Area Forecast Discussion
Issued by NWS Spokane, WA

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273
FXUS66 KOTX 221018
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
218 AM PST Mon Jan 22 2018

.SYNOPSIS...
An active weather pattern is expected through the week. A wet and
mild weather system will bring significant precipitation Tuesday
night into Wednesday and again next weekend with a mix of rain and
snow across the lower elevations and snow in the mountains.
Travel impacts are expected at times, especially over the mountain
passes.


&&

.DISCUSSION...
Today and tonight...Another short wave disturbance visible on
satellite as an area of increasing enhancement on the InfraRed
Satellite loop moving into the Olympics and Vancouver Isle early
this morning will transit the forecast area today bringing another
1 to 3 hour round of snow showers and afternoon valley rain
showers to mainly the eastern reaches of the forecast area while
largely skipping over the deep basin in the lee of the Cascades.
Snow amounts will be light and spotty in the valleys and basin
this morning...but with the potential for a few inches in the
Panhandle mountains with some slippery travel possible across
Lookout Pass this afternoon. Otherwise generally cloudy conditions
across the eastern portion of the forecast with at least partially
sunny skies in the downsloped deep basin and high temperatures
continuing to run above normal for this time of year are
expected. Tonight will be a quiet and cool break period before the
next significant storm system envelops the region. /Fugazzi

...HEAVY SNOW TUESDAY INTO WEDNESDAY ACROSS THE NORTHERN MOUNTAINS...

Tuesday through Wednesday night: A wet and mild storm system will
impact the Northwestern US bringing the potential for heavy
snows, mix of lowland snow and rain, and breezy wind at times. A
warm front will usher in a rich fetch of subtropical moisture
starting Tuesday morning across the Cascades and expanding across
much of Eastern WA and North Idaho by the afternoon hours.
Precipitation within the warm sector will persist well into
Wednesday then end or transition toward showers Wednesday night
with the passage of a cold front. Between half and one inch of QPF
will fall by Wednesday morning along and north of Highway 2 from
Stevens Pass to Sandpoint then an additional quarter to inch will
be possible for the next 24 hours. Snow levels will start off low
and will no doubt rise from south to north with the Inland NW
remaining in the warm sector for 24-36 hours under 25-40kts of
southerly flow around 4000 feet AGL.

The million dollar question is what will snow levels start at and how
fast will they rise? Models continue to forecast a cool air mass
settling in tonight and Monday characterized by 850mb temperatures
of -3 to -7C. This leads to high confidence for snow in the
northern mountains through Wednesday morning then rising some
Wednesday afternoon. In most winters, temperatures of this nature
were a sure bet for snow in the valleys when the calendar read
January but as we have seen in the last few systems, we are
experiencing rather mild lowland conditions resulting in less snow
than models are suggesting. Temperatures will warm into the mid
to upper 30s Monday afternoon. There looks to be some clearing
tomorrow night before the cloud shield begins to thicken. Models
indicate temperatures dip into the mid to low 20s but I am not
buying this. First of all, the 00z guidance is already off by as
much as 8F for locations like Omak and Winthrop. Initialization is
not as bad in Basin and Idaho Panhandle. Nonetheless, I feel the
northern valleys will dip below freezing but sticking with upper
20s to lower 30s vs guidance numbers. This should allow
preciptiation to start as snow north of I-90 but quickly
transition to rain or non-accumulating snow for for most locations
south of Highway 2. Valleys north of the the highway will stand a
better shot of light to moderate snow accumulations before
switching over to rain later Tuesday. If the preciptiation
intensity is to drive down temperatures and hold them steady,
moderate to heavy snow amounts will be possible for these lowlands
spanning from Sandpoint to Chewelah to Omak and into the Methow
Valley. Forecast will need to be fine tuned as we see thermal
profiles Monday night.

The lower Columbia Basin and Palouse will largely be breezy with
rain and lighter amounts than expected across the north. Snow
will fall over the Central Panhandle Mountains and possibly the
higher terrain around the Camas Prairie and Blues Tuesday
afternoon but a transition to rain with snow levels around 4000
feet should lead minimal travel impacts. That is until Wednesday
night when the cold front drives down snow levels and favorable
winds lead to renewed snow chances into Thursday morning. The
strongest winds will occur in the foothills of the Blue Mountains
and around the Snake River Valley. Gusts around 40 mph are
possible. /sb

Thursday through Sunday...An active and wet pattern will continue
through the end of the extended period. Thursday looks like an
unsettled showery day with models in decent agreement in ejecting
what is left of the offshore trough through the region with
probably some shallow instability and cooler temperatures
promoting scattered snow showers with local minor accumulations
especially in the mountains. Increasingly moist westerly zonal
flow on Friday will continue a threat of mainly mountain snow
showers. Saturday the GFS and EC models are in decent agreement
featuring a warm front passage for a threat of widespread
precipitation. The models develop this front during the morning
hours leading to reasonable confidence that most locations will
see light snow at least initially...before the valleys and basin
change over to rain or a wet mix during the afternoon.
However...this storm appears to be an opening act for a
potentially much wetter direct hosing from a sub-tropical Pacific
moisture feed on Sunday...and this potential brings the
possibility of heavier mountain snow and valley wintry mix
transitioning to soil saturating rain during the day...with
attendant small stream rises and mud slide concerns by the
beginning of next week. /Fugazzi

&&

.AVIATION...
06Z TAFS: Post frontal air mass is over the region now behind the
cold front. Moist SW flow in the boundary layer will promote
stratus for the eastern TAF sites tonight into Monday morning.
Models differ on precise restrictions but given some drier air
over SE Washington with 8-11F T/Td spreads at PSC/ALW/PDT as of
05z will side towards mainly MVFR conditions for KGEG/KSFF/KCOE
and keep VFR conditions going at KPUW where low level upslope is
less favorable. For KEAT/KMWH skies have cleared with near
saturated conditions still at the surface. Given the strong
westerly flow fog should have a hard time forming and kept
conditions VFR. A fast moving wave crossing the area Monday will
produce scattered lower elevation rain/snow showers for the TAF
sites with the westerly flow most favoring the eastern TAF sites.
JW


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        37  29  36  35  40  30 /  70  10  50 100  80  70
Coeur d`Alene  37  29  35  34  39  30 /  70  20  40 100  90  80
Pullman        39  30  37  36  42  32 /  50  20  20  80  80  70
Lewiston       45  33  42  41  47  35 /  20  10  20  60  70  60
Colville       37  27  34  33  38  29 /  40  10  70 100  90  90
Sandpoint      36  27  34  33  37  29 /  60  40  50 100  90  90
Kellogg        36  28  35  34  37  29 /  80  40  20  90  80  80
Moses Lake     43  29  39  37  44  31 /  20   0  70  90  60  20
Wenatchee      41  28  35  33  38  28 /  10   0  90  90  70  30
Omak           37  28  34  33  36  28 /  30   0 100 100  90  50

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Storm Watch from Tuesday morning through Wednesday
     morning for Northern Panhandle.

WA...Winter Storm Watch from Tuesday morning through Wednesday
     morning for East Slopes Northern Cascades-Northeast
     Mountains-Okanogan Highlands.


&&

$$



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