Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 222344
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
444 PM PDT WED OCT 22 2014

.SYNOPSIS...
A very moist frontal boundary will produce heavy rain and high
elevation snow for the mountains and steady rain in the valleys
and basin tonight. Breezy and unsettled conditions will continue
through Thursday. A strong cold front will produce windy conditions
Saturday night into Sunday morning. The pattern will continue to
be active into early next week. This includes the potential for
widespread rain into early next week, as moisture associated with
former typhoons makes its way toward the Pacific Northwest.

&&

.DISCUSSION...
Tonight through Thursday...Pacific satellite loop displays a
baroclinic leaf cloud structure developing over western Washington
this afternoon...indicative of a frontal couplet featuring a warm
front running west to east over eastern Washington followed by a
cold occluded front running southwest to northeast just off the
Pacific Coast. This frontal complex is being fueled by a deep and
well directed fetch of sub-tropical moisture. The passage of this
sopping wet frontal complex will drive the weather for the next 24
to 36 hours. The entire forecast area is or soon will be in the
thick of this wet storm...with surface observation already
recording a general 1/2 to 1 inch of rain and high mountain snow
WE over the Cascades in the last 12 hours...with even the
normally dry deep basin locations receiving from a few hundredths
to a tenth of and inch already this afternoon. The surface
pressure gradient is producing a downslope southeast wind field
over the Palouse and points south which is retarding the onset of
precipitation.

Two regimes or rounds will occur over the next 24 hours. The
first round will occur this evening over the west and through
tonight over the east and will be characterized by steady light
to moderate stratiform rain enhancing isentropically over the slow
moving or nearly stationary warm front. Low level upslope flow
into the Cascades will assure continued precipitation even over
the normally dry deep basin. Snow levels will remain around 6000
feet in this warm advective scenario. The currently dry Palouse
and LC valley will succumb to the rain later tonight as the fetch
becomes better directed and in particular as the robust lift
along the occlusion moves through towards dawn.

The arrival of the occluded front will mark the beginning of the
second round. Passing trough the Cascades around midnight and into
the Idaho Panhandle around dawn this front will bring an area of
dense and occasionally heavy showers just ahead of it...followed
by a quick shut off of significant precipitation but a quick
increase in winds especially over the exposed terrain of the
basin. A moist orographic regime will dominate with the usual rain
shadow off the Cascades and continued orographic showers feeding
into the northeast Washington and Idaho Panhandle mountains.

Winds on Thursday look to be solidly breezy with gust potential up
to 30-35 mph or so on exposed terrain but the models are similar
in depicting a gradient that will not justify any wind highlights
at this time. These winds will help keep temperatures on the warm
side of normal in a well mixed and adiabatically dominated surface
layer.

Hydrology Issues...rainfall totals from this storm will range
from over 2 inches near the Cascade Crest to 1 to 2 inches in the
mountains north and east of the basin. Lowland locations will
range from 1/4 to 1/2 inch in the basin to near an inch or so in
the valleys branching off the basin. River levels appear to be low
enough to be able to handle this runoff with no trouble but
smaller streams and tributaries draining the mountains will likely
see significant rises but probably no small stream flooding. The
wild card issue is how the recent burn scars in the Cascades will
handle this soaking rain. Rain rates will not be sufficient to
trigger classic flash floods but as the soil becomes saturated
there is a possibility of denuded slopes becoming unstable and
producing debris flows. These debris flows in gullies and stream
beds could also produce debris dams in the channels and lead to a
higher threat of dambreak flooding or "Ice Jam" type upstream
flooding. There is a great deal of uncertainty and little
experience/case history with these possibilities and these areas
will need to be monitored closely over the next 24 hours. /Fugazzi

Thursday night through Friday: Moist and unstable upslope flow
will produce lingering showers across the ID Panhandle. These
showers will be most numerous Thursday evening and taper off
through Friday. All of the rain from the current weather system
impacting the region will result in substantial boundary layer
moisture. This will increase the threat for fog overnight Thursday
into Friday morning as mid level clouds clear west to east across
eastern WA. Friday will be fairly benign with the region in
between weather systems. High temperatures will be near to or
slightly above normal in the 50s to low 60s.

Friday night through Saturday night: A vigorous shortwave trough
of lower pressure will pivot around the upper level low pressure
system in the Gulf of Alaska on Friday. In addition, a moderate to
strong jet streak will be draped across the state of WA. A strong
vorticity maximum at the base of the shortwave trough and favorable
jet dynamics for large scale lift in the right entrance region of
the jet streak will result in rapid surface cyclogenesis along
130W off the coast of northern CA and southern OR. Models show
good agreement with this surface low tracking northeast into
northwest WA on Saturday. The GFS is on the faster side of the
medium range model guidance available, but good agreement overall.

