Regional and Mesoscale Meteorology Branch

Cheyenne High Wind Events

Introduction

One of the more challenging forecasts for the NWS Cheyenne, WY office is high wind events. We are using satellite climatologies to search for cloud signals to these events.

Unlike other sites we have used wind regime climatologies (see Tallahassee, Wakefield and NWS pages), Cheyenne has a strong, overall tendency towards westerly wind.

1=Calm;2=North;3=Northeast;4=East;5=Southeast;6=South;7=Southwest;8=West;9=Northwest

Note: that 55% of all winds are westerly, 84% when including Northwest and Southwest.

So another approach was needed. Since Cheyenne has recorded the dates and times for high wind events, we decided to use them to our advantage.

NWS definition of high winds

Below 7000 feet MSL = sustain winds of 40mph or greater or gusts to 60 mph or greater

Above 7500 feet MSL = sustain winds of 50 mph or greater or gusts to 70 mph or greater

For this project, snow cover and the need for nighttime composites ruled out using the visible channel (1) on GOES West to determine cloud. The 10.7 micron channel (4) with a threshold method was used instead.

Cloud Cover % during all west winds at 1500 MST, 211 cases

First, a composite for all west wind cases at 1500 MST was generated. This provides a glimpse of what we can expect on average for this regime.

Note:

• Cloud cover % values are mostly in the 30-50% range
• Lower values (10-30%) over south central Colorado Rockies and in Wyoming’s Great Divide Basin area
• slightly higher amounts (50-70%) are seen over Western Idaho, northern Nevada, and central Montana.

Next, look at a cloud composite of only high wind events.

Cloud Cover % during high wind events at 1500 MST, 35 cases

Note:

• Cloud cover amounts lower in Central Colorado and around Montana/Saskatchewan border
• Cloud cover amounts have increased in Idaho and Montana, with some areas over 90%
• Increased cloud amounts also seen over the Rocky Mountains on the border of Wyoming and Colorado, with decreases on either side. This could indicate mountain wave clouds that often form streets over and around mountains during high wind events.

To search for cloud precursors before a high wind event, a composite of the 12 hours preceding each case for a given hour was generated.

Cloud Cover % during pre-event period at 1700 MST, 15 cases

Note:

• The streak of very strong (80-100%) cloud amounts over the Rockies in western Montana
• lower clouds amounts to the south and east of this streak.

Suggested explanations for the streak of high cloud amounts include jet streaks, baroclinic leaves, or mountain wave clouds in the lee of the Montana Rockies. Further research into these possibilities are ongoing.

To check on any possible cloud signals for the end of a high wind event, a composite of the twelve hours after a wind event was generated.

Cloud Cover % during post-event period at 1700 MST, 13 cases

Unlike the pre-event cases, no specific signal can be seen. More work needs to be done to see if any can be found.

This work is on-going.