marginal severe hail, but soon transitioned to a damaging wind event from Redwood Falls, Minnesota to Cornell, Wisconsin. Wind gusts of 60-70 miles per hour were observed in Redwood County, Carver County, and the Twin Cities metro area.

Wind & Hail
Photos
Radar
Storm Reports
Environment

Wind & Hail:

From midnight through 1 am, the thunderstorms appeared to have mainly a hail threat from north of the Twin Cities into western Wisconsin. Ping pong to golf ball size hail was observed in Chisago County for one storm. At 1:23 am, Redwood Falls reported wind gusts of 62 miles per hour. Low radar scans began to show velocity values of 60-70 knots near Carver County. Later, 82 knots was observed just under 500 feet in central Carver County. The bow structure of the MCS then moved east towards the Minneapolis Airport, which gusted to 71 miles per hour. St. Paul experienced wind gusts of 60 miles per hour. Radar signatures of very strong winds started to dissipate near Washington County, and then reappeared in western Wisconsin. The wind gusts caused significant damage to trees, power lines, cars, and some roof damage.

Photos


Tree uprooted by wind in Waconia	Car damage in Edina	Tree damage in Waconia

Radar:


High reflectivity showed strong storms	MCS bow structure moved east	MCS bow structure near Twin Cities	Storm relative velocity loop

Storm Reports

Environment

Elevated lapse rates and the intensity of the low-level jet (40-50 knots) provided ideal conditions for severe weather. Increasing southerly surface winds in southern Minnesota and 0-1 km shear around 10-15 knots also created a favorable environment. The soundings showed an abrupt change in lower level wind speed and direction throughout the early morning.


LAPS sounding at 6Z	LAPS sounding at 7Z	LAPS sounding at 8Z


250 mb 0Z analysis	Storm environment diagram

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Prolonged Atmospheric River in the Northwest; Snow in the North-Central US

Summary of August 28, 2007 Damaging Wind Event