Area Forecast Discussion
Issued by NWS Boise, ID

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161
FXUS65 KBOI 291535 CCA
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
935 AM MDT Wed Mar 29 2017

.DISCUSSION...New 12Z model guidance has very strong cold front
in eastern Oregon at 6AM MDT Thursday, moving through swrn Idaho
including the Treasure Valley 8-9 AM MDT, and the Magic Valley
around 11 AM MDT.  Front will bring heavy rain followed by
intense cold advection and rapidly lowering snow level.  Rain
will change to snow as low as 3700 feet MSL later Thursday, but
after the heaviest pcpn has ended.  Even the lowest valleys may
see mixed rain and snow showers Thursday afternoon.  Snow will
accumulate in the highlands of Owyhee County, and current winter
weather advisory there looks good.  Latest models now show winds
even stronger than before for Thursday, and reaching high wind
threshold in the Upper Treasure Valley, Southwest Idaho Highlands,
and southern Twin Falls County.  These winds are shown along and
well behind the cold front, and appear due to pressure gradient,
since thunderstorms would be limited to the frontal zone.  Previous
models also had 700 mb winds from the north or NNE and not in good
alignment with the Snake Basin.  Now the 700 MB winds are shown
from the NNW and much better aligned.  Also, the eastward speed
of the front as timed above should correlate well with surface
wind speeds.  As a result we will increase winds in southwest
Idaho for Thursday and upgrade current wind advisory to high wind
warning in the southeast part Upper Treasure Valley near Mountain
Home, southwest Idaho Highlands (generally Owyhee County), and
southern Twin Falls County.  Not sure yet whether to include the
Western Magic Valley.

&&

.AVIATION...Mostly VFR. Showers moving through central ID and
northeast OR will increase in coverage after 20z today. Areas of
heavy showers will cause MVFR/IFR conditions. Surface winds,
variable 10-25 kts through 06z Thursday then 20-35 kts with
higher gusts within showers. Winds aloft to 10k feet MSL,
westerly 25-40 kts through 12z Thursday then nnwly 35-45 kts.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Thursday night...Complex weather
situation shaping up across southwest Idaho and southeast Oregon
for Thursday. Warm-frontal precipitation will impact the region
today, especially over the higher terrain through this evening.
Snow levels are expected to be around 8500 feet MSL near the NV
border and around 7500 feet near McCall today. As the upper level
trough approaches, snow levels slowly lower late tonight (to
around 7000 feet) with significant precipitation expected. The
upper level trough begins to split from the parent jet stream and
become a closed low by Thursday afternoon. As with closed lows,
models have a hard time pinpointing the location center, with some
uncertainty. Latest model runs have the center of the trough
passing right over southeast Oregon and southwest Idaho as it
moves into NV/UT on Friday. The positioning of the center of the
low is a major factor in precipitation and wind forecasts.
Nonetheless, significant precipitation is expected to fall across
the region as the center of the trough passes overhead.
Thunderstorm formation is possible along and ahead of the cold
front, and it will depend on where the center of the trough
passes. Currently, models place this region along the OR/ID border
and areas east on Thursday morning. As this front passes through,
wind gusts to 50mph are possible with the front and any
thunderstorms that do form. Snow levels abruptly will fall to
around 3500-4000 feet MSL Thursday morning with the front, and high
temperatures are likely to occur early in the morning as
temperatures fall during the morning hours with slight recovery
during the afternoon. Significant snow amounts are expected on the
mountain peaks above 7000 feet. A strong pressure gradient is
still forecast to develop as the trough moves into NV/Utah, with
winds 25-35 mph and gusts to 45 mph, strongest in the Upper
Treasure Valley on Thursday morning lasting into Friday.
Meanwhile, wrap-around moisture associated with the trough will
favor snow in southern Owyhee county in the southwest Idaho
highlands, where a winter weather advisory has been issued for
snow and wind as travel could become difficult on Idaho State
Highway 51.

LONG TERM...Friday through Wednesday...Lingering showers across the
western Magic Valley and south to the Nevada border as the deep
upper low exits the region. Clearing skies and breezy conditions
throughout the day Friday as an upper level ridge builds aloft. High
temperatures will warm to about normal, rising 8 to 14 degrees from
Friday. Dry conditions persist into Saturday as the ridge tracks to
the east. Showers and breezy conditions will return to the region
on Sunday as the next trough digs south and east into the region,
exiting Tuesday. The coolest day in the extended will be Monday as
the trough axis sits directly over the Intermountain West and snow
levels drop briefly to about 3-4k ft MSL. Precipitation will be
greatest over the mountains. Ridging returns for the end of the
period.

&&

.HYDROLOGY...Significant precipitation expected with high snow
levels tonight. Snow levels are forecast to lower rapidly on
Thursday, with more snow at the higher elevations than
previously forecast. Meanwhile, the Boise River at Glenwood
Bridge continues in minor flood until further notice, and the
small stream flood advisory for central Elmore and southern
Camas Counties remains in effect until 1 am MDT Friday.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Wind Advisory from 9 AM Thursday to midnight MDT Thursday night
     IDZ012-014-016-029-030.
     Winter Weather Advisory from noon Thursday to 6 AM MDT Friday
     IDZ015.
OR...Wind Advisory from 9 AM MDT /8 AM PDT/ Thursday to midnight MDT
     /11 PM PDT/ Thursday night ORZ061>064.

&&

$$

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DISCUSSION...LC
AVIATION.....JC
PREV SHORT TERM...KA
PREV LONG TERM....AB
HYDROLOGY...LC/KA



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