Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 191949
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
149 PM MDT Sat Aug 19 2017

.SHORT TERM...This evening through Monday. Zonal flow continues
through Sunday with areas of high clouds drifting through the
region and midday gusty winds throughout most of the area. A weak
trough will slide through Sunday night into Monday morning with a
lingering high level cloud deck associated with it. Impact problem
then comes down to timing and location of this thin and narrow deck
of clouds as it exits the region through the day on Monday. Trend
from various models this morning show their may be affects of this
band from Wyoming to near Blackfoot Idaho. The 12z run this
morning has it moving eastward a little faster than last nights
run. The next run should help establish it`s final forecasted
resting place for the critical 11 am to noon time-frame. The rest
of the region should not experience any any cloud cover problems.

Gusty winds are possible tomorrow - see impacts below in the fire
weather discussion. We anticipate additional gusty west winds
again on Monday on the back side of the trough.

vp


.LONG TERM...Tuesday through Saturday. An upper level ridge begins
to build onto the inter-mountain region by mid week...while the
weak upper low stays anchored over the southern CA coast. A push
of monsoon moisture is anticipated to return to our region
starting Wednesday morning. Impacts to aviation could be a strong
concern for those leaving after the Eclipse. The thunderstorm
chances are area-wide through Friday morning when a cold front
sweeps through. Based on initial look and our forecaster history,
there may be enough divergence and difluence aloft to possibly
enhance the severe weather threat during this sequence. We will be
providing more confidence levels as we get closer to its return.
By Saturday a large scale ridge shifts over the region with
warming temperatures.

vp


&&

.AVIATION...VFR conditions expected through the period. Gusty winds
through about sunset, particularly at BYI, IDA and PIH with gusts in
the 20-25 kt range from the west-southwest, with the strongest winds
at PIH and IDA. Winds diminish around sunset then increase once
again during the early afternoon hours on Sunday continuing through
sunset. Expect winds from 240 gusting to 25kt at PIH, 210 gusting to
25 kt at IDA with occasional gusts to 20kt for DIJ from roughly 260.
AD/VP


&&

.FIRE WEATHER...Westerly flow is in place across the area. The first
of two waves will move through the area today.  Expect strong wind
gusts of 25 mph in valleys and 30 to 35 mph across ridgetops across
the Central mountains and Arco desert. Relative humidity will remain
low, if not slightly lower than yesterday, ranging from 8 to 18
percent for the area. A Red Flag warning is in effect today from
until 9 PM MDT for East Idaho Fire Weather Zones 422, 475, and 476.
Winds in all other zones, though elevated, are slightly below
critical values through this evening. Haines Index will remain high
today and Sunday. Have also issued a Fire Weather Watch for Sunday
afternoon through 9 PM MDT for East Idaho Fire Weather Zones 410 and
425. For Fire Weather Zone 410, the Arco Desert Region will see wind
gusts around 30 mph Sunday afternoon into early evening, along with
minimum relative humidity values from roughly 9 to 15 percent. The
combination of these factors, along with drying fuels and an
expected influx of people for the Eclipse led to this decision.
Minimum relative humidity values are forecast to be critically low
for Monday and Tuesday for these same areas, however winds are
forecast to remain below critical levels Monday and Tuesday. By
Wednesday, the remnants of a weakening low off of the California
coast combined with an approaching Pacific trough will contribute to
enough moisture increase across the area to allow for the
development of isolated to possibly widely scattered thunderstorms
Wednesday through Friday, with the greatest coverage during the
afternoon and early evening hours. The approaching Pacific trough
should force an increase in winds Thursday into Friday as well,
which bears monitoring. AD/VP

&&


.PIH WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch from Sunday afternoon through Sunday evening
for IDZ410-425.

Red Flag Warning until 9 PM MDT this evening for IDZ422-475-476.

&&

$$



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