Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 232336

Area Forecast Discussion
National Weather Service Spokane WA
436 PM PDT Sun Apr 23 2017

Expect cool, breezy, and showery weather this week. The most
widespread precipitation will likely occur Tuesday night into
Wednesday with the potential for a half inch or more of rain over
portions of the Idaho Panhandle and southeast Washington. A ridge
of high pressure next weekend has the potential to bring the
region a day or two of drier weather.


Tonight through Monday night: The cool and showery weather will
continue with an upper level trough off the WA coast. Moist west
southwest flow will remain over the region with upper level
diffluence. Surface based convection has flared up within this
region but weak surface based instability and cloud cover has
limited any lightning chances. Although there have been reports of
small hail with brief downpours as these storms track to the
northeast. A weak shortwave wave will pivot across the Cascades
this evening and help prolong the convection through the evening
hours across the Columbia Basin into northeast Washington. Some of
the storms could produce gusty winds. Winds, convection and cloud
cover will decrease slightly overnight in the wake of the passing
feature. Temperatures will cool a little more compared to last
night, although areas of stratus and patchy fog are possible in
the southern ends of the valleys. By the Monday, the upper level
trough swings inland and weakens over the eastern Washington. Weak
instability will be available but nothing too exciting. Showers
will increase from mid morning into the afternoon across the
region. QPF looks to be a tenth of an inch or less. Daytime
temperatures will remain cool in the 50s. The showers will taper
off overnight as the trough axis slowly exits to the east. Still
could see showers linger over the central Panhandle into southeast
Washington through the night. Clearing skies across north central
WA and parts of northeast Washington may lead to a return of
valley stratus and patchy fog. /rfox.

Tuesday through Wednesday...This portion of the forecast will begin
dry at least across central Washington as drier air spills over
the Cascades ahead of a shortwave ridge. This drier air will also
be initially coupled with downslope westerly winds in the lower to
middle atmosphere. Meanwhile the far eastern portions of
Washington and most of the Idaho Panhandle will feel the effects
of weak shortwave trough dropping SE out of BC during the day.
This feature will lead to some weak ascent coupled with residual
instability. This will likely result in more showers during the
day but they should be less widespread than what we expect to see
tomorrow. It looks like the instability is a little too shallow to
support thunderstorms however it would not be entirely surprising
if we get a few strikes over extreme NE Washington and the
northern tip of the Idaho Panhandle during the afternoon.

The dry weather regime will be brief as the overall trough pattern
breaks down with swift zonal or westerly flow aloft as a 130kt jet
pokes into extreme SW Washington and Oregon. As this feature moves
into the Pacific NW it will embed a impressive atmospheric river
with most of its moisture headed into southern Oregon and northern
California. This far north we still look like we will see a small
portion of the river headed across the southern half of Washington
and into the southern Panhandle. This is where we expect to see
some periods of moderate rain. 24 hour precipitation amounts
(ending Wednesday evening) across this area could range from 0.40
to 0.80 inches with locally heavier amounts possible over SW
facing slopes. Lesser amounts of precipitation are expected to
occur over the northern half of the Panhandle and NE Washington
with little if any precipitation in the lee of the Cascades. Snow
levels during this period will be around 4500-5000 ft and thus we
still expect to see some snow possible over Lookout Pass, however
unless the precipitation gets very heavy (not expected) we expect
the impacts to be minimal due to warm road temperatures. The
precipitation will cause renewed rises, especially across the
southern Panhandle, however we do not expect to see any flooding
at this time. fx

Wednesday night through Sunday: The showery and cool pattern
continues, but a bit of improvement is possible toward the
weekend. A front slides east of the region Wednesday evening, but
it leaves a deformation axis near the northeastern CWA through
Friday. The flow carries a few mid-level disturbances across the
region through this period, yet with the jet axis south of the
region the core of the lift associated with those disturbances
remains south. Yet any moisture and instability within and near
the deformation axis will provide enough fuel for a threat of
precipitation. The threat will be highest Wednesday evening. The
shower chances continue Thursday and Friday, with the highest risk
in the afternoon hours. Coverage appears to lessen each day as
the deformation axis weakens. Through the period snow levels will
be lower than they have been of late. Wednesday evening they
average between 3.5 to 5kft, then lower to average between 3-4kft
Thursday and Friday. This will mean the potential for snow
accumulation around the mountain passes. Confidence accumulations
will be enough to necessitate any highlights is low, but it is
something to monitor. This includes but it not limited to places
like Stevens, Sherman and Lookout Pass. If there are impacts it
would more likely be Wednesday/Thursday thing. In addition to the
precipitation threat, winds will be in the breezy category for
many outside of the sheltered mountain valleys Wednesday evening
and again Thursday afternoon/evening.

Then between Saturday and Sunday another frontal wave crosses the
region. It is lead by weak high pressure on Saturday with a
limited threat of showers over the mountains and a fair amount of
sunshine elsewhere. Then Saturday night into Sunday the frontal
wave quickly slips by, expanding clouds and another shower threat
across the region. Yet the best risk will be within the mountain
zones. Snow levels rise back to around 4-6kft. /J. Cote`


00Z TAFS: As of 23z this afternoon scattered showers were
occurring mainly over NE WA/N Idaho mountains. The boundary layer
was relatively dry at all TAF sites with temperature/dew points
spreads of 15-25F. Thus VFR conditions are expected to continue
through the evening despite a general increase in light showers
as the next wave enters. By Monday morning as low level upslope
flow occurs for the eastern TAF sites (KGEG/KSFF/KCOE/KPUW) and
the boundary layer continues to moisten with light showers...MVFR
conditions may develop.  JW


Spokane        39  53  38  56  42  53 /  50  70  30  30  50  60
Coeur d`Alene  39  52  38  54  39  52 /  60  80  30  50  60  80
Pullman        40  52  38  55  42  53 /  50  70  50  30  60  80
Lewiston       43  57  42  60  46  58 /  50  50  50  30  70  60
Colville       38  55  36  57  41  54 /  50  40  20  50  60  80
Sandpoint      39  51  36  52  37  50 /  90  40  20  60  70  80
Kellogg        38  49  35  50  37  48 /  70  90  60  60  60  90
Moses Lake     43  59  39  62  45  61 /  40  20  20  10  40  20
Wenatchee      42  57  41  60  43  58 /  30  30  20  10  20  20
Omak           38  58  37  63  43  60 /  30  20  10  10  20  20



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