Post-tropical storm Hilary is on track to impact the Western U.S. this weekend and early next week. While Southern California and Nevada will take the brunt of this storm in the U.S., significant impacts are also expected across Idaho and portions of Eastern Oregon and Western Montana.

Check out the projected 48-hour rainfall totals from Sunday morning through Tuesday morning across this region. The mountains of Central Idaho are expected to take the brunt of this system, including Sun Valley and McCall, while even Boise is expecting upwards of an inch of rain. 

Hurricane Hilary Update: 

As of Friday afternoon (August 18), Hilary is located off the coast of Mexico, southwest of the Baja Peninsula, and is a major hurricane.

Hilary will move northward over the coming days and will weaken significantly as it encounters cooler water. However, it is still projected to maintain tropical storm strength as it reaches California, which would be a rare occurrence. 

Learn More → Hurricane Hilary Takes Aim at the Southwest U.S. and Mexico

As Hilary continues its journey northward, it will become a post-tropical cyclone and will become absorbed into a deepening trough of low pressure along the West Coast – forming into more of a hybrid storm that involves both tropical moisture and mid-latitude storm characteristics. 

As the storm reaches the Interior Northwest, heavy rainfall will be the main impact while gusty winds can also be expected in some areas, especially at higher elevations.

The rain will have both positive and negative impacts for this region. Let's dive into the details...

Heavy Rain – Timing and Potential Flooding Impacts:

A combination of tropical moisture from Hilary along with moisture from the North American Monsoon will surge northward into Idaho and West/Southwest Montana during the day on Sunday well ahead of Hilary's storm center. 

NOAA's Weather Prediction Center has this region under a Level 2 out of 4 risk for flash flooding on Sunday and Sunday night. 

 

On Monday, another surge of moisture will arrive as Hilary continues its trek northward. This second surge is projected to be centered a bit further west, with the main impacts expected across Idaho, Eastern Oregon, and West/Northwest Montana. 

The Weather Prediction Center has Idaho under a Level 2 out of 4 flash flooding risk once again, with the threat also extending into Eastern Oregon and toward the Missoula region of Montana. The Wallowa Mountains in NE Oregon look to be right on the edge between light vs. heavy rainfall. 

The amount of water vapor in the atmosphere (referred to by meteorologists as precipitable water) will be 200-300% of average across this region on Monday, or 3 to 5 standard deviations above average.

In fact, National Weather Service offices in the region that launch weather balloons into the atmosphere (such as Boise) could approach or set all-time records for precipitable water values. 

The substantial amount of moisture available has the potential to result in heavy rainfall rates over long periods of time, especially in mountainous terrain with orographic lift could enhance rainfall rates locally.

Areas of steep & rocky terrain and wildfire burn scars will be the most susceptible to flooding and runoff issues, as will small streams in steep drainages. Mudslides and rockslides will also be a concern in mountainous terrain.

Fire and Smoke Relief For Some

Leading up to this event, fire activity has increased substantially across the Northwest during the past week or so, thanks in part to a heat wave that has taken hold over the region. 

Check out the smoke forecast map for Friday evening, which gives a good idea of the current fire situation:

The good news is that heavy rainfall on Sunday and Monday is expected to fall over ongoing fires in Central Idaho, and quite possibly Western Montana as well. Air quality will also improve over these regions.

Large fires in Northwest California, Western Oregon, and Southern British Columbia are going to be on the fringe and are only expected to see light and spotty rainfall at most. 

While these larger fires won't see much moisture, on the plus side, cooler temperatures and higher relative humidity this weekend should help to reduce fire danger and scale back fire behavior somewhat. Warmer temps are expected to return across BC next week, though.

Gusty Winds on Monday

The center of post-tropical storm Hilary is projected to move into Idaho on Monday, though some uncertainty exists regarding the exact track.

Leading up to its projected landfall in Baja or Southern California, strong winds can clearly be seen rotating around the eye of the storm.

As the storm approaches the California border, it will quickly weaken. However, gusty winds can still be expected, especially over mountainous terrain and on the eastern side of the storm.

As the storm works its way into Idaho on Monday, it will become windy across Central/Eastern Idaho, Northern Utah, Western Wyoming, and SW Montana. High mountain areas and exposed ridges will see the highest wind gusts, while the Snake River Plain could also see some impressive gusts.

Check out the peak wind gust projection from the European Model on Monday afternoon (6-hour period from noon to 6pm).

Storm Uncertainties

The exact tracks of tropical systems are always tough to nail down, and the fact that this one will become absorbed into a mid-latitude system over the Western U.S. adds to the complication.

While models have been in fairly good agreement, there is still enough uncertainty that could influence the eastern or western fringes of projected rainfall, as well as the locations of the heaviest rainfall rates. 

Idaho appears to be the best bet for significant rainfall. If the storm were to track a little bit further west than expected, then Eastern (and possibly even Central) Oregon as well as Eastern Washington could end up receiving heavier rainfall. 

On the other hand, an eastward shift could result in heavier rain across Western Montana and possibly NW Wyoming, with less rain for Oregon.

Alan Smith