Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 162353
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
353 PM PST Mon Jan 16 2017

.SYNOPSIS...
Tonight will remain mostly dry, with cold temperatures. A
prolonged period of active weather develops Tuesday and continues
at least through Thursday. This will start out as a wintry mix,
with the potential for significant snow or ice for some, before
transitioning to warmer, wet weather later Wednesday into
Thursday. Localized flooding will also be possibility.

&&

.DISCUSSION...

Tonight: The Inland NW be relatively quiet this night, before a
wintry mess starts in Tuesday. A strong surface high will keep
cloud cover and/or some fog across the western Columbia Basin and
sheltered valleys. The approaching Pacific system and resulting
easterly flow allowed some clearing over the central Panhandle
to Palouse, portions of the Spokane/C`dA area and some shallower
NE valleys. A forecast challenge be how much the low clouds and
fog re-expand through the evening into the overnight. Either way
middle to high cloud will also be spreading from the west,
especially overnight. The high Palouse south is more likely stay
out of the deeper clouds, thanks to the easterly wind. The wind
into the stratus west of here may squeeze out flurries/isolated snow
shower across the Columbia Basin, especially western and northern
area. It may be mixed with freezing drizzle overnight, but it is
expected to be too light for real impacts. A threat of measurable
precipitation increases in the Cascades, largely near the crest,
and some of the northeastern mountains through the night with the
Pacific system. Overnight low temperatures will be the other
challenge. Most guidance suggests temperatures should remain
steady or slowly rise through the night, but areas where skies
have cleared out and have snow on the ground will again have the
opportunity to drop quickly after sunset. Single digits and teens
will be common, with some 20s in the mountains. /J. Cote`

...SIGNIFICANT ICE ACCUMULATIONS POSSIBLE IN CENTRAL AND NORTHERN WASHINGTON...
...WARMER TEMPERATURES AND HEAVY RAIN MAY LEAD TO LOCALIZED FLOODING ACROSS EASTERN
WASHINGTON AND NORTH IDAHO WEDNESDAY AND THURSDAY...

Tuesday and Tuesday night: An active weather pattern is setting up to say the
least as an atmospheric river barrels into the region and
overruns a very cold air mass in place. This will bring a variety
of winter weather during this period lingering well into
Wednesday. The first slug of moisture arrives Tuesday morning,
roughly 12 hours slower than forecast models were advertising
yesterday. This will be a light precipitation event with most
locations receiving less than a tenth of liquid equivalent however
accompanying p-type will be an issue. We will not see the strong
push of warmer air with this system but if you look at the morning
balloon data from Spokane, you can see that a strong subsidence
inversion has developed under this ridge warming temperatures 5-7k
ft AGL near to above freezing. This is within a degree or two of
falling as sleet or freezing rain, perhaps light snow if
precipitation intensities are heavier enough. Winter highlights
have been hoisted to account for this light wintry mix but the
main focus for ice and snow accumulations will come Tuesday
evening and into the night as the main slug of subtropical
moisture comes inland along an accompanying warm front.

