Jul 20, 2014

Derecho expected for the Northern Plains and Western Great Lakes

This upcoming week will feature some significant and widespread heat and humidity from the east facing slopes of the Rockies to the East Coast. The weather setup for this week  features a sprawling high pressure area surface and altof , acting like a heat pump, extending from the Southwest States to the Great Lakes.

The image to the left is the 500 hPa level (approximately 18,000 feet above the surface of the earth) from Sunday morning 20 July 2014  8AM EDT. The solid black lines are forecast heights. (An upper air chart such as this is called a constant pressure chart; on the chart the pressure is 500 hPa but the 500 hPa pressure is "located" at a different "height" above ground.) Think of upper-air maps as a topographic map of the atmosphere. The solid black lines are the actual heights of the 500 hPa and also can serve or approximate the the direction of the the winds aloft at near 18,000 feet. The distance between these contours can also approximate the wind speed -in general the closer the distance between any two contours, the stronger the winds and the greater the distance between the contours then the lighter the winds. The center of the "heat" producing high is located over southwest New Mexico with an elongated axis of higher heights extending to western South Dakota. From the Pac Northwest States to northern Wisconsin, a flow of air bringing some Pacific moisture prevails along with embedded disturbances that will cause ascent or lift.

The combination of all of these above features is expected to cause a significant severe weather outbreak across the Northern Plains to Wisconsin during Monday into Tuesday morning (July 21-22). Here is the severe weather outlook from SPC for Monday 21 July. The severe weather will initially start over the the border region of North and South Dakota with Montana as scattered but intense supercells during Monday afternoon, capable of producing severe hazards of damaging winds, large hail and a few tornadoes. These supercells are then expected to merge into a strong progressive derecho over the eastern parts of the Dakotas and then move rapidly east across and towards Minnesota to Wisconsin by late Monday night into Tuesday morning.

This derecho is expected to cause extremely strong winds of hurricane force (75 mph) or more in places along its path. The remnants of this system may also  redevelop and perhaps affect our area during Tuesday night, though probably not as intense. Often times these derecho are "episodic" in nature, that is they tend to develop on consecutive days and may even form over the same geographic region or move across the same region as the predecessor derecho did. Progressive derechos during the summer tend to form and become quite intense during extreme heat and or heat waves. The pattern for the development of these derechos is often referred to as "ridge-rollers and the ring-of-fire"