Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 262115

Area Forecast Discussion
National Weather Service Spokane WA
215 PM PDT SUN APR 26 2015

After a few lingering mountain showers this evening, expect a
drying and warming trend through Tuesday. Between Tuesday night
and Thursday a system brings more seasonal temperatures, breezy
conditions and a threat of showers, followed by a general drying
trend by the weekend.


Tonight and Monday...An upper level ridge is sprouting over the
forecast area today. Satellite imagery does not clearly show this
due to a feed of moisture impinging on the northwest coast...but
models are in good agreement in rapidly strengthening this ridge
aloft and allowing the incoming moisture field to be shunted off
to the north of the region tonight. The approaching deep trough
visible far out over the Pacific near 150W will plausibly allow
this strengthening. However...areas of thick high clouds will
transit the region through the next 24 hours as this high altitude
moisture enhances over a mid level warm front. The only real
chance of precipitation will be over the high Cascades overnight
tonight near the crest...closer to the deeper moisture and
benefiting from orographic ascent just over the crest on the west

Otherwise a quiet and cool night is on tap but not as cold as
last night. Residual boundary layer moisture from yesterday`s
shower activity may set the stage for some more valley fog tonight
to the north of the basin but not as extensive as this morning.

Monday will be a dry day with filtered sunshine and light winds as
the ridge strengthens. High temperatures will bounce back up to
slightly above normal at most locations. /Fugazzi

Monday night to Thursday: The Inland NW transitions from warm
and dry to cooler and breezy with shower chances, as an amplified
ridge is replaced by a deep trough. Between Monday night and
Tuesday the ridge axis will be shifting east, while the trough
axis advances to the coast. This puts the region in a stout
southwesterly flow, giving much of the region the warmest day of
the week. Regional 850mb temperatures rise into the lower to mid-
teens, supporting 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 showers and perhaps some thunder will be the
northeast mountains by afternoon, with steepening lapse rates and
some marginal CAPE per some models. More likely, however, these
factors will just lead to afternoon cumulus in this region. The
approaching trough will also lead to breezy conditions by mid to
later afternoon as pressure gradients tighten between the two air

Between Tuesday night into Wednesday a second shortwave rounds
the parent trough and helps propel a cold front across the region.
This will bring the next best chance of precipitation. Clouds and
shower chances expand between late evening and Wednesday morning.
The modest westerly and downslope flow should keep the
precipitation threat to a minimum in the lee of the Cascades.
Additionally, outside of the Cascade crest and northeast
mountains, it appears the precipitation duration and amounts
should be limited because the front appears to pass relatively
quickly. However there is some elevated instability along the
front and I wouldn`t rule out some thunder or at least locally
enhanced precipitation rates. Confidence is only high enough to
place some thunder chances across the northern mountains Tuesday

Wednesday afternoon and Wednesday night the main upper trough
migrates across the Inland NW. There is some disagreement on the
timing and these differences will continue to be ironed out. In
general the trough will be accompanied by another cold pool. It
doesn`t appear as strong of a cold pool as the one that crossed
this Saturday (yesterday), 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 start to diverge over the details of the
pattern. However in a loose way they show a flat ridge migrating
over the region while another shortwave trough approaches
northwest WA. Currently that next trough is weaker and tracks
further north than Wednesday`s. In general look for some
clouds from the west-northwest. A slight chance of showers was kept
near the central and southern Panhandle mountains due to some
model disagreements on how quickly Wednesday`s trough exits.
Shower chances will also spread into the Cascades and skim along
the BC/US border with the approach of that next wave. Otherwise
conditions look mostly dry and seasonal. /J. Cote`

Thursday night through Monday: The general consensus amongst the
medium range models is for progressive westerly flow to setup
across the Inland NW with shortwave trofs rippling through at
times. Models continue to struggle with the details of these
features but overall, this looks to be a low impact weather period
with the potential for light mountain showers and at times, breezy
afternoon winds in the Basin. Temperatures will be very close to
seasonal readings topping out in the 60s to lower 70s. The
GFS/ECMWF have been hinting at the idea of an area of low pressure
branching off from the mean flow and drifting down the Pac NW
Coastline early next work week. If this were to occur, the region
would experience increasing southerly flow increasing the threat
for afternoon T-storms and above normal warmth. At this time,
confidence remains low regarding any particular model details in
that time-frame but definitely something to keep an eye on. /sb


18Z TAFS: A building upper level ridge aloft and weak high
pressure at the surface will promote VFR conditions at all TAF
sites through 18Z Monday. Morning fog will exist through 20Z in
the northern valleys of WA and ID before burning off...with
mountain cumulus through 02Z. /MJF


Spokane        38  66  45  74  43  62 /  10   0   0  10  50  20
Coeur d`Alene  35  67  40  74  42  62 /  10   0   0  10  50  30
Pullman        35  66  41  73  42  60 /  10   0   0   0  20  20
Lewiston       39  72  45  80  48  67 /  10   0   0   0  20  20
Colville       37  70  41  76  42  66 /  10   0   0  10  60  30
Sandpoint      32  67  36  73  40  61 /  10   0   0  10  60  40
Kellogg        34  65  39  72  42  60 /  10   0   0   0  30  40
Moses Lake     40  72  44  77  42  67 /  10   0   0   0  20  10
Wenatchee      45  74  50  74  45  67 /  10   0   0   0  10  10
Omak           38  72  43  74  40  67 /  10   0   0  10  20  10



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