textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Ridge of high pressure and abnormally hot temperatures in control through end of the week peaking Friday.

- Elevated convection spreading northward Friday afternoon into the overnight bringing localized lightning and gusty outflow winds.

- Showers and thunderstorms with heavy rain potential for Saturday.

Surface conditions this morning show localized increases in surface wildfire smoke across western Montana, particularly in west-central Montana near Missoula where surface visibility has dropped and air quality is noticeably poor. Overnight maximum relative humidity recoveries are very poor with teens and 20 percent RH observed on mid and upper elevation slopes across the CWA. Northwest Montana has seen slightly better recoveries with most locations moistening into at least the 40 percent RH range.

High pressure over the Northern Rockies will maintain hot and dry conditions through the end of the week. The hottest day remains expected on Friday with high temperatures in the 90s for many valley locations. Triple-digit heat and limited overnight cooling relief remains expected for the lower river valleys of north- central Idaho. While nocturnal inversions enhance the entrapment of surface smoke in valley locations, daytime thermal mixing will only improve surface air quality and visibility if cleaner air exists aloft. Where elevated smoke plumes persist, mixing will only continue to draw more smoke down to the surface, maintaining degraded visibility and air quality.

Monsoonal moisture advection begins Friday afternoon in Lemhi County and southwest Montana triggering high-based widely-scattered showers and thunderstorms capable of strong convective outflow winds (gusts 30-45 mph). This elevated convection risk spreads further northward through Friday evening into the overnight and Saturday morning increasing the areal threat for isolated dry- lightning and gusty outflow potential. A weak, shallow surface front originating from east of the Continental Divide will enhance an easterly pressure gradient and resulting winds late Friday into Saturday morning most noticeable in the usually-prone east- west canyons.

An incoming Pacific Northwest upper-level trough, west of the region, lifts northeastward on Saturday phasing with a significant monsoonal moisture surge. Precipitable Water values exceeding 1.0 inches will support increasing coverage of showers and thunderstorms capable of heavy rainfall rates and localized concerns for excessive runoff and flash flooding. A cold front crosses through from the west by Saturday evening into Sunday morning ending precipitation and convection chances for most locations and ushering in a much drier airmass. Daytime high temperatures will fall back to near seasonal norms (still quite warm) for Sunday with widespread elevated westerly winds persisting.

AVIATION

Wildfire smoke has noticeably increased in density across western Montana (particularly near KMSO) this morning reducing visibility to near 6SM. Daytime mixing may improve visibility pending upstream smoke plume evolution through the afternoon hours. Hot temperatures will peak Friday afternoon maximizing density altitudes across the region. Elevated convection Friday afternoon near KSMN & KBTM will spread northward through the evening and overnight bringing the threat for localized lightning and temporary erratic outflow winds to 25-40 knots.

MSO WATCHES/WARNINGS/ADVISORIES

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