textproduct: Missoula

DISCUSSION

KEY MESSAGES:

Today: Widespread precipitation, light snow above 5000 feet.

Wednesday: Pass level snow, potential travel impacts.

Wednesday night/Thursday: Strong cold front with snow levels to valley floors but light valley accumulations. Moderate pass level travel impacts continue.

Today through Tuesday:

A band of steady precipitation currently extends from the Bob Marshall Wilderness southwestward into north-central Idaho, driven by an upper-level low spinning west of Salmon this morning. This low will track slowly eastward today, gradually shifting the precipitation band across the region and transitioning it into scattered showers by the afternoon. In northwest Montana, modest instability will support a few isolated thunderstorms this afternoon.

Snow levels this morning range from 5,000 feet along and south of I-90 to near 6,000 feet further north. Travelers should allow for the potential of a brief period of slushy snow accumulations (1-2 inches) on the roadway at Lost Trail Pass. Showers will diminish this evening, giving way to a brief period of zonal flow on Tuesday. This will result in mostly dry conditions for lower elevations, though isolated, terrain- driven showers will persist over the mountains.

Wednesday through Thursday (Strong Cold Front):

Widespread precipitation returns Tuesday evening along the ID/MT border and spreads eastward through Wednesday morning. Initial snow levels will range from 3,500 feet in far northwest Montana to 5,000 feet across southwest Montana and the Grangeville area. Winter driving conditions are expected for Marias, Lookout, and Lolo passes to start Wednesday, with mean snow totals of 2-5 inches likely by midday.

A powerful cold front will cross the region Wednesday afternoon and evening, bringing a focused band of precipitation and gusty winds. Colder air behind the front will cause snow levels to plummet to 1,500-2,000 feet (all valley floors) by Thursday morning. While current guidance suggests the system will have diminishing moisture behind the front, this scenario still allows for scattered valley snow showers but with limited impacts. However, moderate impacts are likely at pass levels; 50th percentile (NBM) projections indicate 3-6 inches of additional snow Wednesday night through Thursday. Higher valleys in southwest Montana may also see slick roads and moderate travel impacts.

Confidence/Alternative Scenarios: Forecaster confidence is high regarding the frontal timing and temperature drop. We are monitoring one alternative scenario (<25% probability) where a closed low develops further south, which would significantly increase snowfall totals for the valleys. But once again, this is a less likely scenario.

Next Weekend (April 18-19):

Brief ridging is expected this coming weekend, leading to a temporary warming trend. However, long-range CPC guidance favors a return to cool and showery weather early next week as another trough approaches the Pacific Northwest.

AVIATION

An upper-level low currently centered west of Salmon, ID, will drift slowly eastward today, shifting a primary band of precipitation across the region. This band, currently stretching from north-central Idaho into the Bob Marshall Wilderness, will transition to scattered showers by this afternoon.

Ceilings/Visibility: Widespread MVFR conditions and frequent mountain obscurations will persist under the main precipitation band this morning. Occasional IFR visibility is possible within heavier rain/snow showers. Conditions are expected to improve to VFR for most terminals by afternoon as precipitation becomes showery, though localized MVFR will remain possible near convective activity.

Convection: Increasing instability this afternoon will support isolated thunderstorms across northwest Montana.

Surface Winds: Southwest winds will increase this afternoon, sustained at 10-15 kts with gusts of 20-25 kts for southern terminal sites. Winds are forecast to diminish significantly after sunset as the boundary layer stabilizes.

MSO WATCHES/WARNINGS/ADVISORIES

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