textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Breezy and warm Fourth of July: Choppy conditions on area lakes

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

This afternoon and Sunday: This afternoon, a weather system passing to our north through British Columbia will flatten the ridge of high pressure overhead, leading to stronger westerly winds. Expect sustained winds of 10 to 20 mph with gusts reaching 20 to 25 mph. This will create choppy conditions on area lakes, so recreational boaters should exercise caution. Fortunately, winds will diminish by dusk, leaving mostly clear skies and calm conditions for evening fireworks and holiday festivities. Additionally, a few weak showers may attempt to develop over the mountains but will struggle to grow or produce any rainfall.

Sunday promises warm and mostly stable conditions for the majority of the Northern Rockies. The main exception will be across the southern parts of our region (specifically southwest Montana and Lemhi County, ID), where moisture slowly moving northward will introduce isolated afternoon thunderstorms. Watch for gusty outflow winds of 40 to 45 mph with any thunderstorms as forecast sounding show an inverted-V profile indicating very dry air in the lower levels of the atmosphere.

Monday and Tuesday: The most impactful weather of the week arrives Monday into Tuesday. Weather models show a strong surge of moisture pushing into the region, driving atmospheric moisture levels well above normal (precipitable water near to just over 1 inch) for early July. As an atmospheric disturbance moves through this moist air during the hottest part of the afternoon, it is expected to trigger organized showers and thunderstorms. Because of these high moisture levels, any storms that develop will be capable of producing heavy downpours, creating localized debris flows in flood prone areas. Additionally, there is a potential for a strong storm or two with gusty winds to 50 mph and small hail. One factor that could limit thunderstorm development is thick morning cloud cover, not expected at this point but something to watch for.

Wednesday through Late Week: Once Tuesday's system clears, the remainder of the week looks warm and relatively uneventful. We will return to a more typical summer pattern, with near-daily chances for isolated, pop-up afternoon thunderstorms largely confined to the mountains of southwest Montana. Breezy westerly winds will also be common each afternoon.

A trend continues toward a ridge of high pressure over the Desert Southwest expanding northward by next weekend, but computer models disagree on exactly where this high pressure will set up and how strong troughing to the west will be. Even hotter temperatures are favored by ensemble clusters if the ridging establishes nearby, meanwhile a minority of clusters show increased moisture and daily thunderstorms coming from the southwest.

AVIATION

VFR conditions will prevail across all Northern Rockies terminals through the weekend. The primary operational impact this afternoon will be an increase in westerly surface winds as a weather system passing through British Columbia flattens the upper-level ridge. Terminals can expect sustained westerly winds of 10 to 15 knots with afternoon gusts reaching 20 knots, which may generate localized mechanical turbulence for light aircraft along and near elevated terrain. These gusty winds will decouple and rapidly diminish by dusk, resulting in light, terrain-driven winds and mostly clear skies for the overnight hours. Breezy westerly winds will once again develop on Sunday afternoon.

A few weak showers may attempt to develop on the terrain this afternoon but weak instability should limit much vertical growth. There will be a slight increase in moisture and instability for Sunday afternoon leading to isolated showers and isolated thunderstorms across Lemhi County and Southwest Montana. Inverted-V soundings indicate dry low-level air and gusty outflow wind potential to 40 kts with stronger storms.

MSO WATCHES/WARNINGS/ADVISORIES

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