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Area forecast discussion 
National Weather Service Spokane Washington
1029 PM PDT Monday Apr 27 2015

Synopsis...
high pressure lingers through Tuesday, producing mild and dry
conditions. However the approach of a cold front will lead to an
increase in winds later Tuesday. Between Tuesday night and Thursday
a passing system will bring more seasonal temperatures, breezy
conditions and a threat of showers, followed by a general drying
trend into the weekend.

&&

Discussion...
current forecast is on track with high pressure over the region
and high level clouds.

Tonight through thursday: the inland northwest transitions from warm
and dry weather to cooler and breezy with shower chances, as an
amplified ridge is replaced by a deep trough. Between tonight and
Tuesday the ridge axis will be shifting east, while the trough
axis advances to the coast. This puts the region in a mild
southwest flow, with regional 850mb temperatures into the lower to
mid-teens (celsius). This supports highs some 10 to 15 degrees
above normal. The exception will be near the Cascades as the
onshore flow and first shortwave rounding the parent trough brings
a relative cool-down here. That shortwave will bring some clouds
and a chance of showers to the Cascades crest too. The other
region that may receive some isolated pop-up showers Tuesday
afternoon will be across the northeast mountains, near the
Canadian border, with steepening lapse rates and some marginal
cape per some models. More likely, however, these factors will
just lead to afternoon cumulus. The approaching trough will also
lead to breezy conditions by mid to later afternoon as pressure
gradients tighten between the two air masses.

Tuesday night into Wednesday a second shortwave rounds the parent
trough and helps propel a cold front and next best shower threat
across the region. The first shower chances expand between late
evening and Wednesday morning with the cold front passage. The
highest threat will be near the Cascade crest in the early evening
and expand east, especially across the northeast mountains, from
late evening (11 pm) through Wednesday morning. The modest
westerly and downslope flow should keep the precipitation threat
to a minimum in the Lee of the Cascades. Outside of these areas
there will be a modest threat of showers with the immediate cold
front passage. However it appears the precipitation duration and
amounts should be limited, because the front appears to pass
relatively quickly. There is some elevated instability immediately
along the front, indicated by a ribbon of 50-150 j/kg of MUCAPE
between 850-500mb and similar ribbon of marginally favorable high
level total totals, which swing into Montana by later morning
Wednesday. So I wouldn't rule out some thunder or at least locally
enhanced precipitation rates.

Wednesday afternoon and Wednesday night the main upper trough and
pool of colder air cross the inland northwest. The cold pool doesn't
appear as intense as the one that crossed this past Saturday, but
it should be enough to bring some threat of hit-and-miss showers
and perhaps some isolated thunder during the afternoon and evening
hours. The main risk will be around the mountains. However a few
showers cannot be ruled out over the eastern Columbia Basin under
the unstable trough. The risk wanes through the evening and
overnight with the loss of daytime heating and a Flat Ridge
nudging in from the west. Expect otherwise breezy conditions, a
few clouds and temperatures cooling closer to seasonal averages.

By Thursday models show a more zonal (west to east) flow over the
region, with a secondary shortwave trough skimming by the Canadian
border. In general look for some high clouds from the west-
northwest, along with some afternoon cumulus. A slight chance of
showers will also be found across the northern mountains,
especially closer to the Canadian border. Otherwise conditions
look mostly dry and seasonal temperatures. /J. Cote'

Thursday night through Monday night: model consensus continues to
advertise a dry zonal or westerly flow across the inland northwest with
a few weak shortwaves rippling through at times. Each of these
waves will bring some increase in winds, especially if the timing
is late afternoon, but little in the way of precipitation with 24
hour model trends continuing toward a dry period. Temperatures
will hover just above seasonal readings topping out in the 60s to
lower 70s. The GFS/European model (ecmwf) have backed off on a closed low early
next week but do indicate more of an open trof and increase in
moisture around midweek. Tough to buy into any solution given the
wavering the last few days so at this time, will just hint at the
potential for a few showers and potentially, another bout of wind.
/Sb

&&

Aviation...
06z tafs: high pressure will continue over the forecast area
through Tuesday afternoon, leading to light easterly winds. A fast
moving upper cold front will ripple east of the Cascades between
22z- 06z tomorrow. This will lead to a narrow shield of mid/high
level clouds and high based showers. Winds will also switch
westerly along and behind the front. Overall, expect VFR
conditions. /Rfox.

&&

Preliminary point temps/pops...
Spokane 45 75 45 62 39 61 / 0 0 50 20 10 10
Coeur D'Alene 40 75 42 61 36 61 / 0 0 40 30 10 10
Pullman 40 74 41 60 36 61 / 0 0 20 30 10 10
Lewiston 45 82 48 66 41 68 / 0 0 20 20 10 10
Colville 40 77 46 66 39 64 / 0 10 60 20 20 10
Sandpoint 35 74 41 61 34 60 / 0 10 40 40 20 10
Kellogg 39 73 41 59 36 57 / 0 0 30 40 10 10
Moses Lake 43 78 42 67 39 68 / 0 10 20 10 10 10
Wenatchee 50 75 45 67 43 67 / 0 10 10 10 10 10
Omak 43 76 42 67 37 67 / 0 10 10 10 10 10

&&

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

$$

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