Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 272329
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
329 PM PST FRI FEB 27 2015

.SYNOPSIS...
Windy conditions will bring cold wind chill temperatures Friday
night and Saturday morning. Dry weather is expected through
Sunday. Another weather system will pass Monday into early
Tuesday with wintry weather possible, especially the Idaho
Panhandle. Much colder air will move into the area behind this
system with temperatures well below average for the middle of next
week. Drier weather, with moderating temperature follows into
the end of next week.

&&

.DISCUSSION...

WINDY TONIGHT WITH LOW WIND CHILL TEMPERATURES SATURDAY MORNING

Tonight through Sunday night...This period will be dominated by
the arrival of a cold and dry Canadian continental air mass.
Currently an upper level wave bearing a surface low pressure is
moving down the Washington/Oregon coast. The forecast area is
being impacted by the fringe of this system with valley rain and
minor accumulations of mountain snow in and near the Cascades.
All the latest model guidance is in agreement and plausible
against satellite extrapolation in taking this upper level trough
far south into the Great Basin (with a tapering off of lingering
precipitation near the Cascades by dawn tomorrow) over the next 12
to 24 hours...leaving the forecast area high and dry under a flop-
over ridge aloft. The northerly flow under this building ridge
will hustle in a shot of dry...cool and dense continental polar
air. Thus evolves the main forecast issue for the next 24 hours.

The arrival of this new air mass will manifest itself as windy
conditions tonight and tomorrow morning through the northern
mountain gaps and into the Columbia Basin as this dense air is
squeezed through the gaps and then receives a gravity assist as
it flows downhill and pools in the deep basin. Gradient analysis
and previous case histories suggest low end Wind Advisories are
prudent measures for the Purcell Trench and the Okanogan Valley
tonight and into Saturday morning.

Meanwhile skies will be clearing. The gradient will slacken during
the day Saturday leading to a night of good radiational cooling
with very cool morning lows by dawn Sunday. Adiabatic mixing of
the low level air mass on Saturday...and on Sunday the lack of low
level snow pack...the higher late season sun angle and the dry
air mass efficiently exploiting the available sun will allow high
temperatures to recover to only slightly below normal readings.

Sunday night the next weather system will approach the region from
the west allowing an increase in clouds and then potential for
some light snow to develop overnight in the Cascades and northern
mountains...while the rest of the area remains dry but cloudy and
thus a bit warmer due to less radiational cooling potential.
/Fugazzi

Monday through Friday: The next system drops across the Pacific
Northwest early in the week, with snow chances and another shot of
cold air, before milder and drier conditions return.

Between Monday and Tuesday night a mid-level trough, currently
pushing into Bering Sea, dives southeast across western Canada and
the northwest United States. How quickly it moves by and whether
it will carry a precipitation threat throughout the Inland NW or a
threat relegated to just the northern mountains and Idaho is where
models disagree. The wettest model by far has been the ECMWF and
it has some run-to-run consistency. However the drier models, such
as the GFS, have also been consistently drier. So right now they
staunchly disagree with each other.

The official forecast goes somewhere in the middle. A broader
precipitation threat is forecast, but the best threat of more
measurable snow will be in the mountains and ID panhandle. And
within that latter area confidence is highest toward the ID
Panhandle mountains, where some snow impacts are possible in the
passes. Chances rise early Monday. Between Monday afternoon and
evening the threat will be ending near the lee of the Cascades,
but chances will remain higher farther east, again particularly
across Idaho. Overall chances then dissipate over the east
overnight into Tuesday as the drier air and subsidence pushes in
with a rebounding ridge of high pressure, with the main chances
lingering across the Central Panhandle mountains and Camas Prairie
on Tuesday. There is also the potential for some showers lingering
across the northern mountains on Tuesday as some weaker ripples
drop south along the incoming jet and interact with some shallow
instability in that region.

Behind the precipitation threat comes increased winds and that
next push of cold air. As there are disagreements in the
precipitation potential, there are disagreements in the wind
speeds. In general, however, look for increasing northerly winds
between Monday afternoon and Tuesday. The speeds abate through the
Tuesday night. Tuesday afternoon temperatures are expected to be
below normal by 5 to 10 degrees, followed by lows dropping some 10
to 15 degrees below normal Tuesday night in an atmosphere more
conducive to good rational cooling (i.e. dry, with few clouds and
lighter winds). This means lows in the singles digits and teens
Tuesday night/Wednesday morning.

Between Wednesday and Friday the rebounded ridge is eventually
dampened again by another shortwave trough moving in from the
west. The primary wave stays north of the Canadian border, but
some weaker ripples do skim in on an the westerly flow. For the
most part the forecast is dry, but some shower chances will return
to the toward the north Cascades and along the Canadian border
toward the end of the work week. The rebounding ridge is also
accompanied by some milder air, so look for temperatures to
moderate back toward normal and, by Friday, slightly above normal
again. /J. Cote`

&&

.AVIATION...
00Z TAFS: A storm system will pass out of the region to the south
between 00Z and 12Z. There will be some residual snow showers over
and near the Cascades through 12Z. Meanwhile a push of cold
Canadian air will squeeze through mountain gaps and drain into
the Columbia Basin through 00Z Sunday. While skies will clear out
for VFR conditions at all TAF sites...winds will become gusty and
locally strong with a general north to northeast winds of 20 to 30
kts at the KGEG...KSFF...KCOE...KPUW and KMWH TAF sites. KLWS and
KEAT will be somewhat sheltered but still breezy. /MJF



&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        23  40  19  42  24  39 /  10   0   0   0  10  30
Coeur d`Alene  22  40  18  44  22  40 /  10   0   0   0  10  40
Pullman        27  40  18  41  23  40 /  10   0   0   0   0  40
Lewiston       29  45  23  45  28  44 /  10   0   0   0   0  20
Colville       25  45  20  46  24  43 /  10   0   0   0  10  30
Sandpoint      22  39  17  40  21  38 /  10   0   0   0  10  60
Kellogg        22  36  16  41  22  37 /  10   0   0   0   0  70
Moses Lake     31  48  22  47  26  48 /  10   0   0   0  10  20
Wenatchee      32  48  27  48  30  48 /  20   0   0   0  10  20
Omak           30  48  22  48  26  47 /  30   0   0   0  20  20

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Wind Advisory until 10 AM PST Saturday for Coeur d`Alene Area-
     Northern Panhandle.

WA...Wind Advisory until 10 AM PST Saturday for Okanogan Valley.

&&

$$



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