Tornadoes and Flash Flood in Idaho, May 31, 2018

Area Forecast Discussion
National Weather Service Pocatello ID
259 AM MDT Thu May 31 2018

.DISCUSSION...Through Friday night. Primary focus continues to be
placed squarely on Thunderstorm potential and associated impacts
today. Here's a look at the ingredients: A trough of Low Pressure
is forecast to move ashore across the Western U.S. this Morning,
advancing eastward towards Idaho while continuing to
simultaneously strengthen. A strengthening Jet core is forecast to
set up shop over Western MT and the Idaho Panhandle this
afternoon while a secondary jet max develops over southern Nevada.
This will provide the necessary background large-scale forcing.
Other ingredients necessary for organized Thunderstorm activity
are also forecast to be present, including favorable Wind Shear,
mid-level Lapse Rates and instability. Not to mention a surface
boundary is forecast to set up in the vicinity of the I-15
corridor this afternoon, acting to serve as a focus for
Thunderstorm development.

In addition, Precipitable Water Values
will be nearly 2x climatology for the date -- indicative of
plentiful atmospheric moisture. This moisture will allow for
locally heavy Rainfall with some storms. Zippy mid-level flow will
promote relatively fast-moving storms, acting to limit overall
Precipitation. However, should training storms develop, some
locally impactful Heavy Rainfall will be possible. All of the
above point to a potentially robust round of Thunderstorms This
Afternoon and early evening, with Large Hail and Damaging Wind
gusts the primary threats. Anticipate robust Thunderstorm
development to begin taking place shortly after 12 PM, continuing
through around 9PM or so. Most favored corridor for Strong/Severe
storms will be the Arco Desert and Interstate corridor from
Southwest MT to the UT/NV border. This includes SE Idaho's
population centers. Given such favorable support for
Strong/Severe Storms, anticipate stronger, more organized activity
compared to Wednesday. However, one limiting factor is Showers
and associated cloud cover currently located over Central NV. This
activity will swing through SE Idaho this Morning, limiting
surface heating potential. Thus, certainty in the scope and
intensity of Thunderstorms is limited a bit as a result. Should
this activity weaken before Noon and allow for robust surface
heating, explosive Thunderstorm development will be possible.
Continue to monitor trends through the Morning.

The Low Pressure Trough/Cold Front will sweep across the area on
Friday. This will allow for the development of Showers and
Isolated Thunderstorms in the vicinity of the MT and WY border
region Friday and Friday evening. Friday will feature Temperatures
dropping some 10 degrees off of Thursday's highs. In fact, should
favorable clearing and light Winds develop, patchy Frost will be
possible across portions of the Upper Snake Highlands and Northern
Snake Plain both Saturday and Sunday mornings. Will monitor this
potential. Expect warming and drying conditions to envelop the
region beginning on Sunday. By early week, the region will reside
sandwiched between a building High Pressure ridge across the Texas
High Country and persistent Troughs ejecting across the PacNW.
This will maintain SW flow across our region along with periodic
Shower and Thunderstorm chances, especially during the
Afternoon/early Evening and across higher terrain areas. This
could be the early signs of a typical Summer-like pattern
beginning to take shape. AD/DMH

&&

.AVIATION...Convection continues to drive terminal forecasts today
and tonight. Showers linger in the SE corner of the state, and
portions of the central mountains but so far terminals are not
affected. Short range models showing similar solutions so will lean
on timing of precipitation with consensus from these especially
through 00Z. Atmosphere supportive of strong storms yet again.
Strongest storms most likely between 20Z-02Z today, especially
focused along the frontal boundary as it moves through the Snake
Plain this afternoon. DMH

&&

.FIRE WEATHER...Another round of thunderstorms expected today, which
could become strong once again this afternoon with strong wind gusts
and hail. Strongest storms likely along the frontal boundary this
afternoon. Precipitation expected to linger tonight and restrengthen
Friday in areas generally along the Divide and WY border. Drier
conditions are still forecast into the weekend as a weak upper ridge
moves across the region. The next system to impact East Idaho
arrives on Monday, though there is some indication that showers
could move into the central mountains as early as Sunday night.
DMH

&&

.HYDROLOGY...High flows and some flooding is still occurring  on
some area rivers and streams. The Snake River near Idaho Falls has
dropped below minor flood stage, so have dropped the flood warning.
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 should drop below minor flood stage by
Saturday/Sunday, but will be highly dependent on the amount of rain
that occurs the next several days. The Henrys Fork near Rexburg and
the Snake near Menan may stay in flood until the middle of next
week. Flows have receded some along the upper Big Lost near Howell.
The Big Lost near Howell has dropped just below minor flood stage
but could rise again tonight back to minor flood stage, depending on
rain amounts. Still have a flood advisory going for the Big Lost
near Howell for minor flooding impacts. Farther downstream, overall
minor flooding continues below the Mackay Reservoir from releases as
well as along Antelope Creek near Darlington. Witnessed a spike in
flow yesterday evening just downstream of Mackay Dam and attendant
increases downstream at Leslie. Will need to confer with area
Dam/Water and Emergency managers to confirm. The Big Wood has
receded some, slightly below minor flood stage. A flood warning
continues for the Big Wood near Hailey. The forecast for the Big
Wood is showing the possibility of going back up over minor flood
stage again the next couple of days with current rain forecast.
Again, future river rises will be highly dependent of rain amounts
the couple of days. Rain amounts will be highly dependent of
thunderstorm tracks, some areas will see light to moderate
rainfall and others receive moderate to heavy, localized,
rainfall. Wyatt/AD

&&

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