Moderate to heavy precipitation intensities will accompany the warm
front Tuesday night bringing the potential for upwards of a half
an inch of QPF or more over the 6-12 hour period. Very cold air in
place will make this one of the most challenging precipitation
type forecasts I have ever encountered but confidence continues to
increase that the cold air will remain firm in the lee of the
Cascades allowing precipitation to persist as freezing rain and
sleet south of Lake Chelan and Omak and mix of sleet and snow
northward. We also expect to see a difference in precipitation
types across the northern mountains ranging from snow and sleet
along and north to snow, sleet, and freezing south. The air mass
is a touch colder than guidance was indicating yesterday and
thinking this is due to the slower onset of atmospheric river and
more time for the subsidence inversion to wetbulb below freezing.
As such, ice accumulations up to 0.30" will be possible from
Nespelem to Springdale to Priest River with 4-10 inches to the
north and in the mountains. With these higher impacts expected
Tuesday night and Wednesday, decided to go with winter storm
warnings for ice and snow at this point and may look into
upgrading some areas to ice storm warnings if the data is
consistent. Further south into the Basin, we are looking at the
potential for significant icing across the Western Basin including
locations like Wenatchee, Chelan, Douglas, Waterville, Moses
Lake, and Grand Coulee. Ice accumulations moving east into the
Eastern Basin and Idaho Panhandle should be significantly lighter
with a switch to rain sooner as winds increase from the S/SE but
this will need to be monitored closely in the event the models are
scrubbing out the air mass too quick. Freezing rain advisories
have been issued for these locations to account for the light
icing expected Tuesday morning/afternoon. At this time, these
advisories will expire early Tuesday evening but we will need to
reassess where cold pockets still exist going into the nighttime
hours with the next slug of moisture arriving after 10PM. SREF
probabilities for freezing rain are very impressive for the
Cascade East Slopes and Western Basin, moderate for the lower
sections of the northern valleys, and into pockets of southern
Shoshone County around Avery, Deary, and Clarkia. We have also
added some freezing rain for cold air trapped in the valleys
around the L-C Valley and Camas Prairie. Precipitation will
largely skip these areas Tuesday but will increase Tuesday night
but with winds expected to be on the rise, confidence is much
lower for ice accumulations. /sb

Wednesday through Thursday: The messy...wet and locally icy
transition period from our current deep winter inversion regime to
a more seasonably normal but continued unsettled regime will
continue through this period. The atmospheric river impinging on
the region will be most favorably directed into the forecast area
with PWAT values upwards of 0.75 inches on Wednesday and Wednesday
night. This moisture will be efficiently enhanced into near
universal precipitation isentropically as it glides over the dense
cooler air in the basin...and helped along by dynamic support
ahead of a migrating trough Wednesday night and Thursday...with
some drying beginning in the Cascades lee Thursday afternoon as
flow turns more westerly and downslope in the wake of the short
wave trough axis passage.

Mountains above 3500 to 4000 feet will confidently remain snow
through this period...picking up a good 8 to 14 inches of snow.
The lower elevations will be a mixed bag of snow...rain...sleet
and freezing rain eventually turning to rain then tapering off
somewhat by late Thursday. Three general geographic areas will
display distinct evolutions.

First...the eastern Columbia Basin and the valleys of northern
Idaho and northeast Washington will be in the process of
transitioning from freezing rain/sleet and some snow to all rain
during the day...probably in the morning over the basin if it
hadn`t already occurred overnight...and in the afternoon in the
valleys near the Canadian border.

The deep basin and Cascades lee zones will be hit hard with
a long run period of freezing rain and sleet from about Quincy
and Grand Coulee and points westward up into the valleys of the
Cascades south of Lake Chelan. The deep cold air trapped against
the Cascades here will be very stubborn to eradicate. Travel over
the Waterville Plateau...the Wenatchee to Chelan corridor and the
highway 2 corridor will likely be extremely slippery and dangerous
on Wednesday. A transition to simple rain will probably occur
late Wednesday afternoon or evening as precipitation begins to
taper off.

The northern valleys of the Cascades particularly the Methow will
feature deep enough cold air to promote snow through Wednesday
with another 3 to 6 inches of accumulation during the day with a
change over period Wednesday night.

Flooding issues from saturated low elevation snow melt and rain
fall will probably become noticeable in low laying areas and urban
drainages by Wednesday evening or Thursday morning especially over
the eastern basin and Idaho Panhandle. At this time no mainstem
rivers are expected to flood from this system.

Friday through Monday...Models are in good agreement in
maintaining an upper low just off the Northwest Pacific coast with
a general trough appending off this mother low laying over the
forecast area Friday and Saturday...and then moist southwest flow
over the region Sunday as the offshore low evolves into an open
trough...with the offshore trough ejecting inland on Monday.

