Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 252144
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
244 PM PDT Sat Mar 25 2017

.SYNOPSIS...
The threat of showers will taper off this evening as a weak ridge
begins to build over the region. The dry weather will come to an
end by late Sunday as another system moves into the Pacific
Northwest. This system will deliver widespread valley rain and
mountain snow, most of which will be fairly light. Showery weather
is then expected to persist into early next week. The next chance
of widespread precipitation will arrive on Wednesday and Thursday.


&&

.DISCUSSION...
Tonight...Upper level trough is making a steady progression
eastward through extreme eastern Washington and north Idaho this
afternoon. Good instability ahead of this feature has resulted in
scattered showers east of a line from Republic to Ritzville. The
instability is generally too shallow to support thunderstorms,
however the latest RUC analysis is showing MUCAPE values nearing
200 j/kg over the Clearwater Mountains and Camas Prairie where the
current dewpoints are in the upper 30s to lower 40s. The
corresponding model soundings show marginal instability depth so
the odds of thunder don`t look good. The showers will rapidly
dwindle toward sunset as the shortwave trough moves east of the
ID/MT border and we lose any solar heating. The remainder of the
night will be dry and relatively cool at least compared to last
night. Dewpoint readings are generally down 10-15F compared to
last night and the winds will decouple during the evening. The
main forecast problem will then pertain to the threat of fog.
Based on the low-level wind fields and lapse rates the best
chances for fog will occur over the protected northern valleys. fx

Sunday to Wednesday: The Inland NW will remain in an active
pattern, with several opportunities for precipitation. First
Sunday morning patchy fog will be possible in some sheltered
valleys early, but otherwise it looks mostly dry. The main
exception to the dry forecast may be in the Cascades as the first
system starts to approach. Then through Sunday afternoon and
evening precipitation is expected to increase from west to east as
a warm front lifts in and cold front pushes up to the Cascades.
Then overnight into Monday morning precipitation decreases from
the west as the cold front pushes through. Monday an unstable but
stretching upper trough will continue to migrate across the
region. This will keep a threat of scattered showers alive across
the mountains and the eastern third of WA and ID. I kept a slight
chance of thunderstorms in the forecast over the southeast CWA,
but confidence is lower than yesterday. Shortwave ridging and more
stable conditions develop Monday night into Tuesday morning,
shifting the main threat of precipitation into the mountains.
Finally Tuesday afternoon into Wednesday the next warm front lifts
in, tapping an increasingly deeper subtropical plume of moisture.
Precipitation chances will increase across the region Tuesday
afternoon and night, with the best chances near the Cascades and
northern mountains. Chances become more likely through the day
Wednesday. /J. Cote`

Wednesday night through Saturday:  A splitting upper-level trof
trickling inland will usher a slow moving cold front into the
region Wednesday night into Thursday morning. This will supply the
lift for one more burst of moderate showers before the rich plume
of subtropical moisture gets nudged to the south. There remains
some uncertainty related to the timing of the front with some
model solutions pushing the front through early Wed night and
others late Thursday morning but the general QPF output through
the course of the event remains similar with an additional quarter
to three quarters of an inch. Due to the splitting nature of the
trof, the most pronounced cooling aloft gets hung up over Western
and Central WA with 500mb temperatures cooling near -29C yet the
trof expand well into Idaho allowing for upper-level to cool near
-23C. As such, the air mass will become increasingly unstable
promoting widely scattered hit or miss showers and a small chance
for a few t-storms.

A flat ridge of high pressure builds into the region Friday and
lingers into the weekend. Disturbances rippling through the ridge
will bring a small chance for mountain showers but at this
time...look to be very light precipitation producers. Model spread
increases into the weekend with some guidance indicating more
amplification of the ridge going into a dry pattern while other
maintain a flat ridge and swift passage of shortwaves allowing for
a continued threat for mountain showers. It appears the region
will be void of heavy precipitation events allowing flooding
issues to recede. /sb

&&

.AVIATION...
18Z TAFs: Main concern during this period will revolve around the
showers over the eastern third of the forecast area. Isolated
showers will occasionally impact GEG SFF and COE. These showers
will bring brief periods of MVFR cigs interspersed with VFR cigs.
Hard to determine of the MVFR cigs will persist more than half of
the time, but based on radar coverage and satellite imagery the
odds are against it. PUW stands the best chance of prolonged MVFR
cigs through most of the afternoon, while LWS has a very small
chance. Conditions should improve at all these sites aft 02z as
conditions stabilize rapidly. For tonight there will be a small
chance of IFR stratus/fog, however with the low-level winds
turning to the SE, we suspect that most of this will remain away
from the airports. fx

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        32  49  38  51  36  50 /  10  60  80  50  20  20
Coeur d`Alene  31  48  36  48  35  48 /  20  50  80  60  40  30
Pullman        34  50  39  49  36  50 /  10  50  70  60  20  20
Lewiston       36  54  41  55  38  56 /  10  30  70  50  20  10
Colville       29  47  36  49  34  48 /  10  20  80  40  10  50
Sandpoint      29  46  33  46  33  46 /  20  10  80  60  30  50
Kellogg        31  46  34  44  34  45 /  20  30  70  90  50  40
Moses Lake     32  52  37  58  38  57 /   0  70  60  20  10  20
Wenatchee      32  47  35  53  36  53 /   0  80  70  20  10  20
Omak           30  47  35  53  36  52 /   0  70  80  20  10  20

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.
&&

$$


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