textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Showers and isolated thunderstorms will impact northwest Montana again this afternoon, primarily near and along the Divide.

- Rapidly building heat will bring widespread 70s and 80s this weekend with temperatures potentially reaching close to 90 degrees by Tuesday.

- Dangerously cold water continues to be the primary hazard, as river temperatures in the 40s and 50s can trigger cold-water shock.

The short-term focus remains on a weak disturbance currently tracking eastward. Similar to yesterday, afternoon instability will lead to scattered showers and a few thunderstorms, primarily across the mountains of northwest Montana. Afternoon surface winds will remain gusty in the 15-25 mph range.

The primary headline is the significant ridge of high pressure amplifying over the region. Confidence is exceptionally high for a prolonged period of mostly dry, unseasonably warm conditions through at least early to middle-part of next week. Tuesday remains on track to be the hottest day of the season so far. Current forecasts of temperatures nearing 90 degrees (25% chance for Missoula and Bitterroot valleys, 70% chance for lower river valleys of Idaho) would begin to encroach on daily high temperature records for many locations.

Despite the "summer" feel of the air, local waterways remain extremely hazardous. Rivers are currently running fast and high with near-peak mountain snowmelt ongoing. Debris and hazards may be obscured or hidden under the water's surface. Water temperatures between 40 and 50 degrees can paralyze muscles and trigger involuntary gasping (cold-water shock) in minutes even for strong and experienced swimmers. Recreationists are urged to wear life jackets and stay back from slippery, unstable banks.

Model uncertainty increases by Wednesday as the ridge could begin to break down. There is currently a roughly 50% chance of a transition to a slightly cooler and more active pattern by late next week.

AVIATION

VFR weather conditions will prevail across the Northern Rockies through the forecast period. A weak disturbance will continue to bring in mid and high-level clouds throughout this afternoon and evening. Scattered showers and isolated thunderstorms are possible between 08/2100z and 09/0200z which could lead to brief reductions in visibility and erratic gusts. Typical afternoon westerly winds will gust between 15 and 20 knots before becoming light and variable overnight.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...None.