This all adds up to a continued unsettled weather period with a
few waves and disturbances migrating through the region...each
capable of producing light snow accumulations at night and mainly
light snow but also a non-accumulating rain/snow mix or just rain
in the lower elevations of the basin during the days. None of the
models develop any organized or significant storm systems...and
the flow regime will promote temperatures generally near normal
for this time of year...well below freezing overnights...but not
arctic-like...and highs in the lower to mid 30s during the days.
/Fugazzi

&&

.AVIATION...
00Z TAFS: High pressure with a strong inversion will continue over
the Columbia Basin tonight with IFR and LIFR stratus filling back
into the KGEG...KSFF and KCOE TAF sites soon after 00Z today. KPUW
and KLWS have cleared out today and easterly winds will resist a
return of IFR conditions however this is a low confidence forecast
for the evening hours and stratus could return for a few hours before
east winds increase overnight. MVFR ceilings at KMWH and KEAT
will probably deteriorate to IFR conditions late tonight and near
dawn Tuesday. On Tuesday increasing moisture from an overrunning
Pacific fetch will promote an onset of freezing rain and ice
pellets form west to east through the day. Timing is low
confidence but the overall threat is high confidence for an onset
of mostly freezing precipitation during the day at all TAF sites
except for KLWS. /MJF

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        12  29  29  40  34  38 /  10  80 100 100 100  50
Coeur d`Alene  16  31  31  41  35  39 /  10  80 100 100 100  70
Pullman        12  34  32  41  35  40 /  10  40 100 100 100  60
Lewiston       12  32  32  44  38  44 /  10  10 100 100  90  60
Colville       15  29  29  39  36  39 /  10  80 100 100 100  60
Sandpoint      19  30  30  40  35  39 /  10  90 100 100 100  90
Kellogg        17  28  28  40  35  39 /  10  70 100 100 100  90
Moses Lake     11  25  25  39  32  38 /  10  90 100  90  80  20
Wenatchee      11  26  25  33  29  37 /  10  80 100  80  70  20
Omak           18  28  28  37  33  37 /  10  70 100 100  90  30

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Air Stagnation Advisory until 10 AM PST Tuesday for Central
     Panhandle Mountains-Coeur d`Alene Area-Idaho Palouse-Lewis
     and Southern Nez Perce Counties-Lewiston Area-Northern
     Panhandle.

     Winter Storm Warning from 7 AM Tuesday to 8 AM PST Thursday for
     Northern Panhandle.

     Freezing Rain Advisory from 9 AM to 4 PM PST Tuesday for Central
     Panhandle Mountains-Coeur d`Alene Area-Idaho Palouse.

WA...Air Stagnation Advisory until 10 AM PST Tuesday for East Slopes
     Northern Cascades-Lower Garfield and Asotin Counties-Moses
     Lake Area-Northeast Blue Mountains-Northeast Mountains-
     Okanogan Highlands-Okanogan Valley-Spokane Area-Upper
     Columbia Basin-Washington Palouse-Waterville Plateau-
     Wenatchee Area.

     Winter Storm Warning from 7 AM Tuesday to 8 AM PST Thursday for
     East Slopes Northern Cascades-Northeast Mountains-Okanogan
     Highlands-Okanogan Valley.

     Freezing Rain Advisory from 9 AM to 4 PM PST Tuesday for Spokane
     Area-Washington Palouse.

     Freezing Rain Advisory from 9 AM Tuesday to 8 AM PST Wednesday
     for Upper Columbia Basin.

     Ice Storm Warning from 7 AM Tuesday to 9 AM PST Wednesday for
     Moses Lake Area.

     Ice Storm Warning from 7 AM Tuesday to 4 PM PST Wednesday for
     East Slopes Northern Cascades-Waterville Plateau-Wenatchee
     Area.

&&

$$



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