Heavy storms from a hurricane could hit Boise this weekend. Here’s when, how much rain

Boise is projected to be in the direct path of remnants of a hurricane as early as Sunday, a rarity that could bring widespread heavy rains and localized flooding.

The storm, named Hurricane Hilary, is just off the Mexican coast and gaining strength. The Weather Channel has projected Hilary to become a Category 3 hurricane before crashing into the Baja Peninsula and traveling through Southern California and the western part of the United States.

A Category 3 hurricane requires sustained winds of 111-129 mph and is considered a major hurricane. The Global Forecast System, a modeling device used widely by meteorologists, expects the hurricane to drop up to 10 inches of rain in Southern California over three days, equating to about three years of precipitation for the region.

How could Hilary impact Boise?

Boise won’t see nearly as drastic of an impact as California, but the City of Trees still may find itself amid a historic event.

National Weather Service meteorologist Josh Smith told the Idaho Statesman on Wednesday afternoon that based on Hilary’s track northward, Idaho has encountered such a direct track from the remnants of a tropical storm or hurricane on only three occasions in the past 102 years: 1921, 1956 and 1998.

Smith said there would be widespread rainfall from the remnants of Hilary from eastern Oregon through Southern Idaho, toward Jackson, Wyoming, and as far north as Missoula, Montana. Thursday morning’s storm track for Hilary features the southwestern corner of Idaho within the probable path cone, something that Weather Service meteorologist Spencer Tangen believes never has happened before.

The Weather Service expects Hilary to provide Boise with rainfall from sometime Sunday through Tuesday, but doesn’t have a precise reading yet on how much could fall. The Weather Service has predicted totals through Sunday night, with the mountains of Idaho receiving up to 1.5 inches of rain and the valleys, including Boise, up to three-quarters of an inch.

“I think that might be a little bit on the low side,” Smith said. “But that’s what we have right now because we don’t have all the days (of the storm) in that total yet.”

Cloud cover from Hilary’s remnants will also help Boise cool significantly. The city was under a heat advisory through Wednesday evening, with high temperatures hitting 105 degrees, but the high will drop to 81 by Monday, according to Weather Service forecasting.

Why will Hilary remnants make it this far north?

Hilary’s arrival on the Baja coast coincides perfectly with conditions favorable for a monsoon, which is when moisture from the gulfs of California and Mexico is pulled northward through the deserts of the U.S. Southwest.

The ensuing storms follow a south-to-north track typically seen during a monsoon, eventually bringing rain toward Idaho.

In addition to Hilary, a high-pressure system to the southwest of Idaho is providing a flow of air toward the state, and a low-pressure system off the coast of California is pushing a lot of moisture toward the Gem State.

“(Hilary’s) forecast track northward for the next four days,” Smith said. The eye of the storm “will reach near California sometime Monday morning, but all that moisture associated with that storm is going to be transported north, and it’s forecast to go right over Idaho as well as Nevada, Montana and Wyoming.”