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Area forecast discussion 
National Weather Service Spokane Washington
451 am PDT Sat may 23 2015

Synopsis...
the Memorial Day weekend is expected to be warmer than average,
with afternoon temperatures in the 70s and 80s. Look for scattered
showers and thunderstorms this weekend into next week. Some
thunderstorms will be capable of producing heavy downpours and
frequent lightning.



&&

Discussion...
Saturday through Sunday night: a thermal trough of lower pressure
will shift slightly more into the basin today. This will set up
a weak northerly gradient over the northeast portion of the
region. The main impact that this gradient will have is to filter
in some drier air into out of Canada. There are some
discrepancies amongst the 00z model guidance as far as how much
drier air will push down. The GFS and rap (a.K.A. Ruc) solutions
are a bit more aggressive with the drier air. The NAM/sref are
more moist with dew point temperatures generally remaining in the
mid 30s to the lower 40s across the northern mountain valleys;
although, both of these solutions show a slight drying trend as
well. As would be expected, the GFS is much less unstable today
compared to Friday with dew point temperatures dropping into the
20s with drier push. The GFS would suggest that the Cascade crest
and the far southeast portion of the forecast area would see
convection today. The NAM, on the other hand, is a bit more
unstable, with convection possible over the Waterville Plateau and
across the northern mountains. I went with more of a compromise
between these two models. Best chances for convection today is
expected along the Cascade crest, over the northeast Blue
Mountains over to the central Panhandle mtns. I also added a
slight chance of showers over the Waterville Plateau and over the
Okanogan Highlands.

Sunday looks to be a better day for more thunderstorm activity compared
to Saturday. Models are in good agreement that an upper level low
pressure system, currently over Graham Island off the British Columbia Mainland,
will drop down toward the region. This disturbance will tap into
some instability across the northwestern portion of the forecast
area. This will result in isolated to scattered thunderstorms
across the east slopes of the Cascade mtns and progressively shift
across the northern half of the forecast area Monday afternoon
into Monday evening. Thunderstorms will be very wet with heavy
down pours likely; small hail and gusty outflow winds will also
be possible with the thunderstorms.

Temperatures will remain above normal through this weekend. Expect
high temperatures to remain in the upper 70s and 80s. /Svh

Memorial day: an upper low is expected to descend into central
Washington Monday afternoon/evening bringing a good chance for
showers and thunderstorms to round out our Holiday weekend.
Model agreement is good that the center of the 500mb low will be
in the vicinity of Omak by late afternoon placing northeast
Washington and north Idaho in a favorable region of upper level
diffluence under an advancing 500mb cold pool. The models suggest
that a dry slot surging into the Columbia Basin into southeast
Washington will suppress showers over the Palouse and basin in the
afternoon. However, as the upper cold pool advances southward in
the evening, widely scattered showers may develop as far south as
Moses Lake, Pullman and Lewiston. Storm motion on Monday will be
more typical (east or northeast) compared to the westerly
propagating cells of the last couple of days. Weak shear and light
storm steering flow Monday afternoon suggest the potential for
locally heavy rain especially under cells over northern Washington
that may become terrain based. Burn scar flooding could be a
concern Monday afternoon and early evening.

Tuesday: chances for showers and thunderstorms have been raised
for Tuesday especially over the Idaho Panhandle and
northeast/north central Washington. There is good model agreement
that the upper low that arrives Monday will move little by Tuesday
afternoon and evening. We may see better convective coverage
Tuesday than we do on Monday. As the upper low stalls, the dry
slot we anticipate Monday will probably be less pronounced on
Tuesday. Once again, storm steering flow will be very weak on
Tuesday setting the stage for locally heavy rain. If the models
place the 500mb low correctly on Tuesday over central Washington,
thunderstorms could be of particular concern for the 2014 burn
scars. This area could be in a prime spot for deep layer moisture
and surface instability in the wrap around region of the upper
low.

Wednesday through friday: for the second half of the week chances
for shower/thunderstorm chances trend down as the medium range
models weaken the upper low and build a warm ridge over the inland
northwest Thursday and Friday. The end of the week has the
potential to be quite warm. The European model (ecmwf) and GFS suggest 850mb
temperatures near 20c which translates to mid 80s to low 90s.
Without a strong push of dry air, slight chances for showers and
thunderstorms remain in the forecast for the mountains into
Thursday and Friday. Afternoon and early evening showers primarily
driven by diurnal surface based instability. /Gkoch

&&

Aviation...
12z tafs: isolated showers will linger near keat and kmwh early
this morning. Diurnally driven showers and thunderstorms will be
possible this afternoon over the higher terrain along the east
slopes of the northern Cascades, over the northeast blue mtns and
over the southern and central Panhandle mtns. Any storms that
develop over the central Panhandle mtns may impact kpuw and klws
taf locations as storm motion will be to the west and southwest;
however, confidence is low since thunderstorms are expected to
dissipate quickly as they move off of the higher terrain. /Svh



&&

Preliminary point temps/pops...
Spokane 81 53 79 55 71 51 / 10 10 10 30 50 50
Coeur D'Alene 80 53 80 54 70 48 / 10 10 10 30 50 50
Pullman 75 47 74 49 70 46 / 10 20 10 10 20 20
Lewiston 81 55 81 56 78 53 / 20 30 10 20 20 20
Colville 83 53 82 53 73 50 / 10 10 20 40 70 50
Sandpoint 80 48 79 51 70 47 / 10 0 10 30 60 50
Kellogg 77 49 78 50 70 47 / 20 10 10 20 60 50
Moses Lake 85 55 83 55 80 52 / 20 10 20 20 20 20
Wenatchee 85 59 82 57 78 55 / 20 10 20 20 20 20
Omak 87 53 83 52 76 51 / 10 10 40 40 50 40

&&

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

$$

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