textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Strong to severe thunderstorms afternoon hours and evening, bringing the 5% threat of strong winds, large hail, and heavy rain from north-central Idaho into southwest Montana.

- Elevated fire weather this weekend from hot, dry, and breezy conditions, especially in northwest Montana.

- Another surge of monsoon moisture expected to arrive by Tuesday. More clouds and/or thunderstorm chances to increase. We may experience further moisture increase from remnants of East Pacific Hurricane Elida by Wednesday or Thursday.

Water vapor imagery shows a nice area of monsoon moisture embedded in southwest flow over eastern Oregon into southern Idaho. This moisture interacted with the very warm air mass that was in place and triggered morning thunderstorms. Their movement has been rather fast, generally 30 to 35 MPH to the northeast which can infer that there is a decent mid-level jet streak associated with the moisture. Current and forecast effective shear depicts 40 to 50 knots could be possible over Idaho County and west- central/southwest Montana into this afternoon. Current conditions over Idaho County would suggest that there could be a couple of supercell/severe thunderstorms that could form, capable of producing large hail or damaging winds. Current boundary layer moisture levels are being underdone by the high resolution models which could be part of the reason they are not resolving that many storms.

Further to the east, the RAP is showing a bulls-eye of 1000 to 1500 Joules/kg of mixed layer CAPE/instability over the Bitterroot Valley and points eastwards. This is the region where we have higher confidence for storm threats.

One caveat to today's storms is the fact that there is a dry layer found over northwest Montana towards central Idaho. This is depicted by looking at the 24-hour relative humidity change on our weather and hazards data viewer. RH's are generally 10 to 30 percent lower from a line from Dickey Lake south and west towards Grangeville. This dry air may shift further east with time today as the westerly wind gradient tightens a bit. With that said, a few isolated showers or thunderstorms could initiate off the mountains east of this line (i.e. Blacktail Mountain, Mission Mountains and points south). If they were to develop, they could become very strong, very quickly and produce hail and gusty outflows.

The stronger storms could also produce torrential rainfall. The precipitable water values(PWAT, measurement of how much moisture is in the atmosphere) are generally around 0.90" which is +2 standard deviations for this time of year. Yesterday one of the storms produced 0.54" in 15 minutes, 0.86" in 30 minutes just southwest of Clearwater Junction.

For Sunday, there is a bit more drying across the region, but the southern Idaho County to southwest Montana could see showers and isolated thunderstorms develop. Working against the shower development could be modest warming occurring aloft.

On Monday, the upper ridge builds more over the western U.S. As it does this, it will also continue to draw up monsoon moisture. Some of this could mainly be high-level cloud cover, while other locations towards Lemhi County could get enough heating to help bring the "Cap" or capping inversion and allow storms to fire off.

On Tuesday, north to east winds may develop. The latest RRFS smoke model shows some of the Canadian smoke moving into the Glacier Region. The upper level ridge axis may be present from Colorado north and west to Spokane. Usually in this pattern, the elevated warmth will suppress or prevent showers from growing very well (more stable). But with PWATs approaching 1.00", all it could take is a weak push of mid-level monsoon moisture and associated shortwave or mountain initiation to fire off a few storms.

Models depict that the remnants of Hurricane Elida may track northwards, west of California by Tuesday. This could bring even more subtropical moisture into the Northern Rockies. How this evolves on Wednesday in terms of showers or thunderstorms will depend on timing of and magnitude of cloud cover.

AVIATION

Thunderstorms are expected again today, mainly in southwest Montana affecting terminals KBTM, KSMN, and KHRF. The HREF ensemble model has significant convective initiation starting starting between 18/19Z and 18/20Z this afternoon between terminals KSMN and KHRF, and all HREF members show storms out of the area by 19/04Z. Heavy rain and hail have been the main impacts from the storms in this air mass the last few days, and that's what we expect again today as well as wind gusts around 40 mph. Models are trending downward on thunderstorm potential for Sunday the 19th.

MSO WATCHES/WARNINGS/ADVISORIES

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