Wind Advisory 5, NWS Missoula, MT

234 
WWUS75 KMSO 220331
NPWMSO

URGENT - WEATHER MESSAGE
National Weather Service Missoula MT
931 PM MDT Mon Aug 21 2023

IDZ009-010-MTZ007-220445-
/O.EXP.KMSO.WI.Y.0005.000000T0000Z-230822T0400Z/
Western Lemhi County-Eastern Lemhi County-Butte/Blackfoot Region-
931 PM MDT Mon Aug 21 2023

...WIND ADVISORY WILL EXPIRE AT 10 PM MDT THIS EVENING...

$$

AL

Area Forecast Discussion
National Weather Service Missoula MT
239 AM MDT Sun Aug 20 2023

.DISCUSSION...Hurricane Hilary remains the big story and the big
impact in the weather from Baja, Mexico all the way up to Montana.
Moisture associated with out flow from Hilary is running into a
ridge centered of the central U.S. and being forced north into
Idaho and Montana. Yesterday, thunderstorms associated with this
moisture plume dropped quarter to half an inch of rain in under an
hour across portions of Idaho and southwestern Montana.

These storms are expected to continue again today ahead of a
broader plume of moisture that will cause more steady rain across
the region. By Wednesday morning the storm total rain will amount
to about at quarter of an inch on the low end of the forecast to
over an inch on the high end. The hardest hit areas in
southwestern Idaho county like Riggins, Whitebird, and Grangeville
could possibly see over 2 inches, though the most likely forecast
is closer to 1 inch for those areas over the 3 day period. With
such heavy rain, flooding is a concern, especially in recent burn
scars where debris flows are possible with heavy rain rates.

Such high rain amounts are possible in August in this region with
thunderstorms, but they are rare and associated with
thunderstorms. So much rain over such a wide area in August is
exceedingly rare. Hilary will impact southern California as a
tropical storm, which last occurred 84 years ago, and we may also
expect the impacts felt in Montana to be similarly rare for
August.

After Hilary passes, models agree well on the ridge reasserting
itself over the region, so by next weekend we return to warm
weather. But after the soaking the region took, there will be a
lot of moisture left around in the soil still that can be recycled
and cause afternoon showers and thunderstorms.

&&

.AVIATION...High pressure east of the Continental Divide is
causing easterly winds and gap winds affecting terminal KGPI this
morning, but those are expected to wane throughout the day.
Moisture from Hurricane Hilary will reach central Idaho and
western Montana again today. Some thunderstorms at the leading
edge of the plume will give way to more steady, stratiform rain
overnight tonight. The leading thunderstorms are capable of
producing rain rates in excess of 1 inch per hour, which is heavy
rain. Most terminals other than KGPI will transition over to
stratiform rain by 21/03Z with lighter rain rates.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...ID...Flood Watch from 1 PM MDT /noon PDT/ this afternoon through
     Monday evening for Eastern Lemhi County...Lower Hells
     Canyon/Salmon River Region...Northern Clearwater
     Mountains...Orofino/Grangeville Region...Southern
     Clearwater Mountains...Western Lemhi County.

&&