River Flood Warning 16, NWS Pocatello, ID

Area Forecast Discussion
National Weather Service Pocatello ID
310 AM MDT Wed May 30 2018

.SHORT TERM...Through Friday. Primary focus through the short-term
forecast period is on Thunderstorm potential Today and Thursday.
An elongated Trough of Low Pressure will stretch NE-SW from the SW
Canadian Provinces through the ID Panhandle SW into northern CA.
This Trough will strengthen on Thursday while edging closer to our
area. This trough and associated Jet dynamics will provide the
lift to aid in Thunderstorm development. Available moisture is
adequate both days. Instability appears better Today compared to
Thursday, although Instability on Thursday is adequate. Wind Shear
parameters are marginal today but strong on Thursday. The bottom
line is: Strong to Severe Thunderstorms are expected this
Afternoon, maximized in the 3PM - 9PM period -- and again on
Thursday for roughly the same time period. Some Convective
Allowing Models suggest Thursday's activity may be even more
robust than Today's given the stronger forcing for ascent and
more favorable Wind Shear. Nonetheless, primary threats both days
will come in the form of potentially damaging Wind gusts and
Large Hail. Locally Heavy Rain will be possible as well, but
faster storm motions than what we've experienced of late will
limit total Rainfall and associated Flooding potential. While
Severe Thunderstorms will be possible anywhere across the Forecast
Area both days, the greatest potential will lie across the Snake
Plain northward into Southwest Montana both Today and Thursday,
with the Thunderstorm threat gradually lessening as one heads
South and East into far Southeast Idaho.

The Severe Thunderstorm threat will gradually wane through
Thursday Evening. The above-mentioned Trough will pass through SE
Idaho on Friday, ensuring a continuation of Shower and
Thunderstorm activity -- with the greatest chances shifting
eastward along the Wyoming border. A threat of Small Hail and
local gusty Winds exists on Friday as well, although the Severe
threat is minimal.

.LONG TERM...Saturday through next Wednesday.
A drying and warming trend is forecast to build into the area on
Saturday. A wave approaching from the Northwest is then forecast
to re-ignite Shower and Thunderstorm activity beginning on or
around next Sunday. Models begin to diverge significantly with
the strength of this approaching wave, with the ECMWF much
stronger than the GFS. Have taken a blended approach, adding
Shower and Thunderstorm chances mainly in the Central Mountains
eastward along the Montana border Sunday-Monday. Have added a more
broad mention of Shower and Thunderstorms Tuesday-Wednesday to
cover growing uncertainty on expected Precipitation placement.
AD/DMH

&&

.AVIATION...A few showers already developing mainly across the
western portions of the state this morning, encroaching into the
central mountains. Would not be surprised to see TS at KSUN by
sunrise, with remaining terminals at least under VCTS threat after
15Z. Conditions worsen through the day as threat for TSRAGR
increases along with 50kt or greater outflows, most likely 18Z-02Z.
For this morning terminal issuance, will maintain -SHRA/VCTS mention
for the day, allowing for amendments to fine tune as convective
development ensues. DMH

&&

.FIRE WEATHER...Thunderstorms will be the expectation for today and
Thursday while the region remains under the influence of an unstable
atmosphere ahead of a cold front. Some of these storms may be severe
with strong outflow winds and hail greater than one inch in
diameter. The cold front is still expected to stall over the region,
allowing more thunderstorms to form on Thursday. The storm system
shifts east Friday, but additional storms might be possible along
the WY border. Drier conditions are expected for the weekend with
passage of a weak ridge of high pressure. DMH

&&

.HYDROLOGY...The Snake River near Idaho Falls will continue to
flirt with flood stage through Today. Otherwise, flooding from
rain and snowmelt will continue for the next several days along
the Henry's Fork near Rexburg and along the Snake near Menan.
Flows have receded some along the upper Big Lost and the flood
warning has been cancelled. However, flooding continues downstream
of Mackay Reservoir from releases and along Antelope Creek near
Darlington. The Big Wood has receded some, a flood warning
continues there. The forecast is for the Big Wood to recede below
flood stage. May downgrade the warning to an advisory on
Wednesday. However, additional rainfall from thunderstorms is
expected which could be significant. So, there's a lot of
uncertainty in the forecasts for the Big Wood and to a lesser
extent the Big Lost.

Valle/AD

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
None.
&&