Area Forecast Discussion
Issued by NWS Boise, ID

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FXUS65 KBOI 230347

Area Forecast Discussion
National Weather Service Boise ID
847 PM MST Fri Feb 22 2019

.DISCUSSION...Current forecast is on track beginning snow tonight
in northern areas and expanding to cover all zones Saturday. Only
light snow (less than an inch) expected in the Treasure Valley.
The really moist storm hits Sunday through Tuesday with heavy snow
in the mountains and rain in the lower valleys. Estimates of 1 to 3
feet of snow during that period in the Idaho mountains north of the
Snake Basin still look good.  No updates for now.


.AVIATION...Snow spreading south and east across the area overnight
and continuing on Saturday, mixing with rain below 3000 feet MSL
after 23/18z. Areas of MVFR/IFR and local LIFR conditions and
mountain obscuration. Surface winds: southeast to southwest 5-15
kts. Winds aloft at 10kft MSL: west 20-30 kts.

Outlook for Sunday...Snow continuing, mixing with rain below 3000
feet MSL through Sunday morning. Snow level rising to around 3500
feet Sunday afternoon. Widespread mountain obscuration and IFR/LIFR
mountain conditions. TAF site surface winds will become SE-SW 10-20
kts, with gusts 25-35 kts in eastern Oregon and in Idaho south of
the Snake Basin. Winds aloft at 10kft MSL will become W-SW 40-50 kts.


SHORT TERM...Tonight through Monday...Clouds are on the increase
tonight as an upper trough drops into the Pacific NW. Snow will
develop ahead of this initial wave with the focus of snowfall across
Baker County and the west-central Idaho mountains through early
Saturday. Still a chance of light snow in the Snake Plain Saturday
morning, especially for sites north of Interstate 84. At this point
up to an inch for valley sites with a tight gradient to higher
amounts as you get into the foothills of the Boise mountains. This
initial wave quickly moves east on Saturday with snowfall rates
tailing off Saturday afternoon in its wake. The next round of snow
ramps up Saturday night into Sunday as an elongated trough develops
off the coast. This will be a strong orographic event as westerly
flow (700mb winds of 30kts or more) channels moisture into the
central Idaho mountains Sunday and Monday. The Snake Plain will see
snow or rain/snow mix through Sunday morning, before changing to all
rain by Sunday afternoon. Sheltered lower valleys in Baker County
along with the Weiser River valley could hold onto the snow into
Monday before there is enough mixing to change these sites to rain.
Snow levels are around 3000 feet on Sunday rising to 3500-4500 feet
on Monday. A complicating factor for the mountains will be an
increase in surface winds starting Sunday afternoon and continuing
into Monday. Similar to recent storms, mountain valleys could see
blowing and drifting snow. Have updated the hazard products to match
current thinking for this prolonged event.

LONG TERM...Monday night through Friday...A deep upper level low
pressure trough anchored off the Pacific Northwest coast will drive
moist southwest flow aloft over the northern Intermountain Region,
with a chance of precipitation each day. A weak upper level high
pressure ridge building over the Idaho-Montana and Wyoming border
will support gradual warming, with temperatures returning to near
normal on Wednesday, and snow levels rising to around 4500 feet.
Forecast confidence decreases after Wednesday as model solutions
diverge, and there has been poor run to run continuity in handling
the offshore trough. So for now we are not showing any significant
changes for the end of the week.


ID...Winter Weather Advisory from 5 AM Saturday to 5 PM MST Monday
     Winter Storm Warning from 5 AM Saturday to 5 PM MST Tuesday
OR...Winter Weather Advisory from 4 AM Saturday to 4 PM PST Monday



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