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Area Forecast Discussion (AFD)

Issued by Boise, ID (BOI)

                            
616 
FXUS65 KBOI 212211
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
311 PM MST Wed Feb 21 2018

.SHORT TERM...Tonight through Friday...An upper trough dropping
into the Pacific Northwest will bring a period of light snow to
the region tonight into Thursday. Models have been consistent in 
showing a band of snow developing across southeast Oregon tonight,
transitioning into southwest Idaho by sunrise. However confidence
in the track of the developing precipitation band is lowest as it
moves into southwest Idaho. The heaviest snow will fall across a
fairly narrow area from s-central Harney and Malhuer counties,
into the Owyhee mountains. In southwest Idaho the focus of 
snowfall will cross the Treasure valley between Ontario and 
Mountain Home and extend northeastward into the central Boise 
mountains. This system will be working with a limited amount of 
moisture (PW values are in the 10-20th percentile during this 
event) so have weighted QPF (and thus snowfall) amounts toward 
model solutions with lower amounts. Temperatures will likely warm
above freezing at sites below 4000 feet Wednesday afternoon, 
though precipitation type will stay all snow through the event. 
Have issued an advisory for southern and western zones starting 
tonight. 

The upper low moves into eastern Idaho Thursday night, which will
cut off the precipitation. Friday is dry but continued cold under
a passing short wave ridge. Temperatures continue to run 10-15
degrees below normal through Friday. 


.LONG TERM...Friday night through Wednesday...The northern 
Intermountain Region will remain under northwest flow aloft, with 
unsettled weather and below normal temperatures. The next upper 
level trough from the Gulf of Alaska will spread snow showers
across  our area Saturday and Saturday night. Snowfall in the 
valleys is expected to be around an inch, with heavier amounts in 
the mountains. On Sunday lower elevations of southwest Idaho and 
southeast Oregon will see brief improvement, with snow showers 
decreasing to isolated and partly cloudy skies. Temperatures will 
average from 5 to 10 degrees below normal. 

Monday evening through Thursday morning...Active troughing 
pattern holding on and continuing to facilitate the ushering in of 
periodic trough-ridge couplets into and across the CWA. The GFS 
and EC models somewhat agree at the start then quickly part ways 
by Tuesday evening with the GFS being the more active. Mostly 
cloudy conditions with widespread snow during most of the time 
frame associated with these translating disturbances with brief 
periods of sunnier skies and milder weather in between. Mild 
northwest winds for the most part becoming breezy with passing 
frontal systems. Temperatures averaging around 5-8 degrees below 
normal.


&&

.AVIATION...VFR through this evening except for local MVFR 
ceilings with isolated snow showers. Deteriorating conditions after 
06z with snow becoming widespread across southeast Oregon and the 
central Idaho mountains and continuing through Thursday. On Thursday 
snow spreading east through the Snake River Valley, but decreasing 
to isolated showers at KBNO and KBKE. Surface winds variable under 
10 kts, then variable 5-15 kts after 06z. Winds aloft at 10k ft MSL 
southwest 10-20 kts. 

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Winter Weather Advisory from 5 AM to 11 AM MST Thursday IDZ030.
     Winter Weather Advisory from 5 AM to 5 PM MST Thursday IDZ015.
     Winter Weather Advisory from 11 PM this evening to 5 PM MST 
     Thursday IDZ029.
OR...Winter Weather Advisory from 11 PM MST /10 PM PST/ this evening 
     to 11 AM MST /10 AM PST/ Thursday ORZ061-063.

&&

$$

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SHORT TERM...DG
LONG TERM....JT/WH
AVIATION.....JT