Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 210948
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
248 AM PDT TUE OCT 21 2014

.SYNOPSIS...

A cold front is very slowly moving through the Inland Northwest
which will bring precipitation to much of the area and an end to
the above average temperatures. The active pattern will continue
through the weekend with the potential for heavy rain continuing
as moisture associated with former typhoons makes its way toward
the Pacific Northwest early next week.

&&

.DISCUSSION...

.Today and tonight...Deep upper level trough and associated cold
front will continue to make a slow eastward crawl through the Inland
NW this morning. As of 200 am...the front was located along a
line from Colville to the Blue Mountains with widespread
precipitation immediately ahead of the boundary. This shield of
precipitation will be the main feature of the weather today over
the region. North of I-90...the precipitation will be somewhat hit
or miss with lesser amounts of available moisture and deep ascent.
Meanwhile...south of I-90...the precipitation will begin to
blossom as the base of the aforementioned shortwave trough tracks
northeast...moving into SW Montana by afternoon. Various model
guidance is all consistent on producing a MISSPEND of heavier
precipitation across the southern Panhandle. The NAM is the most
aggressive solution and produces a broad swath of precipitation
amounts from 0.20-0.40 inches this morning. Some of this
intersects southern Shoshone County...impacting the towns of Avery
and Clarkia. The other solutions are insistent the heaviest
precipitation band will remain over the Clearwater Mountains and
points to the south. This scenario leaves most of the Inland NW
free of moderate to heavy precipitation for this morning.

The base of the trough is forecast to move into SW Montana by late
afternoon/evening which allows shortwave ridging to rebuild over
most of the area. This should bring a brief drying trend to the
region...however residual moisture and instability will keep a
threat of showers going along the ID/MT border as well as near the
Canadian border.

The shortwave ridge is expected to shift from the Cascades this
evening and into NW Montana by daybreak on Wednesday. This
transition will mark an increasing threat of precipitation once
again beginning near the Cascades and spreading into the Okanogan
Highlands and Wenatchee overnight. This round of precipitation
should be wetter than what the current front is delivering. These
differences are related to higher amounts of precipitable water
and much stronger isentropic ascent. The precipitation will also
be aided weak to moderate orograpic ascent from south-southeast
winds in the mid-atmosphere. The threat of precipitation will
persist through most of Wednesday and Wednesday night. fx

Wednesday through Saturday...

...SUBTROPICAL MOISTURE FETCH TO BRING HEAVY RAIN TO THE CASCADES
WEDNESDAY...

Wednesday...Forecast models continue to show an atmospheric river
containing subtropical moisture getting picked up by a deep closed
low off the Central BC coast...with moisture slamming into the
Cascades on Wednesday...with rain also spilling into North Central
and Northeast Washington through the afternoon. 850mb winds out of
the south-southeast may result in significant spill over from the
crest into the valleys with 1-2 inches of rain not out of the
question with up to 3 inches along the Cascade crest. This long
duration rain may be enough to cause small debris flows over the
burn scar areas. With rivers and streams at their climatologically
lowest stages these rain amounts are not expected to cause
flooding. Elsewhere timing of when the moisture plume sags
southeast has slowed a bit with the 00z model runs with the
exception of the ECMWF. Thus POP`s were lowered east of a line
from Ritzville to Spokane to Sandpoint as rain in these areas may
hold off until Wednesday evening. 850mb winds out of the south-
southwest over the northern mountains will provide strong upslope
flow where around an inch of rain is possible. These same winds
will downslope off the Blue and Wallowa Mountains with the
Lewiston area getting the least amount of rain with this system.

Thursday...The atmospheric river shifts south across Oregon
possibly clipping far southeast Washington. Central and Eastern
Washington and North Idaho will be on the cool side of a 140kt
upper level jet supporting more of a showery scenario. With 850mb
winds out of the southwest at 30 kts and lack of isentropic
lift...a more pronounced rain shadow east of the Cascades is
expected. POP`s were lowered for much of the Columbia Basin
extending to Spokane and the Palouse on Thursday. Afternoon sun
breaks in combination with stronger winds aloft should result in
breezy conditions over the Upper Columbia Basin, Spokane area, and
Palouse.

Friday and Saturday...GFS and ECMWF models show the development of
a 990mb surface low off the Oregon coast tracking north to off the
Washington Coast. The GFS shows the low closer to the coast and
shows more precipitation and wind potential compared to the ECMWF.
However both models indicate a warm front lifting north into
Washington and North Idaho late Friday afternoon into Friday night
with easterly flow favoring the East Slopes of the Cascades and
adjacent valleys and POP`s were increased to likely over a broader
area with chance POP`s elsewhere. Then the cold front passage
Saturday will keep precip chances alive especially in the
mountains. Snow levels look to remain high through the period with
the only the higher peaks being cold enough for snow.  JW

Saturday night through Tuesday night: An active weather pattern
remains over the Inland Northwest, with additional chances for
precipitation into early next week. The trough will finally move
inland on Sunday, which may be followed by somewhat of a break in
the wet weather over the lowlands. However, the approach of
another storm system in the Monday night/Tuesday time frame will
bring additional chances for rainfall and high mountain snowfall.
This system looks to ride on another deep moisture fetch, which
could bring heavier rainfall to at least the Cascades.

Another thing to watch, although it doesn`t look like it will be
too much of an issue at this point, are snow levels. Snow levels
may fall low enough that some of the mountain passes could be
affected. Something to watch in the coming days. Otherwise,
temperatures should finally fall back to near normal late October
values, at least during the afternoons. Low temperatures may be a
bit above normal. ty

&&

.AVIATION...
06Z TAFS: Moderate to heavy rain occurring along an elongated
frontal zone slowly passing through Eastern Washington and North
Idaho overnight and early Tuesday will bring MVFR ceilings and
visibilities at times. Wind should shift and be somewhat breezy
behind the front with a lesser chance of showers. There may still
be some possibility of fog and low cloud formation due to this
rainfall either tonight into tomorrow morning and again tomorrow
night. /Pelatti

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        57  43  59  47  59  44 /  20  10  50  90  50  20
Coeur d`Alene  56  40  57  45  58  43 /  40  10  40  90  60  20
Pullman        56  42  62  47  58  46 /  30   0  20  80  60  40
Lewiston       63  43  66  50  62  49 /  40   0  10  20  60  50
Colville       58  43  54  46  58  44 /  30  30  90 100  70  40
Sandpoint      55  40  55  44  55  40 /  50  10  50 100  90  40
Kellogg        51  39  57  44  52  42 /  80  10  20  90  80  50
Moses Lake     65  46  59  49  64  45 /   0  20  80  70  20  10
Wenatchee      63  48  55  46  61  45 /   0  70  80  80  20  20
Omak           61  45  52  45  60  43 /  10  70 100  80  50  10

&&

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

$$




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