Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 211955

Area Forecast Discussion
National Weather Service Pocatello ID
155 PM MDT WED SEP 21 2016

.DISCUSSION...An anomalously moist low pressure system was setting
up along the Oregon coast early this afternoon as previously progged
while regional radar returns were showing increasing shower activity
across the CNTRL mountains. Numerical models continue to show period
of moderate to heavy rainfall over the next 48-hours as the low
shifts inland and gradually lifts east of the divide Friday
night/Saturday morning. Snow levels are expected to drop Friday
allowing accumulating snow to collect across the CNTRL and Southern
mountains above 7500-8000 MSL. Below 7500 MSL,48-hour rain
accumulations of 1.50 to 3.00 inches is anticipated across the SRN
and SE highlands which would pose a significant problem for areas
with recent burn scars (in particular, the Henrys Ck Fire and Grape
Fires among others). Thus a Flash Flood Watch has been posted for
late tonight through Friday for the SE mountain zones. Temperatures
will also drop drastically over the next two days by about 20 deg
possibly impacting outdoor enthusiasts/hunters. Following the
ejecting low, a ridge of high pressure with gradually warming
temperatures are expected early next week. Huston

.AVIATION...Expect deteriorating CIGs and VSBYs later tonight as low
pressure continues its movement into Idaho. It looks a little later
than previous thinking, but it also looks like IFR conditions are
possible, and a shut-down of KSUN during the morning hours is
likely. KSUN should recover to flyable CIG by 22/18Z or so. Wind
direction is one of the more difficult forecasts, but luckily it is
for the most part staying below 11KT during those times. Late night
fog is also a consideration, reducing VSBY slightly, but the
abundant precipitation preventing dense fog. Even worse CIG/VSBY
expected on Thu night and Fri. Messick

.FIRE WEATHER...Low pressure moving into Idaho should bring abundant
moisture with a minimum of 1 inch, and widespread 2 inches in the
mountains of central and eastern Idaho, for the four days from today
through Sat morning. This should strongly curtail fire activity, and
will likely be a season-ending event for most locations. The more
serious concern for land management agencies will be debris flows at
recent burn scars. There will be embedded thunderstorm cells in the
precipitation for tonight and Thu, but it should remain isolated and
with the heavy rainfall and high humidity difficult to get ignition.
After Sat, high pressure will move back in and allow a warming and
drying. However, 80 degree temperatures and humidity below 20
percent will not be seen in the next week. Breezy conditions will re-
develop in the middle of the next work week, on Tue or Wed as
another low reaches the coast. However, this one has considerably
less moisture. Messick

.PIH Watches/Warnings/Advisories...

Flash Flood Watch IDZ022-023-025 for moderate rain over burn
scars beginning late tonight through Friday afternoon.

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