Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 230855

Area Forecast Discussion
National Weather Service Pocatello ID
255 AM MDT Wed Aug 23 2017

.SHORT TERM...Today through Friday. Sat imagery confirms what the
models have been suggesting...monsoon moisture is moving into the
southern highlands early this morn. Radar is showing the result as
showers move northward out of NV and UT. Monsoon moisture will
continue to move northward on southerly flow on the backside of
the upper ridge just to the east. Scattered showers and widely
scattered tstms expected to develop over the mtns this afternoon
and evening as the increasingly moist airmass destabilizes.
Activity will be more isolated in the valleys. Higher dewpoints
and more extensive cloud cover today will keep temps a little
lower. Smoke from wildfires will continue to be an issue in the
central mtns today. A rather vigorous upper trof moves into the
Pac NW on Thu...turning the upper flow more southwesterly. This
effectively cuts off the monsoon flow and pushes most of the
precip to the north and east parts of the fcst area. Another minor
upper wave on Fri is expected to generate a few showers/isolated
tstms over the mtns Fri afternoon and evening.

.LONG TERM...Saturday through Wednesday. Models continue to build
what will become a high amplitude upper ridge over the west. The
ridge dominates thru Wed...resulting in dry conditions and a
return to more summerlike temps. Both GFS and ECMWF suggest some
moisture getting into the ridge on have added isolated
tstms near the Divide. Hedges


.AVIATION...Models showing decent agreement this morning in sct shower and t-
storm activity developing across the Southern Highlands and slowly
expanding NE as the day progresses, leading to at least moderate
forecast confidence. 12Z TAFs will carry VCSH transitioning to VCTS
at all terminals, generally starting between 15Z-18Z. Not out of the
question that a terminal or two may be upgraded to SHRA with 12Z TAF
issuance. Winds today are expected to be light at 12 kts or less,
but we will have to watch for erratic t-storm outflow winds gusting
25-40 kts. As usual, pinning down EXACT locations that may see
storms/outflows and at what time is impossible in this setup. The
above recipe will yield VFR conditions everywhere, with a very
marginal chance of brief MVFR if a storm moves directly over an
airport or strong outflows kick up dust. Wildfire smoke has been
reducing visibilities in the Challis area this morning, but we are
not expecting this to be an issue at KSUN.

VFR conditions with isolated to sct t-storms appear on tap again for
Thurs, although coverage may be a bit less. Gusty outflow winds will
again be the greatest threat to aviation. KSmith/Hedges


.FIRE WEATHER...Widely scattered showers and thunderstorms are
forecast along and south of the Interstate corridor this morning
which will spread northward through the afternoon and evening hours.
Moisture thusly increasing and reflected in the RH forecast through
Friday. Expect all zones to see widely scattered thunderstorm
coverage with Haines indicies dropping from a 4 to 3 during the day.
Haines of 5 will be persistent there across most of zone 476 as this
area has the least chance of seeing thunderstorms today,
additionally, they would occur later in the day if they do form. No
further concern of high Haines in the near term. Thursday will get
breezier with speeds increasing to 15 to 20 mph across the Snake
Plain and Central Idaho. Widely scattered thunderstorms expected
again tomorrow afternoon across Central Idaho zones as well
- it will be less likely across the rest of SE Idaho. Things
dry out by the weekend as the moisture exits the region and high
pressure rebuilds. Expect fairly widespread critical RH values
across our area by Sunday afternoon. NP/Hedges



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