Winter Weather Advisory 39, NWS Missoula, MT

799 
WWUS45 KMSO 220548
WSWMSO

URGENT - WINTER WEATHER MESSAGE
National Weather Service Missoula MT
1148 PM MDT Fri May 21 2021

MTZ007-220700-
/O.EXP.KMSO.WW.Y.0039.000000T0000Z-210522T0600Z/
Butte/Blackfoot Region-
1148 PM MDT Fri May 21 2021

...WINTER WEATHER ADVISORY WILL EXPIRE AT MIDNIGHT MDT TONIGHT...


$$

Area Forecast Discussion...corrected
National Weather Service Missoula MT
257 PM MDT Fri May 21 2021

.DISCUSSION...A upper level closed low positioned across the
Great Basin continues to transport Pacific moisture northward
into the Northern Rockies this afternoon. An area of organized
shower activity has developed across southeast Idaho this
afternoon and continues to lift northward into southwest Montana.
Periods of light to moderate snow will continue to impact areas
along the Continental Divide in southwest Montana (Butte,
Anaconda, Georgetown Lake, etc.) through the early evening hours.
Snowfall accumulations will remain mainly confined to elevated and
grassy areas through this afternoon and early evening, with the
exception of shaded roadways or higher elevation passes which
could become snow covered. Those traveling should be prepared for
quickly changing road conditions and areas of poor visibility,
especially over the passes.

An area of freezing drizzle and light snow is possible along the
I-90 and I-15 corridor in the vicinity of Butte late tonight into
Saturday morning. Those traveling tonight in the vicinity of Butte
and across Homestake and MacDonald passes should be prepared for
winter driving conditions as untreated roadways freeze. Valley
rain and snow and mountain snow shower activity will develop
across Lemhi County, Idaho tonight into Friday morning.
Accumulating snowfall will likely impact travel across Gilmore
Summit in southeast Lemhi County.

The upper level low will begin to lift northward into southern Idaho
Saturday afternoon. Shower activity will remain widespread across
Lemhi County and north central Idaho Saturday. A few
thunderstorms are possible across Clearwater and Idaho counties
Saturday afternoon.

The low pressure circulation that brought cold, snowy, and wet
conditions to the Northern Rockies this week is expected to swirl
back over the region from the south on Sunday. A good portion of the
region will receive 0.50-1.00 inch of liquid precipitation Sunday
through Monday. Also Sunday through Monday, snow levels will drop to
near 5000 feet along the Continental Divide and create hazardous
winter driving conditions over the mountain passes. Precipitation in
mostly the form of rain at lower elevations may cause rocks to slide
on steep slopes.

Looking ahead to the middle of next week, temperatures will trend
back towards seasonal norms. Spring rain showers will continue.
Also, afternoon and evening thunderstorms are possible each day
Tuesday through the end of the week.

&&

.AVIATION...An area of organized shower activity has developed
across Lemhi County, Idaho and southwest Montana this afternoon.
Widespread precipitation will continue across these areas through
at least 22/0000Z. High confidence exists for KBTM to see ceiling
impacts through at least 21/1400Z. Pockets of freezing drizzle are
possible in Butte from 22/0600z to 22/1400z as ceilings trend
lower with upslope stratus clouds overnight. A second wave of
organized showers will move through Lemhi County, Idaho from
22/0600z to 22/1200z. A mix of rain and snow is likely across
Lemhi County valley locations as snow levels range between 4,000
and 5,000 feet MSL depending on precipitation intensity.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...None.
&&