textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- A surge of upper level moisture into the region is causing cloud cover, virga, and increasing humidity across the region today.

- Thunderstorms this afternoon and evening are forecast for southwest Montana including southern Powell, Granite, Deer Lodge, and Silver Bow counties.

- Thunderstorms are in the forecast through the week for much of western Montana and portions of central Idaho. Storms will continue in southwest Montana through the weekend.

Satellite imagery early this morning depicts a broad ridge of high pressure over the Midwest and low pressure spinning over the Pacific off the coast of the US-Canada border. This configuration puts the Northern Rockies under southwest flow aloft. The area of high pressure to the southeast is drawing monsoonal moisture up from the Gulf of California. The moisture feed will bring precipitable water values upwards of an inch to the Northern Rockies this afternoon. With a lack of large-scale forcing for ascent, this will mean widespread increasing clouds and higher relative humidity. One exception will be a weak disturbance over northwest Montana bringing rain later this morning and this afternoon. Another exception will be thunderstorm formation this afternoon and evening, primarily over southwest Montana and Lemhi County. Model soundings still show a dry lower atmosphere despite the abundance of upper level moisture, which will be a good recipe for strong outflow winds associated with any storms that do develop. The trend continues on Wednesday, as precipitable water stays up to around 150 percent of average for this time of year. The Pacific low moves south and becomes positioned off the coast of the Pacific Northwest, strengthening southwesterly flow. High precipitable water and wind shear will be available for developing afternoon storms over southwest Montana and Lemhi County. However, daytime cloud cover should keep instability, a key ingredient for severe storms, at bay.

This general pattern persists into next week. The details will determine which areas see thunderstorms each day, with southwest Montana being the most consistent area. But overall, the general southwest flow pattern with thunderstorms remains through early next week.

AVIATION

Today will bring a variety of weather factors for aviators to consider when flying in the Northern Rockies. A shortwave disturbance will move through northwest Montana later this morning, bringing rain showers to KGPI starting around 14/1600Z. The cooler showers over the northwest Montana will set up a north-south pressure gradient, resulting in strong northerly winds at KGPI, through the Mission Valley, KMSO, and possibly as far south as the Bitterroot Valley including KHRF. Finally, expect convective storms affecting KBTM and KSMN this afternoon. Threats from thunderstorms will be heavy rain reducing visibility, gusty, erratic outflow winds, small hail, and lightning. There will be about a 5-10% chance storms will become severe and produce large hail (one inch or greater) or gusts of about 55 knots or greater.

MSO WATCHES/WARNINGS/ADVISORIES

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