Forecast Highlights:

• Snowpack Update
• Above-average warmth for much of the West on Wednesday and Thursday.
• Cold front arrives late this week with rain for the Northwest on Thursday and Friday.
• Severe thunderstorm threat for the Interior Northwest on Thursday and Friday.
• Active Memorial Day weekend for much of the West with colder temps, rain, and mountain snow.

Synopsis:

Looking at the overall pattern, a relatively quiet stretch of weather is expected during the middle part of this week for a large portion of the West as a ridge of high pressure takes hold over the next 1-2 days.

However, this break will be short-lived for the Northwest and West Central U.S. as a trough approaches late in the week with a cooler and wetter pattern taking hold as early as Thursday and Friday and persisting through Memorial Day weekend.

Snowpack Update:

It's getting late in the season and snowpack percent of average values are starting to become more exaggerated as historical averages drop to low levels late in the season. Still, this map gives a good general idea of what to expect in terms of snowpack conditions relative to average right now.

Snowpack is well above average across the Northwest and Northern Rockies, both of whom have experienced an unseasonably cool and snowy spring. Colorado has also seen a late season boost in snowpack relative to average after a significant winter storm last week. Snowpack remains below average across the Southwest, Wasatch, Great Basin, and Sierra. 

Above-average warmth for much of the West on Wednesday and Thursday

As high pressure builds over the West, temperatures will rise to well-above average levels across California on Wednesday. Areas of the Rockies that have experienced chilly temperatures since last week will also see a noticeable warm-up on Wednesday afternoon and it will be a beautiful day to get outdoors.

On Thursday, California will cool off slightly while above-average warmth expands across the Central and Northern Rockies where another spectacular day is expected.

Warm and dry conditions along with low relative humidity will continue to result in elevated fire danger across New Mexico, who has had a rough spring. The Great Basin (mostly in Nevada) will also see elevated fire danger on Thursday due to warm and dry conditions along with gusy winds as a storm system approaches the West Coast. There is also a threat for dry thunderstorms (i.e. storms with lightning but little rain) across the Great Basin on Thursday.

Showers and thunderstorms for the Northwest on Thursday and Friday (and beyond)

A storm system will work its way across the Northwest on Thursday and Friday with widespread showers expected across the Cascades and Northern Rockies. Thunderstorms should also be expected across the Inland Northwest and Northern Rockies.

We will see a cooling trend in temperature across this region on Thursday and Friday as a cold front moves through but snow levels will remain high for most of this period.

Here are the projected rainfall amounts for Thursday and Friday:

Strong to severe thunderstorms for the Interior Northwest on Thursday and Friday

Ingredients are also coming together for a severe thunderstorm threat across the Inland Northwest on Thursday afternoon and evening – especially Northeast Oregon, Southeast Washington, and portions of the Idaho Panhandle.

This includes the Blue and Wallowa Mountains, where a level 2 out of 5 severe weather threat has been highlighted by NOAA's Storm Prediction Center.

The image below from the Storm Prediction Center indicates a general thunderstorm threat in light green, and a severe thunderstorm threat in darker colors (green = marginal, yellow = slight, etc.).

Convective available potential energy (CAPE) is a measure of atmospheric instability, with values of 1,000 Joules/kg and higher across this region more than sufficient for strong thunderstorms with frequent cloud to ground lightning to develop.

Dewpoint values in the 50's ºF are also expected across the Inland Northwest on Thursday afternoon, which is a significant amount of moisture for this region and is also conducive for strong to severe thunderstorms along with locally heavy rain.

In addition, wind shear (changes in wind speed and/or direction with increasing elevation) will be signifcant, over 40 knots in the regions with significant moisture and instability, and this will support organized clusters of thunderstorms with the potential for severe weather.

The strongest thunderstorms on Thursday will have the potential to produce hail of 1" in diameter and straight-line wind gusts of over 60 mph, along with dangerous cloud-to-ground lightning and locally heavy downpours.

On Friday, the severe thunderstorm threat (level 1 out of 5 risk) will shift into the mountain ranges of Southwest Montana where isolated large hail and damaging wind gusts will be a threat.

Active Memorial Day Weekend Ahead

A stronger and colder storm system will move into the Western U.S. over the weekend, resulting in a cool and wet holiday weekend for the Northwest and Northern/Central Rockies, while the Sierra will likely see some moisture as well.

Check out the 3-day precipitation projection from both the European and GFS Models. Right now, the Oregon Cascades, Central Idaho Mountains, Northwest Wyoming, Utah Wasatch, and a large portion of Montana are in line to receive the heaviest precipitation. 

Colorado will be on the southern/eastern fringe of this pattern but will likely see some scattered showers/thunderstorms at times over the weekend, while Arizona and New Mexico will remain dry.

 

Temperatures will trend downward across the West throughout the weekend as well, with the potential for significant mountain snow on Sunday and Monday, especially across portions of the Cascades and Northern Rockies. Check out the latest projection from the European Model.

By Monday (Memorial Day), temperatures will be well below average across a large portion of the West.

Outlook for May 31 - June 6:

Heading into the first week of June, confidence is low but we are seeing some signs that the Pacific Northwest could head into a somewhat drier pattern along with milder temperatures as high pressure builds off the Northwest Coast. 

Cooler than average temperatures are expected to prevail across the Rockies with near-average temperatures over California. We could see the main focus of showers and thunderstorms shift to areas along and east of the Continental Divide.

Thanks so much for reading! Next update on Friday (5/27).

ALAN SMITH