Snowpack Update:

Warm temperatures in April and early May have resulted in a rapid snowmelt this spring. Many areas that had a near or even above average winter in terms of snowpack are now well below average as of May 13th, especially over the Southern Rockies.

There are some pockets of above-average snowpack remaining in Northern California and Southern Oregon, but most of the West is experiencing an earlier melt this year. 

While it has been a mild start to the spring, we are heading into a 7-day period of cooler and wetter weather, which will include high elevation snow and significant low elevation rain.

Let's dive into the details...

Tuesday (May 13) to Thursday (May 15):

The first of two storm systems is already impacting the West as of Tuesday, May 13th and a much colder airmass is taking hold following several days of unseasonable warmth.

A deep trough is slowly working its way across the West, and an upper low (i.e. a center of low pressure in the upper atmosphere) will track across the Northern Rockies. 

Widespread showers are likely across much of the West from Tuesday night through Thursday night. In the vicinity of the upper low, Southwest Montana is likely to see the heaviest precipitation, with some areas expected to receive 1-2 inches of liquid-equivalent precipitation. 

The colder airmass will also lead to falling snow levels and accumulating snowfall across the higher terrain of the Rockies and the Sierra. 

Zooming in on the Northern Rockies, we can see that mountain ranges in Idaho, Montana, and Wyoming are favored for the deepest snow totals, with some areas potentially seeing up to a foot or so.

View → Forecast Total Snowfall Map

Friday (May 16) to Monday (May 19):

There will not be much of a break before another large and slow-moving low-pressure trough moves into the West. This trough will make landfall in the Pacific Northwest initially, but then is projected to dip a bit further south into Utah, potentially bringing significant precipitation to a larger area of the West. 

The Cascades and the Central Rockies look to take the brunt of this system, with precipitation coming in multiple waves over several days. The Sierra will pick up more precipitation in this pattern as well, and higher terrain in Southern California and Arizona could also see some moisture, which is always a bonus at this time of year. 

While temperatures may warm up briefly in the wake of the May 13-15 system, another blast of colder-than-average air will move through with this second system, resulting in lowering snow levels and accumulating mountain snow.

The Central Rockies look to be favored for the deepest snow totals, while some lower elevation terrain east of the Divide in Wyoming could potentially see some snow as well, depending on the track of the low as it passes across the Rockies (some uncertainty this far out).

To sum it up, the next 7 days are looking very cool and wet across the West. Here is a 7-day precipitation projection from the European Ensemble Model, which represents the average of 50 simulations of this model.

Outlook for May 20th to 28th:

Another weaker system is possible around May 20th-21st, which could bring more showers to parts of the West, especially the Northwest and the Rockies.

Over time, longer-range models are hinting at a high-pressure ridge gradually building back into the West from south to north. This would lead to a gradual warming and drying trend, though weaker disturbances could still bring showers and thunderstorms from time to time.

Warmer than average temperatures are expected to return to California and portions of the Interior West during this window, while cooler than average temperatures are expected north of the Canadian border across BC.

The map below shows projected 7-day precipitation from the European Ensemble Model. This shows that while the overall trend will be drier, occasional showers should still be anticipated, especially across the Pacific Northwest and the Northern Rockies. 

Thanks so much for reading! I will be traveling over the next week, so my next post will be on Thursday, May 22nd.

Alan Smith 

OpenSnow: Your Daily Weather App

As the snow begins to melt and summer conditions quickly take over, remember that you can always use OpenSnow as your daily weather app during the non-winter months.

Getting Started

1. My Location > Weather
2. Maps > Weather
3. Favorites > Weather

View your current location conditions under the "My Location" screen, avoid poor air quality and incoming storms with our summer-focused map overlays under the "Maps" screen, and check the "Weather" tab under any location screen for hourly temperature, precipitation, wind, cloud cover, and lightning forecasts.

You can also view the hourly forecast for the next 10 days for any location on Earth in OpenSnow.

1. Go to the "Maps" tab.
2. Tap anywhere or search for a location.
3. Tap "View Forecast".

View → My Location

Alan Smith