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Area forecast discussion 
National Weather Service Spokane Washington
258 am PDT Wednesday Aug 20 2014

Synopsis...
cooler temperatures and the occasional threat of showers and
thunderstorms will linger the rest of the week and into the
weekend. However, the vast majority of this precipitation will be
in the north Idaho Panhandle.

&&

Discussion...
Wednesday and thursday: an upper-level trof will continue to deepen
over the inland northwest during this time-frame bringing cooler temperatures,
areas of showers and thunderstorms, and locally breezy winds.

A strong northwest to southeast oriented jet stretching from the Gulf
of Alaska to Astoria, or will feed shortwave energy into the longwave
trof with three distinct waves expected within the next 48 hours.
The first wave is currently crossing through ern Washington and utilizing
a rich and unstable air mass expanding from NE Oregon...through
much of the Idaho Panhandle...and across northeastern Washington. This can be
traced well on the GFS 700mb Theta-E surface. The presence of this
air mass and midlevel forcing is clearly evident when viewing the
2am radar which continues to show isolated thunderstorms going
strong through Kootenai and Shoshone counties. A midlevel dry slot
is slowly working its way east through The Gorge into ern Washington and
should stabilize the air mass over southeast Washington and southern portions of
the Idaho Panhandle throughout the morning hours.

Meanwhile...the rich Theta-E air will remain anchored over the
northern mountains and upper reaches of the basin mainly north of
Hwy 2 delivering another afternoon and evening of showers and
thunderstorms. Instability parameters do not differ much from
Tuesday which implies brief heavy rain, small hail, and wind gusts
to 40 mph will be the main threats. There is some forcing expected
to translate through southern areas of the County Warning Area but the atmosphere
does not look to recover following this morning's dry slot and
will continue a mainly dry forecast for the afternoon and evening.
Any showers and storms for the Cascades looks to be north of Lake
Chelan but this even carries some uncertainty. This second
shortwave which passes through this afternoon is difficult to make
out on satellite this evening but can generally be traced to the
area of darkening entering northwestern Washington. This wave will
continue to deepen as the day progresses as it passes through an
area of increased ascent within the left exit region of the
approaching jet streak. The shortwave is expected to make a quick
passage through southern Idaho by early evening. As this occurs, the
axis of instability and deeper moisture across the northern
mountains will be pulled back to the S/southeast and renew the threat for
showers and storms into the upper reaches of the Columbia Basin,
Spokane-cda area, and Idaho Panhandle.

On Thursday, a third and stronger shortwave trof will dig into northwest Washington
by the afternoon and cross the Cascades by early evening. This
will bring the strongest forcing of the next 48 hours. We will see
some drying of the air mass as it continues to cool pushing
precipitable water (pwats) down from 0.90" on Wednesday to 0.75"
Thursday across NE Washington while the Cascades and Lower Basin generally
reside between 0.40-0.50". The greatest risk for thunderstorms
will continue along the northern mountains from northwest Montana to the
northern Cascades with a decreasing risk for the upper Columbia
Basin. Dry conditions will prevail for the Columbia Basin,
Wenatchee area, Palouse, and blue mtns. The synoptic pattern will
transition from the passage of open shortwave trofs toward more a
closed low over the region. This does raise some concern for near
stationary storm movement, especially near burn scars. The
moisture is not as plentiful as we typically see with such events
but if afternoon heating is strong enough and surface based cape
exceeds 900-1000 j/kg...flash flooding and mudslides will be a
large concern.

Temperatures will cool back near seasonal readings or upper 70's to
80's which will end the current four stretch of above normal
warmth. /Sb

Thurs nt through Sunday nt: we're now seeing a clear westward
trend in run-to-run model guidance in the path of the deep upper
low that digs south through British Columbia and into central WA by thurs nt.
The result will be an increased pcpn threat across the Cascades
thurs nt and Fri. However, the Cascades will only be subjected
to the first band of pcpn forced at least partly by the vort Max
that coincides with the dry slot over the Columbia Basin. But
after this first wave occludes, we're now seeing new cyclogenesis
well to the south ahead of the upper low that quickly produces a
significant band of pcpn across southeast WA and the Idaho Panhandle Fri
nt through Sat nt. This band of pcpn, associated with a well-
defined trowal, will be much stronger than the first... which is a
change from what model guidance has been showing the last few
days. That said, we trended (once again) to add more pcpn across
the southeast WA and north Idaho zones. A third wave embedded in the northwest
steering flow quickly moves southeast into the region Sunday nt with yet
another round of showers and embedded thunder. These waves will
continue to keep the fcst relatively cool. Bz

Monday through Tuesday night: models have come into decent
agreement suggesting the cool and showery pattern will come to an
end early next week as the jet stream finally slides back north.
This would place the region on the warmer and more stable side of
the jet stream leading to more Summer-like conditions. The final
wave of energy looks to pass Monday into Monday night keeping
shower/T-storm chances alive. Best chances will be in NE
Washington and the Idaho Panhandle, especially in the higher
terrain of these regions. Winds will also be on the increase with
the passing wave as mid level winds will have a better potential
of mixing down to the surface.

By Tuesday much drier air makes it into the region as a ridge of
high pressure takes control in turn drastically limiting the
chances for precipitation. With the ridge, temperatures will be on
the rebound pushing back into the 80s for most by midweek under
clearing skies. Overall this time frame appears to be the rebound
period as temperatures shift back above normal and the chance for
precipitation greatly diminishes. /Fliehman

&&

Aviation...
06z tafs: isolated showers and thunderstorms will slowly shift
east across northeast Washington into north Idaho through the
overnight hours, remaining north of kgeg and kcoe. Meanwhile,
isolated high based showers may creep over the Blue Mountains to
the klws area overnight. Winds in keat and kmwh will gradually
subside, but expect them to pick up again with daytime heating
after 20z. Surface based instability will increase Wed afternoon
with the best chance of convection across the northern mountains.
Some of these storms may approach the kgeg-kcoe corridor after
21/00z. /Rfox.

&&

Preliminary point temps/pops...
Spokane 80 56 79 52 75 52 / 20 20 10 10 20 30
Coeur D'Alene 79 55 77 50 74 50 / 20 20 10 10 30 40
Pullman 80 50 79 46 76 51 / 10 10 10 10 20 40
Lewiston 85 59 85 57 83 58 / 20 0 0 0 20 30
Colville 81 54 79 52 78 50 / 40 30 50 50 40 30
Sandpoint 77 52 73 51 72 50 / 40 30 50 50 50 30
Kellogg 75 53 73 51 71 49 / 30 20 20 20 40 50
Moses Lake 86 56 84 53 82 56 / 0 0 10 10 20 20
Wenatchee 84 60 83 59 80 59 / 0 0 10 10 20 20
Omak 86 59 84 55 80 55 / 10 10 20 40 30 20

&&

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

$$

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