Flood in Idaho, May 12 to 22, 2018

Area Forecast Discussion
National Weather Service Pocatello ID
233 PM MDT Fri May 11 2018

.SHORT TERM...Tonight through Monday. Center of closed low parked
over Southwest Idaho this afternoon per satellite imagery. General
track of the low through Monday is to basically wobble around the
triple point of Idaho/Utah/Nevada. This slow wobble keeps East
Idaho under a wet and cool regime through the entire short term
period. Showers and an occasional afternoon thunderstorm will be
the rule. Despite periods of moderate QPF, snow levels remain high
enough to keep any accumulation impacts limited to back-country
users. Will hold off on headlines but highlight potential for
trouble for recreationists with a Special Weather Statement.
Overall QPF amounts through Saturday night still appear to run in
the 0.50-1.00 inch range MOST areas. The exception could be the
region from Stanley to Challis, far enough away from the upper low
to escape the heaviest rainfall.

.LONG TERM...Monday night through Friday night. As mentioned
previously, all model guidance remains in stellar agreement through
the extended period, lending above average confidence in the
forecast. Typical uncertainty will be in place regarding placement
and timing of heavier Precipitation due to the convective nature of
Precipitation. General theme will feature a weakening upper low over
NE NV/South Central Idaho/NW Utah, which loses its identity by
Wednesday. Meanwhile, heights will gradually build across the Great
Basin, resulting in a gradual warming trend through week's-end.  A
new Trough/Closed Low is then expected to form off of the CA coast
and wobble inland by late-week. The end result of all of this will
be continued Shower/Thunderstorm chances through the week, with
Mountain areas favored. Best Precipitation chances are Tue with the
Upper Low lingering in the area, and again by Fri with the
appearance of an approaching new Low. AD/DMH

&&

.AVIATION...Area of Low Pressure will take up residence over South-
Central ID/NE Nevada/NW Utah through the period, ensuring plenty of
cloud cover and Precipitation chances. The Devil will be in the
details. The bulk of the Precipitation through the remainder of the
Afternoon/early Evening will be confined to the Mountains, with
MVFR/VFR Stratus for BYI, IDA and PIH. DIJ will see the greatest
chance of CIGs falling to MVFR (and possibly IFR) by this Evening
due the emergence of Showers and northerly upslope flow in the Teton
Valley. After this Evening, forecast confidence diminishes due to
some uncertainty in placement of heavier Rain. Used a blend of high-
resolution model guidance to create the TAFs. This blend resulted in
the placement of a light/moderate Rain band for BYI, IDA, PIH and
SUN by 12Z Sat. Moderate potential for IFR CIGs and some reduced VIS
as a result. Expect some improving conditions after 18Z Sat most
areas, with CIGs becoming more variable thereafter with the
potential for the development of isolated to scattered TS with
embedded heavier Rainfall. Expect winds to veer N to NE most areas
as the surface low takes up residence just to our South. AD/DMH

&&

.HYDROLOGY...Wet and cool weekend beginning for East Idaho. Cooler
temperatures, especially at night at higher elevations, will
moderate the ongoing spring snow melt. However, bands of
precipitation will rotate through the region, and the
precipitationmay be moderate to heavy at times. Expect continued
rises on rivers, creeks and streams through the region. The
Henry's Fork at Rexburg is expected to reach flood stage late this
weekend, but heavier precipitation amounts could push that
timeline sooner. The Snake River, Wood River and Portneuf River
are all at bank full stages, but at this time are expected to
remain that way with minor, if any, impact. The takeaway should be
that the rivers all remain high and fast, and will be hazardous
even if not in flood. DMH

&&

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