The warm front is progged to push north across the region Friday
night. This will result in increased low level flow out of the
east. The front itself does not appear to be particularly strong
or moist. Easterly flow down the east slopes of the ID Panhandle
Mountains will result in some downward motions across these areas
out into the eastern basin. This will counteract the lift along
the warm front somewhat. Best chances for precip will likely be
across the western portion of the forecast area, and more so in
the East Slopes of the Northern Cascades where easterly flow will
create some orographic enhancement. Temperatures will likely be
fairly mild across the eastern portion of the forecast area
behind the warm front on Saturday.

Cold front passage with this system is expected Saturday night.
This will result in increasing chances for precipitation from west
to east along the front. Precipitation with this system is not
expected to be much of a concern with light to moderate amounts
anticipated. The greater concern with the front will be the
winds. There is still quite a bit of uncertainty still at this
time, but the surface low will be pushing inland across northwest WA
near its peak intensity. The 12Z GFS and NAM model guidance has
come in with an 850 mb jet of around 50-55 kt winds. There will be
some decent cold air advection with the front, so I anticipate
these winds will have a chance to mix down to the surface even
though it will push across after sunset. These solutions are much
stronger than previous runs, so confidence is still only moderate
at this time. A wind highlight may be needed if these stronger
solutions continue as the event approaches. /SVH

Sunday through Wednesday: Models continue to depict an active
pattern, with temperature held closer to seasonal averages;
however agreement over the details decreases in the new work week.

Sunday and Sunday night a surface low over southwest Canada
tracks east. A trough trailing it sags across northeast WA and
north ID, weakening and pulling away by Monday morning. In tandem,
the supporting upper trough migrates from the Pacific NW to the
northern High Plains, with the steering flow switching from
southwest to west-northwest. Moisture and low-grade instability
wrapped up in these features will continue providing shower chances
across much of the Inland NW. Yet the best chances will be near
the Cascade crest and the mountains of northeast WA and ID; the
lowest chances will be in the lee of the Cascades and L-C Valley.

From Sunday night into Monday the threat will retreat to just the
mountain areas and if anything falls it looks light. There will
be a stratus and patchy fog threat over the eastern Columbia Basin
into the eastern mountain valleys in the night and morning hours
too, given the lower level southwest flow and moisture provided
from recent rains.

From Monday night through Tuesday night the next system comes in,
ostensibly with another modest moisture fetch which includes the
remains former tropical system Ana. However there is wide
variation amongst models over the precise track and timing of the
system. Some runs bring a defined low toward the central BC coast
and others bring a smaller scale low toward the northern Oregon
coast. The further south solutions offer higher precipitation
amounts for the Inland NW and the further north solution shows
more diffuse, lighter precipitation. These details will continue
to be monitored and fine-tuned as we get more information. Right
now look for thickening clouds on Monday night with the main rain
chances increasing over the Cascades. The precipitation chances
increase across the rest of the area going into Tuesday and
Tuesday night as the system moves inland. By Wednesday models
depict a shortwave ridge builds in, leading to a relative decrease
in the precipitation threat and some stratus/patchy fog threat
again over the eastern third of WA and north ID, especially in the
sheltered valleys. Rain chances will begin to increase from the
west again late as that next system approaches. /J. Cote`


&&

.AVIATION...
00Z TAFS: A very moist stream of Pacific moisture will enhance
over a slow moving developing warm front draped west to east
across the region today with gradually deteriorating conditions at
all TAF sites except KPUW and KLWS...where dry downslope winds
will promote increasing clouds but generally dry conditions today.
MVFR ceilings likely in RA at KEAT and possibly KMWH tonight. At
the KGEG area TAF sites mostly VFR showers are expected with
steady stratiform rain and occasional MVFR ceilings developing
after 04Z. Tonight a trailing occluding front will sweep through
the region with heavier rain along and immediately behind the
front. A moist boundary layer even after the FROPA will likely
promote continued MVFR (and possibly IFR) stratus ceilings
through 20Z Thursday at the KGEG area TAF sites and KEAT. Gusty
winds will help to lift low stratus after 20Z Thursday. /EK


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        48  59  43  57  44  63 / 100  50  50  10  20  20
Coeur d`Alene  46  58  41  57  41  64 / 100  70  50  20  20  20
Pullman        48  59  45  59  47  69 /  90  80  50  10  40  10
Lewiston       53  63  48  64  46  72 /  40  60  50  20  40  10
Colville       48  57  41  58  41  60 / 100  60  50  10  50  30
Sandpoint      45  55  38  56  38  60 / 100 100  70  20  30  20
Kellogg        46  54  41  53  42  62 /  90  80  80  30  20  10
Moses Lake     50  64  43  61  45  63 /  90  20  10  10  40  20
Wenatchee      49  61  44  57  47  58 / 100  30  10  10  60  40
Omak           48  59  40  58  44  58 / 100  40  10  10  60  40

&&

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

$$




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