Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 230955
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
254 AM PDT WED APR 23 2014

.SYNOPSIS...
Occasional precipitation chances, breezy conditions and relatively
cool temperatures dominate the weather pattern. Passing rain and
snow chances Wednesday are followed by steadier rain chances late
Wednesday night into Thursday, followed by a threat of more
scattered showers, and perhaps some thunder, Thursday afternoon.
Showers will remain a threat into the weekend, before a relative
lull comes for the start of the new work week.

&&

.DISCUSSION...
Today and tonight: A threat of rain and snow showers returns
to the Inland NW today, while steadier rain develops late tonight.
This morning a tight 500mb shortwave was entering the Cascades,
with weak 850-700mb theta-e ridge leading it into central and
eastern WA and north ID. That latter feature will provide focus
for the moisture and increasing precipitation across the Cascade
crest and the Okanogan to Upper Columbia Basin, spreading into the
eastern third of WA and north ID this morning. There is some
disagreement in how quickly precipitation develops or how
widespread it will actually be. The HRRR actually shows very
little in that way of significant precipitation; however the
GFS/NAM/EC/SREF are more generous. At 930Z clouds were starting
to thicken near the Cascades and expand east, while a few light
radar returns were developing east of Othello. I don`t see any
evidence that anything is actually reaching the ground at this
time, but leaning toward the more dominant model solutions suggest
this will change. So I gradually increase PoPs to likely through
the morning, especially across southeast WA to the central
Panhandle.

For late this morning to early afternoon the 500mb shortwave
comes on the heels of the 850/700mb wave, before shifting into
Montana in the late afternoon. With 500mb temperatures in the core
of this disturbance in the -25 to -31 C range and a swath of
100-200 J/kg of CAPE and marginally negative LI values, this will
mean a continued shower threat across the eastern third of of WA
and north ID this afternoon, before dissipating in coverage by
early evening. I included a slight chance of thunder toward the
northeast mountains, but even then I think any thunder that
develops would be more isolated in nature, if any develops at all.

Looking at current wet-bulb zero heights and regional temperatures,
the precipitation will have a modest opportunity to fall as snow
across a large portion of the region, save for the lower Palouse
to the deeper Columbia Basin and the L-C valley. The threat for
more than an inch of snow, however, will remains toward the
Cascades and the Panhandle Mountains, and perhaps toward the
higher Idaho Palouse. The threat for accumulating snow outside of
the mountains will also be limited to early in the day. However
the convective or unstable nature of the atmosphere under the
trough will mean the potential for a rain/snow/graupel/small hail
mix with any showers that occur into the afternoon.

Late tonight into Thursday morning a warm front comes in from the
southwest. This will shift the region to more stable regime.
Strengthening isentropic ascent and subtropical moisture plume
come the warm front as the night wares on. PWATs rise to between
0.50 to 0.75 inches (or ~150-200% of normal). This favors another
swath of precipitation, enveloping much of the region from the
Columbia Basin toward the northern mountains between 04Z-11Z (or
~9 PM to 4 AM). I increased precipitation chances to likely for a
large portion of the region going past 06Z. By this time the
milder air surges in, pushing wet-bulb zero heights and snow
levels up toward 3500-4000 feet around the mountains of northeast
WA and north ID and to around 4500 to 5500 feet elsewhere. In
other words the next system looks to be more of a valley rain and
high mountain snow event. /J. Cote`

Thursday through Friday...Chances are very good that the recent
spurt of wet weather will continue through this period as well.
The latest water vapor and shows a developing baroclinic band
near 150w with a nice tap into sub-tropical moisture per the
blended precipitable water product. Models in very good agreement
that this atmospheric river will slide into SW Washington
Wednesday night and gradually spread northeast through Thursday
morning in the form of a warm front. Precipitation will likely
impact most locations...including the lee of the Cascades due to
moderate isentropic ascent resultant of the mid-level southwest
winds. Most of the stratiform precipitation will fall in the
morning...as the atmospheric river is expected to shift east of
the region by afternoon. Before it shifts east most of the region
will see measurable rainfall with amounts ranging from .20 to .40
inches over most valley locations...with heavier amounts over the
mountains of north Idaho and the Cascade Crest. This bout of rain
will be followed by a weak negatively tilted trough during the
afternoon...somewhat similar to yesterday. Whether or not the
moisture from the morning rain can combine with afternoon heating
to produce deep convection and possible thunderstorms is the
question. Yesterdays window of opportunity was too short to tap
into the moisture and thus we never saw thunderstorms. The
Thursday setup isn`t ideal but it should be a little better than
what we saw. Thus we will continue to mention a chance of
afternoon and early evening thunderstorms. The best chances will
occur east of a line from Republic to Ritzville. Model instability
isn`t supportive of any strong convection as the CAPE values are
too low for strong updrafts.

