Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 231223

Area Forecast Discussion
National Weather Service Spokane WA
420 AM PST Thu Feb 23 2017

A cooler and continued unsettled pattern will descend over the
region for remainder of the week. The main precipitation threat
will be mainly mountain based snow showers, but a stronger system
on Sunday may bring minor snow accumulations to the valleys and
basin. Temperatures will be cooler than normal for this time of
year with overnight lows in the teens and 20s and daytime highs
only in the mid 30s or so.


Today and tonight...Compared to the weather of the past several
days, today`s will actually be quite benign. The main focus today
will feature an elongating shortwave trough which is currently
located along a line from SE British Columbia toward the
Waterville Plateau. This trough is expected to continue to drop SE
though the day but it should do it quite slowly as a result of a
stronger trough plunging southward off the WA coast. By afternoon,
the axis of the trough might only be as far east as a Sandpoint to
Yakima line and by tomorrow morning it may not be much farther
east than that. This suggests the forcing ahead of it will be
quite weak so it should not be a big weather maker. More important
will be the instability associated with the cold pool of air
aloft. 500 mb temperatures should be on par with what we saw
yesterday (around -35c to -37c) which when combined with daytime
heating will once again yield some steep lapse rates. However
unlike yesterday the moisture availability will be somewhat
lacking, especially for areas near and west of the trough. In
fact, the only areas which stand a fair chance of seeing
precipitation today will be over far SE Washington and across the
southern half of the Idaho Panhandle. Moisture in this area will
pale compared to yesterday, however model soundings suggest there
will be enough to trigger some snow showers when subject to ascent
associated with the mid-level northwest flow. Any snow showers
which form should be fairly weak given the shallower instability
compared to yesterday. By evening and into tonight the shower
activity will wind down rapidly. Aside from the showers the main
challenge will be fog forecasting. The latest fog product is
showing a growing band of stratus and fog extending from the
northern Panhandle to the NE Columbia Basin near Davenport, Grand
Coulee and the West Plains. Much like yesterday this should burn
off with the daytime heating as the low level inversion should not
be strong under the presence of the upper level trough.
Temperatures today will continue their string of cooler than
normal readings. In fact, they should be anywhere from 1-5F cooler
than what we saw yesterday which equates to highs in the mid 30s
to lower 40s. Nighttime lows will likely continue the cooling
trend as drier air continues to filter in from the NNW. Look for
overnight lows in the mid teens to lower 20s for most valley
locations. fx

Friday through Wednesday: The Inland NW transitions to a
cooler, showery west-northwest flow. A long-wave trough settles
over the western U.S. and no deep moisture taps are indicated. Yet
shortwave troughs in that flow will still provide some risk for
showers, largely in the form of snow. Friday to Saturday a weak
shortwave passes. This will bring a threat light hit-and-miss snow
showers throughout the region Friday, before the main risk
retreats to the ID Panhandle and far southeast WA into Saturday.

Between late Saturday night and Monday a better threat of snow
passes and it may cause some impacts to the late weekend and
Monday travel periods. With that said, model consistency is poor
to fair. Yesterday at this time models showed a weak, transient
wave passing in that time frame with limited snow amounts. Two
days ago models didn`t show much at all passing. But today they
show a more robust system that tracks more directly across the
region. Still the moisture tap is not impressive with PWATs around
0.15 to 0.30 inches (50-100% of normal). If the current track and
timing holds there would be an increase in snow across the region
later Saturday night into Sunday, before the threat shifts toward
the central and southern Panhandle and far southeastern WA going
into Monday. Again if models maintain this depiction some light to
moderate snows are possible.

Tuesday and Wednesday the better threat of snow retreats to the
mountain zones, with more hit-and-miss snow shower chances across
eastern third of WA and rest of ID. The night/early morning will
have some patchy fog threat. Otherwise expect a mix of clouds and
some sun. Temperatures will be notable much of the period, with
values below seasonal norms by some 6-11 degrees. So winter
continues. /J. Cote`


12Z TAFs: This will begin as a low confidence forecast and it will
end as one as well due to the rapid development of IFR/LIFR
conditions due to fog stretching from COE-SFF-GEG. We have high
confidence that this will stick around through the morning, but not
sure when it will improve to VFR conditions. We will go with timing
similar to yesterday (17-20z window), however it could be a little
quicker since it should be shallower. For PUW and LWS the problem of
low clouds and fog is much less since there is an MVFR deck of
clouds in that area. This should prevent IFR fog from forming, but
the deck of clouds isn`t continuous so there will be alternating
periods of MVFR/VFR conditions until late morning. EAT and MWH are
seeing drier conditions due to north winds and this should largely
eliminate any chance of fog through the period. Late tonight the fog
could return to the eastern sites, but confidence in that is low
since the northerly winds could deliver the drier air across the
entire forecast area. fx


Spokane        34  19  33  19  34  22 /   0   0  20  10   0  40
Coeur d`Alene  36  17  34  16  34  20 /  10   0  30  10  10  40
Pullman        36  20  36  20  36  24 /  10  10  20  10  10  30
Lewiston       41  25  40  25  40  27 /  10  10  20  10  10  20
Colville       37  17  35  18  35  23 /   0   0  30  10  10  50
Sandpoint      36  19  33  17  34  21 /  10  10  20  10  10  50
Kellogg        33  19  33  18  33  24 /  30  20  30  20  20  50
Moses Lake     37  19  35  19  35  23 /   0   0  20  10   0  20
Wenatchee      36  19  33  19  34  22 /   0   0  30  20  10  20
Omak           36  18  33  19  33  22 /   0   0  20  10   0  30



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