Severe Thunderstorm Warning 9, NWS Boise, ID

Area Forecast Discussion
National Weather Service Boise ID
901 AM MDT Thu Apr 30 2020

.DISCUSSION...Atmosphere is already unstable as seen by early
morning thunderstorms near McCall and Sun Valley.  Radar at 8 AM
MDT showed scattered showers in Idaho mainly east of McCall -
Mountain Home - Owyhee/NV.  No echoes is eastern Oregon this
morning.  Weather should stay quiet until midday.  Pacific cold
front will move across eastern Oregon around noon PDT, i.e., a
little later than previous forecast.  Thunderstorms will develop
along the front as it moves east into the more unstable air over
Idaho this afternoon.  Thunderstorms will become strong in western
Idaho near the front especially between about 2 PM and 5 PM MDT.
SPC again has our area in a marginal risk of severe thunderstorms.
The area now covers all of southern Idaho, western Wyoming,
southwest Montana, and a small part of Malheur County/OR, i.e.,
a little east of where SPC had it yesterday.  Again, this is a
marginal risk, but even that doesn't occur often in our area.
Front will exit east early this evening and showers and storms
will end.  Patchy fog toward morning then dry Friday.  No updates.

&&

.AVIATION...VFR. Showers and thunderstorms will become more
widespread this afternoon. Periods of MVFR in stronger thunderstorms
along with small hail this afternoon. Showers tapering off this
evening. Surface winds: generally northwest 10-20 kt with gusts 20-
30 kt, except gusts to 35-50 kt possible near thunderstorms. Winds
aloft at 10k ft MSL west to southwest 25-40 kt.

Weekend Outlook...Showers and thunderstorms through the weekend.
Breezy northwest winds along with a threat of gusty outflow
winds from thunderstorms.

&&

,PREV DISCUSSION...
SHORT TERM...Today through Friday night...The atmosphere will
remain somewhat unstable through the morning, allowing for
continued showers and isolated thunderstorm activity. A cold front
will pass through the region today, across SE Oregon mid-morning
and entering SW Idaho around noon. Supported by steep lapse rates
and cold temperatures aloft, this feature will cause rapid
destabilization and provide enough lift for thunderstorm
redevelopment across the region this afternoon. The highest
instability will set up over southern Idaho, in which storms will
form over higher elevations and propagate eastward. Models are
consistently hinting at strong storms initiating over the Owyhees
and moving into the Snake River Plain, though exact locations and
timing remain uncertain. Ample shear will allow for long-track
storms capable of producing small hail and outflows up to 50-55
mph. The SPC has issued a marginal risk for severe thunderstorms
across SW Idaho and may increase the risk as more data comes in
this morning. Showers and thunderstorms will dissipate in the
evening while clearing will allow for cooling and patchy valley
fog. A chance for showers over higher elevations remains for
Friday afternoon, though some showers across higher elevations
will linger through Friday.

LONG TERM...Saturday through Thursday...Models continue to be in
good overall agreement through the extended. The next strong cold
front will enter the Pacific Northwest Saturday morning and
move across the forecast area through Sunday. Breezy conditions
are expected ahead of the cold front along with a threat of
showers and thunderstorms over a majority of the region. A
secondary wave will push through Sunday and Monday,  but little
in the way of instability. Therefore removed the thunderstorm
threat for that time period but showers are expected.  That
system will kick eastward Monday, but a threat of lingering
showers will continue across the eastern portions of Idaho's
central mountains. Well above normal temperatures expected
Saturday then dropping below normal Sunday and Monday. The upper
ridge will rebuild for the middle of next week and bring well
above normal temperatures to all zones.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...None.
OR...None.

&&

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DISCUSSION...LC
AVIATION.....ST
PREV SHORT TERM...AL
PREV LONG TERM....CR