textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Today: Isolated strong to severe thunderstorms with localized flash flooding possible, especially in north-central Idaho and northwest Montana.

- Saturday through Monday: A slow moving weather system will bring prolonged, heavy rainfall. Expect significant river rises, potential minor flooding, and a return to winter-like conditions in the high backcountry.

A similar pattern to the past few days will develop again today. Southerly flow will bring in instability and produce showers and thunderstorms, some storms could be strong with isolated severe. Cloud cover and cooler afternoon temperatures may limit activity south of I-90 and east of US-93. North-central ID and northwest MT, the main threat will be gusty outflow winds, hail up to 0.50" with a storm or two potentially attaining severe status and producing 1" hail. Additionally, heavy showers could produce isolated flash flooding, this threat is quite low as storm motion will keep storms moving off to the north at 35 to 40 mph.

A slow moving low pressure system, will transit south of our area before moving north along the Continental Divide. This will allow for a prolonged period of precipitation over western MT, moderate to heavy at times.

The US-93 corridor and east to the Divide have a 80 to 100% probability of receiving at least 0.50" by 6pm Monday. Along the ID/ MT border the probability lowers 50 to 60%. Receiving at least 1", is 60 to 70% probable in the Missoula Valley, 70 to 80% in the Flathead Valley, and east of the these locations is 80 to 100% chance. Two inches or more, there is a 60 to 70% chance in the highest peaks of the Mission, Flathead, and Whitefish Mountains, along with Glacier National Park.

Due to these precipitation amounts, we continue to have hydrologic concerns and will extend all our Hydrologic Outlooks an additional 24 hours, except Flathead and Lake Counties which we have upgraded to flood watches. Additionally, the following point locations are being closely watched for river flooding; the Flathead River at Columbia Falls which is forecast to reach minor flood stage Saturday morning, the Clark Fork above Missoula which is forecast to enter minor flood stage after midnight Monday, and the Swan River near Bigfork which is forecast to reach minor flood stage around noon Monday. Just 18" of fast running water across a road can push a SUV or truck off the road, Turn Around Don't Drown.

There has been an eastward trend in the center or coldest part of the low pressure. We have increased snow levels approximately 1000' this forecast issuance to between 6500 and 7000'. That said, anyone with plans for back country adventure should plan accordingly for cold and wet conditions, to avoid hypothermia. A few to 6 inches of wet snow accumulation is possible above 6500', on Sunday morning. Continue to monitor future forecasts as closed lows are notorious for altering their tracks, snow levels could decrease and snow amounts increase.

AVIATION

Widespread VFR conditions is giving way to expanding convective activity early this afternoon, with thunderstorms just beginning to initiate. Initial development will track south- to- north across north-central Idaho and west- central Montana, spreading into northwest Montana through the afternoon. Terminals, most notably KGPI and KMSO, will face potential periods of MVFR/IFR conditions due to torrential downpours, sudden visibility reductions, and low ceilings. The primary hazard to aviation will be strong, erratic convective outflow winds exceeding 45 knots and localized hail, with the highest probability for severe criteria centered over northwest Montana (KGPI). Additionally, heavy rain from training storms along the Idaho corridors could produce prolonged localized flight restrictions and mountain obscurations.

Widespread rain is expected to increase on Saturday as an upper low moves into the Yellowstone region. Ceiling and visibility reductions will be common in areas experiencing persist rainfall.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Flood Watch from Saturday evening through Monday morning for Flathead/Mission Valleys...West Glacier Region.

ID...None.