textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Gusty winds with thunderstorms this afternoon in Lemhi County and in the vicinity of Butte.

- Localized heavy rainfall/gusty outflow wind threat Monday and Tuesday with showers and storms.

This Afternoon/Evening...Warm and mostly stable conditions will prevail across the majority of the region today. However, mid- level moisture advancing northward into southwest Montana and Lemhi County, Idaho, will trigger isolated afternoon and evening thunderstorms. Deeply dry low-level air (inverted-V sounding profiles) will favor dry microbursts capable of producing localized severe outflow winds exceeding 50 mph, primarily across Lemhi County and the Butte area. The Storm Prediction Center maintains a Marginal Risk (5-15% chance) for these damaging gusts.

Monday and Tuesday...The week's most impactful weather arrives as a robust surge of subtropical moisture pushes precipitable water (PWAT) values well above early-July climatological normals. An upper-level disturbance interacting with peak afternoon heating on Monday will spark organized thunderstorms across north-central Idaho and west-central into southwest Montana.

Primary hazards with Monday's activity include: * Frequent lightning capable of igniting new wildfires in dry fuels. * Erratic convective wind gusts of 40 to 50+ mph. * Heavy rainfall rates of 0.50 to 0.75 inches per hour, introducing localized flash flood and debris flow risk, especially on recent burn scars.

Thick morning cloud cover on Monday may initially limit daytime destabilization, but strong afternoon dynamic forcing and pockets of instability are expected to overcome this convective inhibition. By Tuesday, low levels begin to dry out, but lingering mid-level moisture and instability will trigger a second round of scattered afternoon thunderstorms, focusing primarily from Interstate 90 southward.

Wednesday through Friday...Following the departure of Tuesday's system, the region transitions back to a classic summer pattern. Expect warm, dry conditions with breezy westerly afternoon winds. Isolated, terrain-driven afternoon thunderstorms will remain possible but will be largely restricted to the mountains of southwest Montana.

Next Weekend and Beyond...Looking ahead to next weekend and into mid-July, forecast models indicate a strong ridge of high pressure currently over the Desert Southwest will expand northward. While the exact placement of the high pressure remains uncertain, the prevailing trend strongly favors a shift toward well above normal temperatures. As this prolonged heat builds late in the week and through the weekend, the risk for heat related illnesses will increase. This will be especially true for vulnerable populations and those recreating outdoors without adequate hydration and shade. Forecast models also suggest this high-pressure setup could periodically draw moisture up from the southwest (beginning around the 13/14th) to trigger daily thunderstorms.

AVIATION

Predominantly VFR conditions will prevail across western Montana and north-central Idaho through Monday. The primary aviation concern today will be isolated afternoon and evening thunderstorms tracking into southwest Montana and Lemhi County, Idaho.

Impacts will be localized to the KSMN and KBTM terminals: * KSMN: VCTS/TSRA potential from 05/21z to 06/01z. * KBTM: VCTS/TSRA potential from 05/22z to 06/02z.

Due to a deeply dry lower atmosphere, these high-based storms will carry a 30 percent chance of generating erratic outflow winds gusting over 30 knots and a 5-15% probability of 50+ knots. Elsewhere, dry and stable conditions will maintain high-confidence VFR flight categories with no thunderstorm impacts expected.

Mid-level clouds and light showers (mostly virga) will increase on Monday morning as sub-tropical moisture moves overhead. Forcing from an advancing shortwave and modest instability should be enough for scattered showers and thunderstorms on Monday afternoon, especially from I-90 southward. Brief heavy rain, lightning, and gusty outflow to 35 kts are the main hazards.

MSO WATCHES/WARNINGS/ADVISORIES

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