Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 180840

Area Forecast Discussion
National Weather Service Pocatello ID
240 AM MDT Tue Jun 18 2019

.SHORT TERM...Today through Thursday evening...Influence from weak
low pressure to our northeast will exit our area today. Central
Idaho will have a differing weather forecast in comparison to
Eastern Idaho. Expect partly cloudy or mostly clear conditions in
Central Idaho throughout most of the day today with late afternoon
isolated thunderstorms across the Wood River Valley, Pahsimeroi
Valley and the mountains in between. Meanwhile, Eastern Idaho can
expect a partly to mostly cloudy day with rain showers and
isolated thunderstorms this afternoon and evening. PW`s in the
region are around 0.80 inches, which might be enough for areas of
heavy rainfall. Small hail and gusty winds are also possible with
these thunderstorms once again. A few thunderstorms may linger
into the overnight hours as well, so have kept the threat for
thunder in the forecast until midnight. The chances for
precipitation widely drop tomorrow. Broad low pressure over Canada
begins to push southwards tomorrow, increasing our pressure
gradient and allowing our wind speeds to increase pretty
substantially. Expect an increase in southwest and west winds by
mid-morning in the 15 to 25 mph range Wednesday and 25 to 35 mph
winds with stronger gusts the remainder of the daylight hours.
With these kind of conditions, a Wind Advisory is sure to be
issued as long as forecast trends persist. As we look at the
progression of the low, it will likely split into multiple low
pressure centers across the Pacific Northwest, progressing into
Montana/Idaho going into Thursday. With that solution, expect
another day of strong winds Thursday afternoon and evening,
therefore another day of a Wind Advisory. We may start to see
hints of precipitation back into the region over the Divide and
Central Idaho by Thursday as well. One last change in the forecast
on Thursday will be the 10+ degree temperature drop thanks to the
accompanying cold front alongside the low pressure system. NP

.LONG TERM...Friday through next Tuesday. We are still watching for
a deepening storm to drop across the state on Friday. The storm may
not be reaching as far as south as forecast 24 hours ago. This means
places like the Magic Valley and the South Hills/Albion Mountains
may see little to no chance of precipitation. The best chance will
be across the central mountains and eastern highlands, especially
farther north toward Driggs and Island Park. Temperatures and snow
levels are still expected to drop Friday. Snow levels in the morning
should be around 6500ft, but quickly rising to around 8000-8500ft by
afternoon. Afternoon temperatures will be running 10+ degrees below
average, with only 60s in the lower valleys and 40s/50s in the
mountains. If you have plans to head to places like Borah and
similar elevations, be ready for snow, wind and temps likely stuck
in the 30s! The low scoots east quickly on Saturday, with lingering
showers and storms though in similar places as Friday. We followed
the ECMWF and Blend of Models which spread showers farther south
toward the Utah border, although the trend is starting leave those
places drier each model run. We should see the flow switching to
westerly and eventually southwest next week, in response to another
low dropping into the Pacific Northwest. The GFS and ECMWF quickly
get out of sequence with that storm, as the GFS drops in along the
coast toward northern California by next Tuesday. The ECMWF keeps it
offshore of Washington and Oregon during the same period, and
therefore is drier. Often in these cases, we lean toward the Blend
of Models because it tries to give us the best of all worlds in
these types of scenarios. That is producing an increasingly wet
forecast with more widespread showers and thunderstorms.  Keyes


.AVIATION...For the most part, expect VFR weather across eastern
Idaho. However, with more showers and thunderstorms in the forecast
today...that means any particular TAF site could see something move
over or pop up on top of that particular airport. Expect at a
minimum, VCTS later today and evening for TAF sites EXCEPT BYI. Even
the higher resolution models want to keep it dry around there. For
the moment, we will follow that trend...but that could change
especially if outflow ends up cranking out some storms. Like
yesterday, small hail and locally heavy rain are the main risks.
Wind gusts could push above 25kts in a few cases. Keyes


.FIRE WEATHER...Primary forecast concern continues to be forecast
strong winds both Wednesday and Thursday. Winds today will be
breezy but less than 20 mph. Expect an increase in southwest and
west winds by mid-morning tomorrow in the 15 to 25 mph range
Wednesday and 25 to 35 mph winds with stronger gusts the remainder
of the daylight hours. Similar wind speeds are forecast for
Thursday. Tomorrow will also be relatively dry, with RH values
around 15 percent across portions of Central and South Idaho. Zone
425 is most impacted by these dry conditions with coverage not
widespread for the remaining zones. NP


.HYDROLOGY...The warm June temperatures along with light to
moderate rain has continued to melt the high-elevation snow,
keeping some rivers and streams running high. The three main
concerns continue to be the Big Wood River at Hailey, the Big Lost
River at Howell Ranch, and the Big Lost River below Mackay
Reservoir. The Big Wood at Hailey is in a Flood Advisory, which
is expected to remain moving back and forth between minor flood
stage and action stage through Wednesday morning before dropping
back into action stage and staying through the end of the week. A
Flood Advisory has been continued for the Big Lost at Howell, as
it`s expected to hover near minor flood stage through Wednesday
morning. The Mackay Reservoir is getting close to filling. It is
currently at around 99 percent full. The outflow for the Big Lost
below Mackay reservoir has been increased to 1400-1500 cfs, just
below flood stage. The reservoir should fill tonight into tomorrow
morning with current inflow. Flow may increase slightly above
1500 cfs maybe up to 1700 cfs Tuesday through Thursday after the
reservoir fills and then begin to go back down by the end of the
week with cooler temperatures. TW



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