FXUS65 KBOI 210237

Area Forecast Discussion
National Weather Service Boise ID
837 PM MDT Tue Mar 20 2018

.DISCUSSION...Moist band oriented east/west across our CWA this
evening will shift north overnight and Wednesday morning with
scattered light rain and snow showers in eastern Oregon and
the west central Idaho mountains. More significant moisture
will spread northeast through Calif and western Nevada and
will reach our southern-most zones in the pre-dawn Wednesday.
The moisture will overspread the rest of our CWA Wednesday
morning as a warm front moves in from the southwest. Showers
will continue Wednesday night behind the warm front, then
increase again Thursday as a major upper trough consolidates
off the west coast and advects more moisture into our area from
the southwest. Current forecast is more in line with NAM
details than with the GFS, with pcpn beginning in the Snake
Basin just after 12Z (6AM MDT) Wednesday. No updates.


.AVIATION...A warm front crossing the area late tonight and
Wednesday morning will bring rain and snow showers with low VFR
and local MVFR ceilings and areas of mountain obscuration. Snow
levels 5k-6k ft MSL this evening rising to 6k-7k ft MSL Wednesday
afternoon. Surface winds variable 10 kts or less becoming southeast
to south and increasing to 10-20 kts Wednesday afternoon. Winds
aloft at 10k ft MSL southwest 15-25 kts.


SHORT TERM...Tonight through Thursday...A trough off the West
Coast combining with a trough developing in the Gulf of Alaska
will impact our weather during the short term. Significant
moisture is being directed toward the CA coast today. A warm
front associated with the trough off the West Coast is expected to
move through from the southwest on Wednesday morning, bringing
showers to the region, with the best chances of precipitation
generally southeast of a line from Rome, OR, to Stanley, ID.
As the warm front moves northeast, precipitation will taper off
by Wednesday afternoon, with mild temperatures (lower 60s in the
Snake River Plain). Breezy southwest to southeast winds will also
develop. A secondary wave arrives on Thursday morning, with
widespread precipitation and high snow levels (7000-8000ft)
during the morning. Significant precipitation is expected over
the central ID mountains, with up to 1.0 inch of water possible
on Thursday. May see substantial river rises due to snowmelt
with rain on snow. Winds will increase substantially ahead of the
cold front on Thursday afternoon, with a lull in precipitation
and mild temperatures. Winds briefly approach advisory criteria
ahead of the cold front in southeast OR, Baker County, OR, the
southwest Idaho highlands, and the Treasure Valley Thursday
afternoon. A vigorous cold front will move through southeast OR
on Thursday afternoon and southwest ID Thursday evening, bringing
showers and isolated thunderstorms. Small hail/graupel, gusty
winds, and heavy rain are possible with any thunderstorms that
do form along the front.

LONG TERM...Thursday night through Tuesday...Upper trough axis lifts
through the region Thursday night with drier air in its wake. The
main upper low remains positioned off the BC coast on Friday, and
will refocus precipitation development over southeast Oregon and the
central Idaho mountains. The low opens up and moves into the Pacific
NW on Saturday bringing another round of widespread rain/snow. While
airmass temperatures are cold enough to support snow to valley
floors on Saturday, temperatures will limit accumulations to above
4k feet. Cool and showery conditions continue into Sunday as the
base of the low tracks through NV. Temperatures fall from near
normal Friday to around 10 degrees below normal for Sat/Sun.

Sunday evening through Wednesday morning. A brief break in the
action as remnants of the recent troughing exit to the east and
relatively drier ridging builds in from the west. Models in
agreement with ridging continuing to build into the area; however,
expect patches of upstream moisture embedded in the ridge to make
their way into the CWA and moisten things up by Wednesday. Snowfall
anticipated primarily in the northern half of the CWA with the
exception of our southern mountains. As we gradually trend toward
warming, precipitation will turn to rain starting in the
northwest section by Wednesday morning. Temperatures start around
5 degrees below normal and increase to near normal by Wednesday.






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