Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 182000

Area Forecast Discussion
National Weather Service Pocatello ID
200 PM MDT Tue Apr 18 2017

.SHORT TERM...Tonight through Thursday night. Satellite
imagery was showing a fairly vigorous wave ejecting NE through
California from the main low positioned off the NW coast. Numerical
models indicate that as this wave lifts NE through Nevada this
evening, a band of enhanced showers will develop across the SE
Highlands (including the Pocatello Area) as an upper jet couplet
passes through the region. Although the models were showing fairly
stable conditions, I wouldn`t be surprised to see a line of strong
thunderstorms develop under the influence of the upper jet forcing.
Looks like we see a short break late tonight through Wednesday
morning with some weak instability showers developing in the
afternoon favoring the mountains. The models continue to show the
remains of the upper low ejecting inland Thursday as a negatively
tilted trough.  Some fairly decent jet forcing will accompany the
trough passage but for now, the models appear to be lacking
sufficient instability to support severe thunderstorms. We will
watch to see if this changes over the next several model runs. Huston

.LONG TERM...Friday through Sunday night. A trough of low pressure
is forecast to be positioned from western MT south to western WY
Thursday night. Cyclonic flow and increased moisture associated with
this system will allow for a continuation of showers extending from
the Continental Divide region southward along the WY border. This
activity should continue into Friday morning before gradually
diminishing by Friday evening. Snow levels should be low enough to
allow for light snow at pass level. A ridge of high pressure is then
slated to move into the region this weekend, promoting dry
conditions and a gradual warmup. By Sunday, temperatures should be
at or just above climatological normals. By late Sunday, the next
trough approaching from the west will promote the development of
showers across the Sawtooths and extending eastward along the Divide
and WY border region. Higher snow levels by this time should allow
for snow to fall mainly above passes, with perhaps the exception
being Galena Summit. Anticipate breezy conditions from Sunday
afternoon into Monday morning which may create some difficult travel
as well. AD/MH

Monday through next Tuesday night. Low amplitude longwave ridge over
Oregon-Washington will be easily overcome by a shortwave moving
through Mon night or Tue, depending on whether one believes the the
ECMWF or GFS, respectively. Snow levels could get as low as 6000
feet. Wind with the front should be at least windy (20 to 30 mph
sustained), possibly stronger. The ECMWF brings in a second, weaker
shortwave before the end of this extended period, while the GFS keep
the original trough over eastern Idaho through Tue night; either
way, it will be wet for the end of the extended period. Messick

.AVIATION...Showers and thunderstorms still expected as clouds thin
out and surface heating powers the convection for the afternoon and
evening hours, although more stable air moving in during the early
evening to put an end to the thunderstorms. Potential is for
marginal VFR in rainshowers for the late afternoon. NAM guidance
indicating that with the amount of rainfall that fog and stratus
should be a problem at KDIJ and KIDA, possibly KPIH as well. With
all the upward vertical motion expected, have replace reduced VSBY
due to fog with similar CIG conditions. Not a lot of confidence in
the morning fog/stratus situation, but it at least it seems
reasonable at this time. Messick

.HYDROLOGY...Today`s precipitation coupled with another round
Thursday and warmer temperatures this weekend should support
forecast rises on rivers through the latter half of the week and
into the weekend. The Portneuf at Pocatello will remain in flood
stage throughout the week possibly reaching moderate flood stage by
Wednesday morning. The Snake at Blackfoot has fallen below flood
stage and will likely remain high but below flood stage throughout
the remainder of the week. The Bear river at the Wyoming border is
forecast to remain below the official flood stage of 8.6 feet
through the week but we did notice issues near Dingle and Pegram
when the stage was at 7.38ft back on March 20th and it looks as if
by Thursday afternoon we may be running near that level again per
the latest river forecast. Huston


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