The threat of thunder will wane Thursday evening...and most of
the area will see a brief drying trend...with one exception. The
upper level trough responsible delivering the atmospheric river
will shift onto the Washington coast by morning...which is
expected to send a shortwave trough in from the southwest. All the
models are showing a similar scenario...with the track of the
shortwave expected to impact the southeast corner of Washington
into NC Idaho...due to the proximity of a low-level theta-e ridge
axis. Moderate rainfall amounts will be possible in this region
with locations such as Lewiston...Pullman...and the Silver Valley
seeing the possibility of up to a quarter inch. Across the
remainder of the forecast area the possibility of significant
rainfall is considerably lower. That notion will likely continue
until afternoon at which time the upper level trough will likely
shift into the Cascades. This should result in diurnally
developing convection...however lapse rates aren`t really all that
impressive. Most of the convection will likely fire over the
mountains due to terrain forcing.

Friday night through Monday...The upper level trough or some
semblances of it will likely remain fixed over the Pacific
Northwest through this period as well. There are no signs of
seeing a good fetch of moisture...or strong synoptic forcing at
least through the weekend. So we are primarily looking at an
unsettled weather pattern with generally dirunally driven shower
activity. Most of the convection will fire near the Cascades and
the Idaho Panhandle. By Monday there is some support for a weak
shortwave ridge. This should lead to slightly drier weather but
based on the amplitude of the ridge...we don`t want to go
completely dry. fx

Monday Night through Wednesday: The extended part of the forecast
which yesterday featured an amplifying ridge, now has gained some
uncertainty. Most recent GFS model runs now have the ridge sliding
further to the east over MT allowing the offshore trough to advect
an abundance of subtropical moisture into our region leading to
increased chances for more precipitation. While the GFS has
trended wetter, the Euro still advertises the ridge remaining
entrenched right over our area keeping the storm track to the
west. This would lead to the dry conditions the models were
showing yesterday. Due to the model uncertainty I did increase
POPs slightly to include the mention of showers for higher
elevation locations along with the eastern third of WA and the
Panhandle. Confidence is not high with this change, but thought it
would be worthwhile to at least move POPs towards climo for the
time being. Other than POPs, not many changes were made to the
period due to lower confidence. Many factors for this portion of
the forecast will depend on the verifying model and will have to
be fine tuned once model agreement is better. Given the models
right now, the active GFS would bring continued wet conditions
with below normal temps while Euro would bring above normal temps
and drier conditions. For the time being we will just have to wait
and see how the models trend over the next few runs. /Fliehman

&&

.AVIATION...
06Z TAFS: An upper-level trof is currently anchored over the
region. There is a break in the precipitation at this time but
clouds/showers will begin crossing the Cascades btwn 6z-
9z...spreading toward the ID/WA border 13-15z. The heaviest pcpn
with this feature looks to focus across SE WA and the lower ID
Panhandle. Snow will be possible down to 2000` but most
accumulations will focus at elevations of 2500-3000` and higher.
Showers will carry the potential for MVFR cigs and any intense
snow shower could brielfy lead to IFR conditions. Any snow should
have a hard time accumulating on area runways. Snow levels will
rise above all terminals aft 17z with breezy winds and sct -shra
through 23z. A small break arnd 00z will be followed by lower cigs
and incr -ra chances aft 03z as another moist warm front lifts
into the region. /sb



&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        51  39  55  38  54  37 /  80  70 100  40  40  20
Coeur d`Alene  50  38  53  37  53  37 /  80  60 100  70  50  20
Pullman        50  41  54  39  50  37 /  80  70 100  50  50  20
Lewiston       56  45  60  44  55  42 /  60  50 100  60  70  20
Colville       57  35  59  35  61  36 /  50  50 100  60  40  20
Sandpoint      49  36  51  36  54  37 /  70  50 100  80  50  30
Kellogg        45  36  50  36  52  36 /  80  60 100  70  70  40
Moses Lake     61  44  65  40  61  38 /  20  70  70  10  10  20
Wenatchee      60  45  64  43  61  41 /  20  60  70  10  10  20
Omak           59  39  62  36  62  35 /  20  50  80  10  20  20

&&

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

$$




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