BOZEMAN — A slow-moving upper-level low pressure system over the Pacific Northwest will drift east Friday, bringing rising chances for showers and thunderstorms through the day. Cold air higher up in the atmosphere will create enough instability for a few stronger storms to develop. Where wind shear is strongest, there is a small risk for hail. Overall, the main concern for today will be slow-moving storms capable of dumping heavy rain.
From Friday night through Saturday, a combination of favorable upper-level winds, lingering instability, and the approaching system will set up a prolonged period of rain. This steady rain will primarily affect areas south of a line from Helena to Lewistown, down toward Dillon and Big Sky. Because the rain will last for an extended period, there is a heightened risk for excessive rainfall and localized flooding, especially on recent burn scars and in urban areas. Through Sunday morning, the I-90 corridor from east of Butte through Bozeman has roughly a 90% chance of picking up at least 1 inch of rain. Probabilities taper off as you move further north or south of this corridor.
Rain will gradually taper off and move east late Saturday night into Sunday morning as a weak, disorganized high-pressure ridge moves in for Sunday and Monday. Most of next week is shaping up to be dry, though a few weak systems could pass through and trigger occasional hit or miss showers.
