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Area forecast discussion 
National Weather Service Spokane Washington
419 am PDT Monday may 4 2015

Synopsis...
warm and dry conditions will persist today with widespread 70s
returning to the region. A strong cold front is expected tonight
and will likely return our temperatures into the 60s Tuesday.
Cooler and breezy weather is in the forecast for Tuesday. Spotty
showers will be possible Tuesday and Wednesday as well as a chance
of thunderstorms on Wednesday. The weather will generally dry and
warm through the end of the workweek, except for a small chance of
mountain showers over the Idaho Panhandle. Temperatures could
moderate a bit by the weekend as another weak disturbance
approaches the inland northwest.

&&

Discussion...
today through Tuesday...weather changes are ahead. Satellite shows
a well defined closed low off the southern British Columbia coast. Models are in
good agreement that this low will track over Vancouver Island
tonight...reaching the Washington Cascades Tuesday evening. This
system will usher in a strong cold front passage tonight bringing
high temperatures back down to typical early may values on Tuesday.

For today...the warmest day of the upcoming week is expected.
Prefrontal southwest flow and continued abundant sunshine for most
will lead to additional warming with widespread valley highs in
the 70s...with the warmest spots such as Moses Lake and Lewiston
in the low 80s. Prefrontal southerly winds will channel up the
Okanogan Valley this afternoon with breezy conditions expected.
Regarding convection, models show two areas of interest ahead of
the cold front. First being the Cascades late this afternoon and
early evening. Cape values are marginal...around 200 j/kg so only
a slight chance is warranted for the Pasayten Wilderness area.
Second this evening from the Blue Mountains to the central
Panhandle mountains including the Palouse and Lewiston area.
Models continue to show an area of elevated instability with most
unstable cape values around 100 j/kg. Again values are marginal
for thunderstorms...but a few strikes can not be ruled out.

For the remainder of the area a large dry slot coming around the
low will mix down dry air behind the cold front. Wind gusts of
15-30 mph in the valleys and 25-40 mph in the mountains will be
common with the front passage tonight. Most of central and
northeast Washington into the north Idaho Panhandle will remain
dry under this dry slot through Tuesday afternoon. As the cold
pool aloft approaches the Cascades Tuesday afternoon models show
increasing instability once again with scattered showers expected
to develop over the mountains. The Okanogan Highlands may also see
some of this activity with the rest of the region dry Tuesday
afternoon. Southwest flow ahead of the low may allow some of
these showers to come off the high terrain into the valleys.
Isolated thunderstorms are also possible. Temperatures will be
about 15 degrees cooler on Tuesday compared to today behind the
cold front. Snow showers will be possible over the Cascade passes
as snow levels lower to 3500-4500 feet...but accumulations are not
expected. Jw

Tuesday night through Thursday...unsettled and relatively cool weather
will persist through this period as cold upper level low shifts
east of the Cascades. Models in good agreement that the low will
shift into the Columbia Basin on Wednesday afternoon with 500 mb
temperatures plummeting into the -28 to -30c range. This upper
level cooling will outpace the cooling in the lower atmosphere
resulting in the potential for deep convection and the resultant
threat of thunder. Where this thunder will occur is uncertain,
however the GFS would place the best chances along the northern
locations near the Canadian border as well as near the Cascades.
The NAM and sref are much broader in areal coverage and would
suggest a chance for thunder over most areas except perhaps the southeast
quarter of Washington and adjacent portions of north Idaho. Based
on the broad pool of cold 500 mb temps, we will hedge more toward
the broader coverage. Any thunderstorms which form will be nothing
more than the small pulse type variety as the vertical wind
profiles show with little directional shear while the MUCAPE
values are quite low (generally less than 500 j/kg). The threat of
thunder will likely wane quickly by Wednesday evening and into
Thursday as the 500 mb cold pool and upper level trough shears
south and east with the upper level jet dropping into northern
California. Despite these trends there will likely be just enough
residual instability combined with weak lift around the northern
end of the trough in southeast British Columbia to keep some showers in the forecast
for Thursday. Most of the activity will occur over the the
Panhandle and extreme eastern Washington. The chances are not
great as upper level forcing will be weak with limited potential
instability.

For Friday and beyond the 500 mb pattern changes very little...as
the inland northwest will lie between another sharp upper level trough
moving off the Washington coast while the northern end of the
current trough moves into prairie provinces and carves out a
stronger trough to our east. This pattern will present a fairly
unusual may pattern as cold air drops on the east side of the
Continental Divide with a thermal ridge forming over the Columbia
Basin. This will result in a prolonged period of low-level
northeast to north winds. This is a similar wind pattern to what
we commonly see with an wintertime Arctic intrusion, however in
this case we will not see the typical cooling. Yesterdays model
runs were suggesting the 850 mb winds would peak between 30-40
kts, but that notion has been backed off significantly. Only the
Canadian model is now showing winds above 20 kts. So we will
subsequently back off on the breezy wind forecasts, but keep the
NE orientation. As for precipitation, it appears the best chances
will remain near the Idaho/Montana border although chances will be small.
Meanwhile the offshore trough handling is quite uncertain. Some
model solutions keep it offshore through the weekend, while others
take it through the Cascades as a weakening negative tilted
trough. In either case we will put some small pops in the forecast
for the weekend to account for these differences.

As for temperatures, we will likely see a slow warming trend
through the work week with highs warming through the 60s on
Wednesday and into the upper 60s and 70s on Friday. By the weekend
these reading could moderate slightly depending on what the
offshore trough decides to do. Fx

&&

Aviation...
12z tafs: clear skies will continue through this afternoon for
most of the area. A strong cold front may trigger isolated showers
in the north Cascades after 21z today...and over southeast
Washington and the Lewiston area tonight. An isolated thunderstorm
also can not be ruled out in these areas. Given only isolated
coverage of showers expected at kpuw/klws confidence is not high
of either of these taf sites being impacted. Wind gusts of 15-25
kts are expected with the cold front passage tonight with the
strongest gusts at keat. Jw

&&

Preliminary point temps/pops...
Spokane 76 44 61 36 60 40 / 0 10 0 10 30 20
Coeur D'Alene 75 42 60 35 60 37 / 0 10 10 10 30 20
Pullman 75 41 60 34 58 37 / 0 20 10 10 30 20
Lewiston 81 49 66 39 64 43 / 0 20 10 10 30 20
Colville 77 45 65 38 63 38 / 0 0 10 30 40 30
Sandpoint 73 41 61 32 60 34 / 0 10 10 20 30 20
Kellogg 75 41 59 34 58 37 / 0 40 30 10 30 20
Moses Lake 80 44 66 38 65 40 / 0 0 0 20 20 10
Wenatchee 79 49 64 43 64 43 / 0 0 10 20 30 10
Omak 78 39 64 36 65 37 / 0 10 20 20 40 20

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...none.
&&

$$

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