Moist southwest flow favors afternoon thunderstorms

The West has been in an active pattern this week with numerous thunderstorms at times for most major regions, save for the Desert Southwest. Some thunderstorms have been strong to severe, and in fact, a tornado touched down in Northeast Nevada near the Ruby Mountains on Wednesday.

Check out our lightning density map from mid-afternoon on Thursday, which revealed widespread coverage of thunderstorms from the Sierra to the Northern Rockies. 

A similar pattern will remain in place heading into the weekend as a trough of low pressure lingers near the West Coast with a strong ridge of high pressure east of the Continental Divide. The consistent southwest flow pattern will continue to supply moisture, and showers and thunderstorms, to the same areas. 

Thunderstorm coverage will be highest during the afternoon and early evening hours with better weather windows likely for most areas during the morning to midday hours. 

Check out the 10-day and hourly forecast for Bozeman, which gives a good idea of what to expect over the holiday weekend in Southwest Montana – one of the more active weather regions currently.

Note: This Weather Tab view is currently available for any location on the OpenSnow website, and will soon become available on the OpenSnow app. 

Forecast for Friday (May 26):

Scattered to numerous showers and thunderstorms can be expected across the Sierra, Great Basin, and Northern Rockies with the eastern slopes of the Divide in Montana expected to be the most favored area for heavy rain. Severe thunderstorms are also possible just east of the Rockies from Montana to New Mexico, with large hail and strong wind gusts expected to be the primary threats. 

Showers and thunderstorms will be more isolated in coverage across the Colorado Rockies, as well as the Cascades in Oregon and Washington and across the BC Interior.

Forecast for Saturday (May 27):

A similar pattern is expected with thunderstorm coverage favoring the Sierra and Northern Rockies, with stronger storms capable of producing heavy rain. Isolated showers and storms will be possible across the Pacific Northwest, Western Canada, and Colorado Rockies.

Severe thunderstorms will be possible east of the Rockies once again from Montana to New Mexico, with large hail and strong winds possible.

Forecast for Sunday (May 28):

A stronger disturbance that will have contributed to Friday and Saturday's active weather will weaken and move east of the Rockies on Sunday. The large-scale pattern will remain similar from a geographic standpoint with a southwest flow in place, but thunderstorm coverage and rainfall intensity should be a bit less compared to prior days. 

Forecast for Memorial Day Monday (May 29):

We're not expecting much of a change to the pattern on Monday with isolated to scattered thunderstorms across the Northern Rockies and to a lesser extent the Southern Rockies. However, moisture and shower/thunderstorm coverage are expected to pick up a bit across the Sierra and Northern California (and possibly Southern Oregon) with locally heavy rain possible. 

Outlook for Tue, May 30 to Sun, June 4:

The large-scale pattern is expected to be similar next week with a dominant ridge of high pressure just east of the Divide and extending up into Canada, while stubborn low pressure near the West Coast/California will keep a persistent and moist southwest flow going into the West.

The result will be additional shower/thunderstorm chances with a wetter-than-average signal showing up from the Sierra to the Great Basin and into portions of the Rockies. 

Temperatures are expected to be cooler than average across California and the Southwest and warmer than average across the Northern Rockies, though a consistent supply of moisture with afternoon cloud cover should preclude any early season heatwaves from occurring. 

Thanks so much for reading! Next update on Tuesday (May 30).

Alan Smith