Discussion: Portions of north-central Montana from time to time experience high wind events. Typically winds downslope off the Northern Rockies creating these downslope wind storms which are high impact events for motorists, pilots, etc. The setup for December 18th-19th showed signs of such an event and prompted high wind warnings by the NWS (light gold color) across a large section of central Montana. This article will examine a way to predict strong downslope winds.
This image above is from the article “An Operational Technique Used to Detect ‘Mountain Wave Signatures’: A Forecast Methodology for Severe Westerly Winds in the Mountains of West Texas” by Todd and Coty Lindley, and Jeffrey Cupo, NWS WFO Lubbock, Midland, TX.
One method to forecast a potential downslope wind storm is to take a cross section of the atmosphere perpendicular to the mountain range and analyze the vertical motions present. Based on the article referenced above the vertical cross section should display strong descending air coupled with weaker ascending air just downwind of the mountain peak with an upwind tilt with height. The image above is a cross section down in Texas through the Guadalupe Mountains, notice the upward and downward vertical motions with an upwind tilt in each event with the surface gusts noted below each image.
The cross section starts at point A and runs northeast to point B. Downward vertical motion is represented in grays and whites, upward in pinks. Winds are plotted, equivalent potential temperature, as well as relative humidity. The terrain and elevation are plotted on the bottom of the image with the associated pressure level on the left-hand side.
Luckily, tropicaltidbits.com has a function to create cross sections. This cross section is produced from the GFS at 18Z on the 18th (location marked in the upper right corner point A to point B. Right after the start time of the high wind warning the coupled strong downward and upward vertical motion downwind of the highest terrain shows up in the model in a similar fashion to the previous example.
What did the actual wind observations read during this downsloping windstorm? Attached below are the observations from Blackfeet, MT. Surface winds gusted (purple numbers on the right-hand side) into the 40s around 11AM local time, 18Z, with gusts into the 40s and 50s over the next few hours and even a 60-mph reading at 2PM local, 21Z.
The setup for downsloping winds also create conditions that result in mountain waves. Not only are the strong surface gusts hazardous to trucks and other vehicles on the roadways but mountain waves are extremely turbulent and dangerous for aircraft aloft.
To learn more about high wind events and North American weather in general click here!
©2018 Meteorologist Joe DeLizio