Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 212119
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
219 PM PDT Sat Oct 21 2017

.SYNOPSIS...
Expect active weather through the weekend. Rain and high mountain
snow will continue through Washington and north Idaho tonight,
before decreasing from the west and northwest Sunday. For some
it will also be rather windy, as speeds increase tonight into
Sunday. Gusts of 30 to 50 mph will be possible across portions of
the open wheat country of Washington and the Idaho Palouse. Mild,
dry conditions Monday and Tuesday will be interrupted by another
threat of precipitation and winds Wednesday, before drying out
again later in the week.


&&

.DISCUSSION...
Tonight and Sunday: Wet, windy conditions tonight gives way to
somewhat better weather Sunday. A warm front lifts across the
northeastern CWA this evening, while a cold front starts to move
in from the northwest overnight into Sunday. A plume of moisture
riding into this system (with PWATs near 200% of normal) will help
provide rain and mountain snow to much of the region this
evening. The lower lee of the Cascades and deeper Columbia Basin
will have a more limited threat where some shadowing will occur.
Overnight the threat of precipitation will start to decline in the
lee of the Cascades and central Washington, backing up against
the Cascade crest. Chances will remain high across much of eastern
to southeast third of WA and ID through Sunday morning, with the
main exception being in the L-C Valley where increasing southwest
winds are more apt to shadow out the region. Then Sunday afternoon
the precipitation retreats to the mountain zones and southeastern
WA and turns showery, while some sun breaks out elsewhere.

Precipitation amounts between a quarter to a three-quarters of
inch will be possible in the valleys, with locally over an inch in
some of the mountains areas, while some of the deeper Columbia
Basin may not see much more than a tenth of an inch. The brunt of
this is expected to fall tonight, although some moderate amounts
are still possible across the central Panhandle through Sunday.
Much of this will fall as rain, with snow levels rising tonight.
However there will still be some snow impacts early on near the
Cascades and Okanogan Highlands before those snow levels rise.
Snow advisories remain in place through the early evening hours.
The only change I made was to restrict the Okanogan Highlands
advisory to areas above 3500 feet. Webcams and area temperatures
suggest, even if any snow mixes in, it will be difficult to
accumulate below this level.

Winds will be breezy to windy across portions of the region much
of this period. The speeds and gusts are expected to become
particularly strong as we progress through this evening into
Sunday morning from near the Blues Mountains into the Palouse,
Upper Columbia Basin, Spokane/C`dA area. Wind advisories remain in
place for these spots. As the cold front approaches the
strengthening surface gradient and strengthening 850mb LLJ will
contribute to the increasing winds. The LLJ speeds increase to
around 60-70kts, per the GFS/NAM and up to around 50kts per the
ECMWF. The timing of isn`t favorable for full mixing up to tap
such speeds, but precipitation could drawn some of this momentum
down. Speeds of 15-30 mph with gusts near 40 to 50 mph will be
possible. After about 18Z Sunday that LLJ and pressure gradient
starts to slacken, so speeds should start to decrease through the
day. /J. Cote`

Sunday night through Tuesday night...Good model agreement exists
for this period depicting a building ridge over the forecast
area. This will promote a strong improving trend with residual
showers remaining over the Idaho Panhandle Sunday evening but
quickly giving way to a dry period region wide by Monday with
decreasing clouds...light winds and temperatures running a few
degrees above normal for the first half of the week. The only
issue will be some patchy fog and low clouds developing each night
in the sheltered valleys surrounding the basin.

Wednesday and Thursday...On Wednesday the next weather system will
break over the forecast area with both the GFS and EC models and
to a weaker extent the Canadian model depicting a northwesterly
trajectory short wave beating down the ridge...likely dragging a
cold front. This front will be fed by a moderately deep Pacific
moisture feed...but this moisture plume will only be favorably
directed into the region for a short time...with a shot of dry low
level air invading from Canada immediately behind the front.
Thus...while showers (pretty much rain showers with snow levels
remaining only on the highest peaks) will affect the mountains
and perhaps the northeastern basin along the front...this does
not look like a big precipitation event. The main effect may be a
period of post frontal windy conditions down the Okanogan Valley
and into the basin late Wednesday and into Thursday. There are
model differences between the GFS and EC regarding the strength
and orientation of the post frontal northerly gradient...with the
forecast trending toward the more aggressive ECMWF based on it`s
run to run consistency. Another upper ridge will aggressively
build off the coast on Thursday placing the forecast area in a
dry northerly flow under the east flank of this feature...for a
dry...moderately breezy and increasingly sunny day on Thursday.
/Fugazzi

Friday through Sunday: We remain on the eastern side of an
incoming ridge for Friday and Saturday, which will continue to
bring dry and cool Canadian air south. Winds will be light and
temperatures remain cold. Just how cold is uncertain, as the EC
and GFS disagree on the strength of the CAA. EC has widespread
highs in the upper 40s on Friday, nearly 10 degrees below the
GFS. For now, looking at highs in the low to mid 50s to start the
weekend, right around normal for late Oct. /bw

&&

.AVIATION...

18Z TAFS: A warm front moving onto the Washington Coast at 18Z
will move northeastward to northeast washington and north Idaho
by 06Z tonight with a trailing cold front passage early Sunday
morning. This frontal complex is fed by a deep and well directed
Pacific moisture plume and will promote widespread MVFR ceilings
and visibilities in RA at most TAF sites through the next 18 to 24
hours. Widespread mountain obscurations in snow are likely as
well around the region. By 09Z-15Z LLWS is expected at the eastern
TAF sites in the warm sector ahead of the approaching cold front.
Very windy and gusty conditions are expected at eastern TAF sites
Sunday morning as the cold front passes. /MJF


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        45  59  40  55  38  57 / 100  30   0   0   0   0
Coeur d`Alene  41  58  39  55  36  57 / 100  60  10   0   0   0
Pullman        45  59  42  58  40  62 / 100  90  10   0   0   0
Lewiston       49  65  45  62  43  64 / 100  60  10   0   0   0
Colville       39  59  35  56  35  55 / 100  10   0   0   0   0
Sandpoint      38  55  34  53  33  55 / 100  90  10   0   0   0
Kellogg        39  51  37  51  35  56 / 100 100  20  10   0   0
Moses Lake     48  64  37  60  35  60 /  90  10   0   0   0   0
Wenatchee      41  61  40  59  39  60 /  90  10  10   0   0   0
Omak           38  60  34  57  34  57 / 100  10  10   0   0   0

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Wind Advisory from 9 PM this evening to 1 PM PDT Sunday for
     Coeur d`Alene Area-Idaho Palouse.

WA...Winter Weather Advisory until 7 PM PDT this evening above 3500
 feet for East Slopes Northern Cascades and Okanogan Highlands.

     Wind Advisory from 9 PM this evening to 1 PM PDT Sunday for
     Lower Garfield and Asotin Counties-Spokane Area-Upper
     Columbia Basin-Washington Palouse.

&&

$$



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