textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Daily showers/thunderstorm chances continue this week.

- Gusty winds 40 to 50 mph and hail are possible with thunderstorms, isolated severe threat each day.

- Warmer latter half of this week, then return to seasonable temperatures this weekend.

An active convective pattern is iminent through the rest of this week as an upper-level low meanders through California and the Great Basin, allowing deep southerly flow to transport moisture across the Northern Rockies.

As of early afternoon, a weak cold front into north-central Idaho and considerable mid- level clouds across west-central Montana have limited instability. Showers are developing from lift on the top side of the low and significant warming has occurred across northwest Montana with areas of partial clearing which should help with thunderstorm initiation. Shower/thunderstorm activity will shift from south to north into this evening. Instability and shear may be sufficient to support a few strong- to severe thunderstorms through mid- evening, with wind gusts exceeding 50 mph and hail as the primary threats. High- resolution guidance highlights the greatest potential across northwest Montana. Brief, locally heavy rainfall is also a threat; high- resolution models show pockets of 20-30% probability for hourly rates exceeding 0.5 inches, especially in northwest Montana. This introduces an isolated risk for flash flooding and debris flows over vulnerable basins, including recent burn scars.

Wednesday through Friday, the overall stagnant pattern yields little day-to-day change, keeping daily afternoon showers and thunderstorms in the forecast. Model trends suggest increasing instability but weaker deep-layer shear on Wednesday and Thursday, with less shower/thunderstorm coverage overall. However, a localized severe weather threat will persist and will expand back into north-central Idaho. Daytime temperatures will warm to well above normal levels once again.

For the weekend, ensemble clusters are in good agreement that the upper low will lift northeastward on Friday, reaching the Yellowstone region by Saturday and maintaining daily shower and thunderstorm chances. A weak cold front on Friday will temporarily return temperatures to seasonal averages for the weekend before a building ridge early next week drives temperatures back above normal.

AVIATION

A weak cold front into north-central Idaho and considerable mid- level clouds across west-central Montana have limited instability as of early this afternoon. Showers are developing from widespread lift and significant warming has occurred across northwest Montana with areas of partial clearing, which should help with thunderstorm initiation. Shower/thunderstorm activity will shift from southeast to northwest into this evening. Ample instability and shear will support up to a 15% chance for both severe convective wind gusts exceeding 50 kt and hail larger than 1.00 inch mainly for northwest Montana (including KGPI). While the bulk of high- resolution model guidance favors more common outflow gusts in the 30-40 kt range, an isolated severe threat remains.

Wednesday will bring another round of afternoon and evening convection, though overall shower/storm coverage appears lower than today. Strong, erratic outflow winds and hail will remain the primary operational hazards near any mature cells.

MSO WATCHES/WARNINGS/ADVISORIES

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