Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 220916
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
316 AM MDT Wed Mar 22 2017

.SHORT TERM...Today through Friday...Rain showers are expected
across the much of the Snake River Plain and elevations below
6500 ft across the Caribou, Eastern and Southern Highlands today.
A surface boundary will be passing through Southeast Idaho,
particularly the extreme Southeast corner, this afternoon.
Instability parameters are highest in areas like Preston, Grace,
Soda Springs, Mink Creek and Montpelier between noon and 9 pm MDT.
Be aware that gusty winds, lightning and heavy rainfall could
occur in these areas. An average of a quarter inch of rain is
forecast over these locations with some areas receiving higher
amounts of rain in heavier downpours. Rain showers are likely to
continue into the overnight hours, but not be severe, and
gradually dissipate Thursday. The surface boundary will be
shifting wind directions in this region with sustained winds of 15
to 25 mph, with higher gusts likely; independent of any
thunderstorm gusts. For the rest of Southeast Idaho Thursday, rain
showers are not likely with a ridge building in from the west.

Meanwhile, through Thursday morning snow showers are forecast
from 6500 ft or so with 1/2 inch of snow possible in the Central
Mountain today, although confidence is low to moderate on these
snow showers occurring. What is more confident is the new snow
in the Southern Highland/Wasatch Mountains with 1 to 4 inches
forecast. Emigration Pass, Geneva Summit and Georgetown Summit can
expect snow showers beginning this afternoon.

For Friday, a new weather system slowly enters Idaho from the west.
Initially, areas like the Central Mountains should be impacted
with snow showers in the afternoon and evening. It would appear
that forecast models have slowed down a bit in their timing of the
arrival of this system; therefore stay tuned for fine tuned
details on this system. This system then persists into the
overnight hours across Southeast Idaho with snow on the mountains
and rain in the valleys. NP/JAM

&&

.LONG TERM...Sat through Tue night. Storm of the week peaks on Sat
with moderate to heavy precipitation and a breezy afternoon. There
is partial clearing in the west on Sat night, then just slight
chance for precipitation by Sun. Starting with Sun night the
agreement between the ECMWF and GFS begins to break down. The GFS is
slower bringing in the next storm, only spreading precipitation
about half way through the forecast area from the west; the ECMWF
brings the entire forecast area into moderate to heavy precipitation
for Sun night. The uncertainty with the forecast grows as the two
solutions diverge in timing. This is likely due to the weakening
longwave pattern that develops after the weekend storm,. Not only is
the 500mb 5-wave much weaker in the gradient aloft, but the flat,
zonal flow means shortwaves will determine the story. So do not look
for model comparisons to show much agreement and watch for large
run to run swings until it gets closer in time. Messick

&&

.AVIATION...With the 22/00Z guidance, there was some concern about
IFR potential in the afternoon for the three plain airports, but the
22/06Z guidance has put an end to the concern with a drier boundary
layer. Thus expect VFR conditions with possible marginal VFR this
morning during the rain event that develops this morning. Believe it
mainly affects KBYI. Gusty winds are only possible in the vicinity
of thunderstorms, which are a low threat with low to mid-level CIGs
expected to linger into the early evening. Messick

&&

.Hydrology...Flood watches and advisories continue until further
notice at this time. The Portneuf River is slightly below
moderate flood stage just under 10 feet and a flood warning is in
place. The Snake River at Blackfoot is near flood stage. Bear
River gauges continue to climb but forecasts keep river just
below flood stage so will continue to watch. Rain and
thunderstorms are forecast for today which could add significantly
to the flooding in the Bear Lake regions. QPF amounts of 1/4 or
more are currently forecast with potentials for more over the next
24 hours and this area will need to be watched beginning at noon
and through this evening for thunderstorms; as heavy downpours may
bring flooding areas already impacted by floods. NP/JAM

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Thursday morning for IDZ017>019-022>025-031-
032.

&&

$$



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