textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Summer-like conditions through Thursday with highs in the 80s, 90s in lower valleys of north-central Idaho.

- Showers and thunderstorms increase in coverage each day through Friday.

- Dramatic shift to colder and wet weather this weekend, as temperatures fall 20-30 degrees.

- Significant hypothermia risk across backcountry and high elevation areas with "Winter-like" conditions and snow down to 6000-7000 feet.

The Northern Rockies will be under the influence of a westerly flow pattern aloft the next few days. Pacific disturbances will move through the region, with large scale ascent bringing an increase in shower and thunderstorm activity. Forecast soundings and surface observations suggest the highest instability by this afternoon and evening across far northwest Montana and across Lemhi County into southwest Montana. Lightning and gusty outflow winds are the primary storm threats given moisture and instability centered aloft and dry air in the low levels. Furthermore, high- resolution models suggest a tightening pressure gradient along the divide by sunset, leading to higher confidence for easterly winds to develop on west facing slopes of mountain ranges along the divide. This happened yesterday evening as well, with easterly wind gusts of 20-25 mph observed along the east shore of Flathead Lake. This evenings pressure gradient appears stronger, bringing the risk for gusts of 30-35 mph on Flathead Lake, Swan Lake and in the Many Lakes region within Flathead County.

Stronger thunderstorms enter the picture late Thursday morning through the afternoon as atmospheric instability increases (CAPE of 750-1000 J/kg). A Pacific disturbance will focus activity across northwest Montana (I-90 northward) and along the Continental Divide. These storms will be capable of brief heavy rain, small hail (up to 0.50 in diameter), gusty winds, and lightning.

A significant change remains on track Friday through the weekend as a closed low drops southeastward from the Gulf of Alaska across the Northern Rockies. Confidence is high for temperatures to drop 20-30 degrees by Saturday and Sunday, with falling snow levels near 6,000-7,000 feet. Those heading into the backcountry and higher elevations should prepare for winter-like conditions. Cold rain, and or wet snow, will bring an elevated risk for hypothermia. Furthermore, the weight of heavy wet snow may lead to downed branches, trees, with prolonged rain bringing a higher risk for rockfall across backcountry roadways. This system will bring widespread rain, with valley areas seeing generally 0.25-1.25 inches of precipitation. Higher elevations, especially along the divide and in Glacier Park may see higher totals, with high-end (90th percentile) NBM guidance suggesting up to 3 inches of precipitation. Precipitation forecasts remain difficult to pin down given variability with mesoscale banding features along the N-NW track of the upper level low. Nonetheless, the main message remains the same, those with outdoor plans should prepare for a significant weather change this weekend!

AVIATION

There will be two areas where shower or thunderstorms could form this afternoon and evening: Far northwest Montana and also southern Lemhi County. The thunderstorms will have limited instability, but impacts will be lightning and gusty winds to 30 knots. An easterly wind gradient tightens around 25/0300Z(9 PM MDT) across western Montana and then wanes after midnight. KMSO has a 40% chance for wind gusts exceeding 25 knots between 9 PM this evening and 2 AM MDT Thursday. An upper wave will trigger showers and a few thunderstorms capable of wind gusts near 25 knots later tonight, possibly impacting KGPI and KBTM. More showers and thunderstorms will develop by 25/1700Z(11 AM MDT) which could be more widespread lasting into the afternoon hours Thursday. Storms could produce lightning, small hail, gusty winds and brief heavy rain.

MSO WATCHES/WARNINGS/ADVISORIES

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