Summer daytime down drainage winds in the Salmon River drainage

The Salmon River drains a very large area in central Idaho beginning in Central Idaho near Stanley, drains north toward Salmon, then westward towards Riggins. There are steep mountains on all sides of the river, and it flows through narrow canyons in many areas. As fire severity and frequency continues to increase across the region, this will in turn place more and more fire fighters possibly in harms way when fighting fires in the Salmon River drainage.

One would expect that, on a daily basis, the winds in this mountainous area would act in a normal diurnal trend: upslope in the morning, then turning up valley in the afternoon. However, based on several observations from fire fighters and forecasters, this expected pattern is overcome, and the winds often blow down-valley during the afternoon in many of the side drainages of the main Salmon River. In one case, two separate fires burning at the same time in the nearby Selway River on opposite sides of the river moved in totally opposite directions; the south aspect up drainage and the north aspect down drainage.

Understanding what causes this wind reversal in the drainages, as well as where they can be expected, will provide forecasters with a tool to forecast these conditions. Don Wagner (2005) noted that in 9 of the 12 fires he studied in and around the Salmon and Snake River area found that fire burned further down drainage compared to up drainage during the afternoon. Wagner also noted that during smoky or cloudy conditions, fires burned as expected – upslope / updrainage during the afternoon. This wind reversal was displayed on the Skull Gulch RAWS during the Butte Fire in August of 1985, where 73 firefighters had to deploy there shelters when a crown fire overtook the river drainage. Our hypothesis is that the large drainage area of the Salmon River is producing so much up-valley flow during hot summer days that it cannot be contained within its narrow canyons. This produces a broad area of up-valley flow at low-levels that spreads horizontally away from the main river. The strong afternoon heating and mixing that occurs in nearby side drainages taps into this broad area of up-valley flow associated with the main Salmon River – often producing flow that appears down-valley with respect to the smaller side drainages. Preliminary results will shown where remote weather stations afternoon down drainage winds do occur on relatively clear, sunny, summer days.