textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Warming and drying trend expected through the weekend.

- Showers and thunderstorms on Memorial Day (Monday). Any lightning, sudden wind, or rapid temperature drop could be impactful to outdoor recreationalists. Cold front moves through by Tuesday morning.

- Tuesday could be quite the wet day as a large upper low becomes situated to our southwest. There's a 50 to 70% probability for widespread rainfall amounts exceeding 0.25" by Wednesday morning.

- Upper low keeps unsettled weather going next week, so keep that rain gear handy!

This Afternoon:

Scattered showers are expected into this evening across northwest Montana and along the Continental Divide. Impacts will be limited to brief, localized rainfall and gusty winds up to 30 mph. However, marginal instability may be sufficient to trigger an isolated thunderstorm, primarily within Glacier National Park.

This Weekend/Water Safety:

A significant warming trend will bring summer-like temperatures to the region for the holiday weekend. These warm conditions will likely increase outdoor recreation on and around area lakes and rivers. However, local waterways continue to run dangerously cold due to ongoing mountain snowmelt. Plunging into cold water without acclimatization can induce cold-water shock drastically altering breathing, heart rate, and blood pressure, creating life- threatening conditions. Always wear a life jacket and exercise extreme caution. For more information on cold-water hazards, visit weather.gov/safety/coldwater.

Monday (Memorial Day):

Ensemble guidance has converged significantly regarding Monday's weather pattern. A large upper-level low is forecast to drop south and east into the Pacific Northwest by Monday. This setup places the region under south-to-southwest flow aloft, a pattern historically favorable for shower and thunderstorm development. While instability appears marginal, it will be sufficient to support lightning, small hail, and gusty outflow winds. As the flow shifts more southerly later Monday, increasing cloud cover will likely limit greater diurnal instability. Boaters and paddlers on area lakes should remain alert for sudden wind shifts and choppy waters.

Monday Night through Tuesday Night:

By Monday night, wrap-around moisture will overspread the region as a cold front moves into western Montana. Ensemble clusters strongly agree on broad, large-scale ascent through Tuesday night as the upper-level low meanders across Oregon, Idaho, and Nevada. Consequently, the National Blend of Models (NBM) highlights this timeframe for the highest probabilities of widespread rainfall, showing a 50 to 70 percent chance of exceeding 0.25 inches during the 48-hour period ending Wednesday morning. With extensive cloud cover and precipitation, Tuesday will likely be the coolest day of the week, with highs in the 50s and 60s (5 to 10 degrees below seasonal averages).

Wednesday through Friday:

As the cutoff low stalls to our south and west, a plume of Gulf moisture is projected to advect northward across the Plains and into western Montana by Wednesday and Thursday. Ensembles highlight Precipitable Water (PWAT) values of +2 to +3 standard deviations, or over 150 percent of normal. This unusually moist airmass will heighten the risk for heavy rain-producing showers and thunderstorms.

By Thursday, ensemble clusters depict a slow eastward progression, positioning the center of the low over northern Nevada. The ensemble means show the system accelerating slightly by Friday; however, model discrepancies remain, with the ECMWF ejecting the low toward the Yellowstone region while the GFS ensemble maintains a more southerly track. While a below-normal mountain snowpack means mainstem rivers have ample capacity to handle this rainfall, smaller creeks and streams could be highly sensitive to localized heavy downpours. This hydrologic threat will be closely monitored in upcoming forecast cycles.

AVIATION

Scattered showers are expected this afternoon into early evening across northwest Montana and along the Continental Divide. Sufficient instability may support an isolated thunderstorm near Glacier National Park. Localized wind gusts of 20-25 knots will be possible near any stronger shower or thunderstorm activity, with potential impacts at KGPI. Otherwise, light and diurnally driven winds will prevail at all terminal sites. VFR conditions are forecast to persist through Saturday as a warming and drying trend establishes across the region. Westerly winds do increase on Saturday afternoon with gusts of 15-25 kts.

MSO WATCHES/WARNINGS/ADVISORIES

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