Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 222343

Area Forecast Discussion
National Weather Service Spokane WA
442 PM PDT MON SEP 22 2014

Warm temperatures are expected again on Tuesday, but fall type
weather is on the way for the end of the week. A slow moving cold
front will bring good chances for rain to central Washington on
Wednesday and Thursday. Cooler temperatures will also accompany
the cold front Thursday and Friday. A slight warming and drying
trend is expected to occur next weekend.


Tonight and Tuesday...Satellite imagery shows several waves coming
around an upper level trough in the Gulf of Alaska that will
impact the region this week. The first one is the weakest as it
moves into an upper ridge. This wave this afternoon is over
southern British Columbia and earlier brought isolated
thunderstorms over the East Slopes of the Cascades. However early
in the afternoon this convection weakened with mainly virga or
light showers falling as of 2 pm from Ritzville to Omak. This
trend should continue into this evening.

Next wave of concern is a bit stronger and approaching the south
Oregon coast. Increased lift with this wave and surface based CAPE
values of 300-500 J/kg over NE Oregon has led to some cells
popping up on radar and a few of these may develop over the
Washington Blue Mountains early this evening. The brunt of the
lift with the wave however will pass through overnight into
Tuesday morning. Model qpf varies quite a bit due to varying
degrees of mid level instability with the NAM and GFS solutions
generating quite a bit of elevated convection. The GFS focuses the
Blue Mountains, Palouse, Lewiston area, and Central Panhandle
Mountains while the NAM focuses further west from Ritzville to
Spokane to Sandpoint. With the ECMWF also favoring an axis of
convection similar to the GFS...will favor this solution although
thunder can not be completely ruled out over Northeast Washington
into the Northern Panhandle.

After this wave passes...attention turns to the Cascades and
northern mountains Tuesday afternoon. Isentropic lift will be on
the increase with an approaching warm front. Light rain or showers
may develop...and over the Idaho Panhandle isolated thunderstorms
are possible with uncapped CAPE around 200-400 J/KG. Whether or
storms develop or not will depend on the amount of sun breaks in
the late morning/early afternoon hours. JW

Tuesday Night through Friday: The morning model runs are in decent
agreement for the Wednesday through Friday time frame. The 12z GFS
and 12z ECMWF show a large upper level trough moving into western
and central Washington Wednesday, getting hung up Wednesday night
just east of the Cascades, then limping eastward Thursday and
Thursday night. If this scenario verifies, portions of central and
north central Washington will receive soaking rains Wednesday and
Thursday. The north Idaho Panhandle and the eastern third of
Washington would probably receive rain of a more spotty nature
Thursday and Friday as the front undergoes a great deal of
shearing and stretching. The GFS and ECMWF dig the slow moving
upper trough deeply into the desert southwest by Friday
leaving places like Spokane, Kellogg, and Sandpoint mainly dry
until potentially Saturday. Since the shortwave disturbances that
will be diving into the trough Thursday and Friday are coming off
the Asian coast today, expect to see some model variability. These
disturbances will be over a relatively data sparse region for a
day or two. Even though the 12z GFS and ECMWF are in reasonable
agreement, forecast confidence is below average for the second
half of the week. However, confidence is high that our much warmer
than average weather will come to an end Thursday and Friday.
Summer will take a serious hit by this incoming trough. /GKoch

Saturday through Monday: We start the period under a trof of low
pressure. This feature begins to migrate east as a second wave
comes off the Pacific. Eventually, the second trof of low pressure
moves over the Inland NW. The presence of the pair of troughs is
well agreed amongst the medium range models...however how fast
this transition occurs carries high uncertainty. What looked to be
a 48 hour break between the features has shrunk with model trends
this morning showing signs of phasing these features. So what does
this mean? A forecast leaning toward the GFS, which maintains two
distinct features, indicates moisture and light pcpn to mainly
skirt the ID Panhandle and far eastern reaches of WA Saturday
giving way to dry conditions region-wide Sunday under shortwave
ridging. The second wave and associated cold front bringing the
next chance for light showers would arrive Monday. On the other
end of the spectrum, the ECMWF which now phases the two waves,
favors unsettled weather over the entire region through a majority
of the weekend. The only exception would be the deep Basin due to
localized downsloping off the Cascades. A closer examination of
ensemble means and run to run consistency suggest the GFS is
trending toward a weaker "in between" ridge but still illustrating
somewhat of a small break. I think the models will meet somewhere
in the middle over the next few days and opted not to make
wholesale changes yet...thus a mostly dry forecast is in place for
Sunday. Even following the ECMWF solution, Central WA is under the
western periphery of the longwave trof, influenced by a northerly
jet and this does not bode well for precipitation. Some agreement
by Monday comes with all guidance trending toward a more
progressive flow with the incoming wave. This would strengthen
westerly flow and continue to bring a rain shadow in the lee of
the Cascades. Temperatures have been lowered slightly and remain
on the warmer end of guidance but could be a handful of degrees
lower if the forecast trends toward a pure ECMWF solution in the
coming days. /sb


00Z TAFS: A couple weather disturbances pass through the Inland NW
over the next 24 hours. Isolated -shra will be a risk early this
evening, through 02-03Z, with a few -tsra possible near PUW/LWS in
the afternoon heating. The 1st weather disturbance with a broader
shower threat comes in between late evening and Tuesday morning,
with scattered showers possible at all TAF sites. A brief lull is
expected in the mid to late morning, before the second disturbance
approaches in the afternoon and keeps the threat of showers alive.
VFR conditions are projected, but lcl MVFR conditions possible in
any heavier showers. /J. Cote`


Spokane        60  76  58  75  52  72 /  40  30  10  30  20  20
Coeur d`Alene  58  77  56  78  50  73 /  30  30  10  20  20  20
Pullman        57  79  54  78  50  73 /  40  20  10  10  10  20
Lewiston       63  85  59  84  56  79 /  50  20  10  10  10  20
Colville       57  76  55  74  49  75 /  30  30  20  60  40  50
Sandpoint      54  72  52  74  48  73 /  20  40  20  20  20  20
Kellogg        56  75  54  79  49  75 /  20  30  10  10  10  20
Moses Lake     61  80  57  75  53  74 /  20  20  10  50  40  50
Wenatchee      62  76  60  72  56  72 /  10  20  30  50  40  50
Omak           58  75  58  74  53  73 /  20  30  30  60  40  50



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