Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 191201
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
501 AM PDT Tue Sep 19 2017

.SYNOPSIS...
Showers will bring much needed rain to the Inland Northwest
through Friday. Our best chances for rain will be today with the
passage of an upper level disturbance. Portions of southeast
Washington and the southern Idaho Panhandle will also have a good
shot of rain on Wednesday. Hit and miss showers are expected
Thursday and Friday before a drier and warmer pattern returns for
the weekend.

&&

.DISCUSSION...
Today through Wednesday: A cold upper level trough of lower
pressure will remain over the region. A couple shortwave
disturbances will push inland off of the eastern Pacific on the
back side of the trough. The first is already making its way east
of the Cascades. A weak surface low will spin up right over the
Waterville Plateau this morning. Moist isentropic ascent will
increase along the eastern flank of this low across the Upper
Columbia Basin to the ID Panhandle. Lift will be weak, but not
much will be needed to generate precip since lower levels of the
atmosphere are near saturated. Precip amounts will be light up to
around a tenth of an inch or so across the eastern basin.
Orographics will help to squeeze out a bit more precip over the
Central Panhandle Mtns where closer to a quarter of an inch of
precip is expected this morning. This will then push up into
northeast WA and the Northern Panhandle around late morning with
stratiform precip transitioning to showers around this time.
Afternoon heating will also result in pop showers behind this
band across much of the region. Isolated thunderstorms will be
possible for the afternoon with best chances expected on the
Palouse into the Central Panhandle Mtns and over the Okanogan
Highlands into the Northeast Mtns.

Showers will decrease in the evening with the loss of daytime
heating. There looks to be just enough clearing that fog could be
a potential issue for the mountain valleys of northeast WA. There
is some uncertainty that mid level clouds will hang around, so
fog coverage may only be patchy. The next shortwave disturbance
will move in overnight into Wednesday. An upper level jet streak
on the back side of the broad upper level trough is expected to
dig this disturbance further south. There has been some
uncertainty with where the center of this low will track, but
models are coming into better agreement that low will track across
southern WA. The best moisture tap with this system looks to
remain over Oregon. We will see some moisture wrap up around the
low into the southern portion of our forecast area through for the
morning hours on Wednesday. Better chances for precip further
north will likely come in the afternoon as the atmosphere
destabilizes again with surface heating.

Temperatures will remain below normal and continue to trend
colder today into Wednesday. Some areas in the Panhandle will
only warm up into the upper 40s to near 50. /SVH

Wednesday night through Friday...Wednesday night a secondary short
wave will dig south around the main upper level low. This will
split the trough, with the main energy ejecting east into the
Canadian provinces and a weaker wave dropping into Oregon and
northern California Thursday, then pulling away from the region
late in the day Friday. Temperatures will remain 10-15 degrees
below normal. Plenty of low and mid level moisture will combine
with a conditionally unstable atmosphere to support isolated
showers in the basin and scattered showers across the higher
terrain circling the Columbia basin.

Saturday through Tuesday...The low will slowly kick east on
Saturday as high pressure builds into the region from the west.
This will put the forecast area in a cool and dry northerly flow
an Saturday. Showers will still be possible across the eastern
zones through Saturday evening. We will have to keep an eye on
just where the ridge axis sets up Sunday and Monday. If it sets up
further west the Idaho Panhandle could see a few showers along the
Montana border both days. But fr now the forecast will remain dry.
temperatures will slowly warm through the weekend and finally warm
up to about average Monday and Tuesday as the ridge of high
pressure strengthens. Winds will be diurnal/terrain driven and
less than 10 mph. Tobin

&&

.AVIATION...
12Z TAFS: A shortwave disturbance embedded in a cold, unstable
upper level trough will spread light rain across much of eastern
WA and the ID Panhandle this morning. Cigs will lower with MVFR
conditions across the Palouse, Spokane-Coeur d`Alene corridor and
in northeast WA and the ID Panhandle. Cigs will improve in the
afternoon lifting to around 5 kft agl after 20Z. Expect clouds to
obscure mountains region wide. Widespread showers will continue
into the afternoon however as daytime heating destabilizes lower
levels of the atmosphere. Some cells may develop into
thunderstorms in the afternoon, but these will likely be isolated
with low confidence that any will impact the terminals. /SVH

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        54  41  52  40  56  40 /  80  50  50  20  20  10
Coeur d`Alene  51  40  52  37  55  37 /  90  70  70  20  30  20
Pullman        53  41  50  37  55  34 /  80  60  80  40  30  20
Lewiston       59  45  55  42  59  42 /  70  60  90  50  30  20
Colville       56  38  57  36  59  35 /  60  60  40  30  30  20
Sandpoint      50  40  53  35  55  33 /  90  50  50  30  50  20
Kellogg        47  39  46  34  52  34 /  90  90  90  60  60  30
Moses Lake     63  42  58  39  63  41 /  20  20  40  10  10  10
Wenatchee      63  43  58  44  62  44 /  20  20  30  10  20  20
Omak           61  39  63  40  63  40 /  30  30  20  30  20  20

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.

&&

$$



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