Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 211758

Area Forecast Discussion
National Weather Service Spokane WA
1058 AM PDT Wed Mar 21 2018

A change in the weather is expected as a deep upper level low
digs offshore today. This will pool moisture into the region from
the south resulting in an increasing chance of precipitation
beginning this afternoon and continuing at times through Thursday.
Most of this precipitation will fall as rain however snow will be
possible over the higher mountains. A cold front late Thursday
will bring gusty winds and deliver significantly cooler conditions
from Friday through the weekend with the threat of rain and snow
showers each day.


Today and tonight...The dry weather of the past few days will come
to an end as we enter an active period of weather. This weather
change will be a result of a digging trough of low pressure
currently off the off the northern BC coast. This system is
expected to dig south into the offshore waters of northern Oregon
by late afternoon and into the northern California offshore waters
overnight. This digging will back the mean mid-level flow to
southerly orientation which rapidly pulls moisture into the Inland
Northwest. The current precipitable water values over the region
are around 0.30 inches and should more than double during the next
24 hours. This increasing moisture will no doubt result in an
increasing threat of precipitation, however pinpointing where
exactly this will occur today is not a sure bet as model solutions
are quite varied. Current radar imagery from northeast Oregon
shows a small band of light precip associated with a mid- level
warm front. This feature will continue to work northward this
morning and could produce some very light precipitation over the
southern Columbia Basin and Palouse. By afternoon it will shift
north of Highway 2 and the combination of moderate mid-level warm
air advection and orographic ascent from S-SE winds at 850 mbs
should result in a fairly rapid expansion of precipitation.
Precipitation amounts will likely remain light through the day.
There are some hints of deep convection and possible thunder just
south of our forecast area this afternoon, however model soundings
suggest the threat of thunder over the SE portion of our forecast
area (near the Blue Mountains) is too small to mention in our

For the first portion of tonight model solutions remain quite
varied with some suggesting we could see a lull in the
precipitation as the main ascent shifts into BC. Not sure what to
believe as other keep the precipitation threat going over the
northern third of the forecast area. Confidence grows overnight as
another surge of moisture as ascent pushes into the region. This
time the major ascent attributed to strong warm air
advection/isentropic ascent will focus most of the precipitation
over the north Cascades, Waterville Plateau, and Okanogan
Valley/Highlands after midnight before drifting eastward

QPF values differ markedly during the overnight period. The GFS is
by far the wettest with over an inch of precipitation forecast
over the western half of Okanogan County and up to half an inch
over Chelan County. While the placement of this precipitation has
been consistent, the amounts have not been whatsoever. We will
continue to lean toward lesser amounts of precipitation suggested
by the EC and NAM solutions. This still means western Okanogan
could see precipitation amounts from 0.25-0.50 inches which is
still a good amount of precipitation. Meanwhile amounts across the
remainder of the forecast area should considerably lighter. Most
of this precipitation will fall as rain, however snow is expected
above 4500 feet so moderate amounts are possible over the
Cascades. There is also a small chance that snow could impact
Sherman Pass tonight, however snow levels will rise above the
Pass toward morning. fx

Thursday: A wet cold front tracks west to east through the Inland
NW Thursday producing a period of widespread precipitation. The
bulk of the precipitation falls along and ahead of the cold front
with snow levels between 5000-6500 feet. Added upslope flow into
the East Slopes of the Cascades and northern mountains will
enhance precip amounts with 0.4-0.8 inches of rain possible. This
combined with snow melt and saturated soils may lead to minor
flooding issues or cause debris flows. A flood watch remains in
place. This will be a strong cold front passage occurring near
peak heating and mixing potential with 850mb winds of 30-40 kts.
These will likely mix down across the Upper Columbia Basin,
Spokane area, and Palouse. The mountains will become windy as
well. Behind the front snow levels plummet...but so down
precipitation chances as drier air moves in.

Friday and Saturday: A cold upper trough moves in with cool and
showery conditions. Models have trended stronger with a wave
tracking up through the region overnight Friday into Saturday
morning. There are some differences with its exact track but ECMWF
and GEFS argues for the best chances for precipitation across the
Eastern third of Washington and north Idaho. Snow levels will be
down to the valley floors, and some towns may see light to locally
moderate snow accumulations Saturday morning.

Sunday: A pair of waves to watch but the brunt of these look to
miss us as one tracks along southern BC and the other tracks
across southern Oregon. Could still see some showers but these
should be more isolated/scattered in nature and mainly over the

Monday: A short wave ridge moves in on Monday with just a slight
chance of mountain showers. JW

Monday Night through Wednesday: A shortwave ridge will continue
press into the Inland Northwest. This will allow for a drying
trend for the region. Isolated mountain snow showers are possible
but models aren`t too high them occurring. Temperatures around
the region will be in the 40s to low 50s for highs and into the
upper 20s and 30s for lows. /JDC


18Z TAFs: VFR conditions will prevail at all TAF sites through
08Z Thursday. A mid level warm front will bring showers to the
region through the day as it lifts to the north. Aft 05Z the
stratiform precip will move into the Cascades and then spread east
through the night and into early Thur morning. Rain will be
moderate to heavy at times, which could produce MVFR cigs or vis.


Spokane        52  41  53  31  46  30 /  30  50  80  80  30  40
Coeur d`Alene  51  38  51  30  44  29 /  40  50  80  90  40  50
Pullman        53  43  54  32  45  31 /  30  50  80  80  50  60
Lewiston       60  46  60  36  53  36 /  20  30  60  60  40  50
Colville       52  38  52  31  47  28 /  50  70  90  90  50  40
Sandpoint      48  37  48  31  43  29 /  40  70  80 100  40  40
Kellogg        48  36  50  29  42  28 /  40  60  70 100  50  60
Moses Lake     56  42  59  31  53  29 /  30  50  60  10  20  20
Wenatchee      50  40  52  31  48  29 /  30  80  70   0  40  20
Omak           50  41  53  31  48  28 /  50  80 100  20  60  30


ID...Flood Watch from this evening through Thursday evening for
     Northern Panhandle.

WA...Flood Watch from this evening through Thursday evening for East
     Slopes Northern Cascades-Northeast Mountains-Okanogan
     Highlands-Okanogan Valley-Wenatchee Area.


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