Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 220321

Area Forecast Discussion
National Weather Service Pocatello ID
921 PM MDT SAT MAY 21 2016

.UPDATE...Radar imagery shows last gasp of convection moving
through eastern highlands and decreasing in intensity this
evening. Satellite imagery shows broad low with center rotating
into western Idaho this evening. Second batch of weaker showers
rotate into the central mountains overnight associated with low
center. Another shortwave rotating north along Nevada/Utah border
also still has weak convection associated with it and should shift
into the southeast corner of the forecast area after midnight.
Current forecast trend dries out the Snake Plain while maintaining
some mention of showers over higher elevation areas in line with
aforementioned trajectories. Main concern then remains the
potential for fog and how far temps will drop in cleared areas.
Overnight lows forecast around or slightly lower than current
surface dewpoints and think this is reasonable. Fog will be just
as difficult to pinpoint and will leave out of forecast for now.
Have made no changes to going forecast this evening. DMH


.PREV DISCUSSION... /issued 241 PM MDT SAT MAY 21 2016/

SHORT TERM...This afternoon through Tuesday. The upper low
responsible for the recent cool, wet weather was centered over
eastern OR this afternoon. The more organized/intense band of
convection was focused across MT and WY early this afternoon in
zone of stronger mid/upper dynamic forcing. However, forcing
associated with the upper low will remain sufficient across
southeast ID this afternoon into early this evening, especially
when combined with weak instability resulting from diurnal heating
beneath cold temps aloft, to continue to generate scattered to
numerous showers and embedded thunderstorms. Extensive cloud cover
has limited instability, however a few storms have briefly pulsed
up in pockets of clearing, and the possibility for a few strong
storms will continue through early evening. Precipitation
coverage will decrease overnight as things stabilize further in
the absence of stronger dynamic forcing, with the central
Mountains and eastern Highlands most favored to see any lingering
rain and snow showers overnight. Snow levels will fall to around
6000 ft tonight, but any snowfall accumulations at these higher
elevations are expected to be light. It will be a chilly night
for late May, and lows across portions of the Snake Plain will
fall into the mid 30s with some spots flirting with freezing
temperatures. Actual lows will largely hinge on the degree of
clearing overnight. Currently expect some mid clouds to hang
around, and given this along with the expectation of light
south/southwest surface flow and moist conditions from recent
rains, will hold off on the issuance of any frost or freeze

The cool and unsettled pattern will continue Sunday through
Tuesday. The responsible upper low will slowly trek northeast
across the area through Sunday night, with remnant troughing then
remaining in place through Tuesday as a new circulation spins up
along the CA coast. The net result will be continued periods of
showers and embedded thunderstorms and high elevation snow showers,
along with below normal temperatures. Southwest winds will
strengthen across the Snake Plain on Sunday as the pressure gradient
tightens, likely necessitating a Lake Wind Advisory for American
Falls Reservoir.


LONG TERM...Tuesday night through Saturday.
General trend is one upper low exits and another approaches by mid
week.   European and GFS models differ a bit with European dropping
low over Idaho by next Saturday while GFS keeps it off shore. Either
case will likely keep a chance for showers and thunderstorms through
the week with the GFS likely a bit milder solution keeping southerly
flow aloft over Idaho.  Temperatures should trend slightly below
normal through the period and split the difference between cooler
European and warmer GFS.

AVIATION...Upper low continues to spin to west and pump moisture
into region.  AFter morning band of showers moved through a second
one developed and brought showers and MVFR conditions to all TAF
sites briefly.   Thunder did occur at PIH and kept in at least
vicinity thunder at all sites this evening.  Expect activity to die
out around sunset.  Expect afternoon showers and thunderstorms again
tomorrow afternoon and wind speeds in the 15 to 20 knot range at
BYI...PIH and IDA as well.


.PIH Watches/Warnings/Advisories...

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