Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 262344

Area Forecast Discussion
National Weather Service Spokane WA
343 PM PST WED NOV 26 2014

Mild and breezy weather is expected tonight and Thanksgiving with
areas of dense fog. Snow levels should rise above the mountain
passes. Motorists with plans to travel over the mountain passes
should experience good conditions this afternoon through Friday
afternoon. The arrival of a cold front Friday night into Saturday
will bring winter driving conditions to the mountain passes and
much cooler weather for the weekend.


Tonight and Thursday: The region remains under a dirty upper-level
ridge. In general, the ridge is bringing much warmer air into the
region (at least in the midlevels) and is shoving the moist
frontal boundary northward into southern BC. Because the ridge has
fully translated east, some of this moisture is coming down the
eastern periphery of the ridge and continuing to bring light rain
and high mountain snow into the northern Idaho Panhandle. The
biggest impacts from the warming aloft is a moist inversion over
the Basin delivering widespread fog and low clouds. Dense fog has
plagued the Upper Columbia Basin through much of the day and will
continue for much of the night. As of 2PM...the worse visibilities
were found along Hwy 2 from Airway Heights to Waterville and
northward along Hwy 395 between Spokane and Colville. As such, a
dense fog advisory has been issued.

As we go into the night and Thanksgiving, the ridge will flatten
and the moist frontal boundary will sag back into Washington and
Northern Idaho. We will not see much in the way of cooling north
of the boundary but rather an increase in precipitation. The good
news is winds should begin picking up and we will lose the pockets
of cooler air in the northern valleys resulting in snow levels
between 5-6K ft. Modest westerly flow within the 850-700mb layer
will deliver somewhat of a rain shadow in the Basin but add lift
to the already strong isentropic omega over the northern and
eastern mountains.

Under this pattern, a wide range of temperatures will be found
across the region. Today for instance, we are seeing highs in the
30`s north to 60`s in the foothills of the Blue Mtns. Overnight
lows will drop very little from the current readings (with the
exception of the 60`s in the southeast) then tomorrow will have
the potential for widespread 40s and 50s with a few 60`s if the
winds can efficiency mix to the valley floors.  /sb

Thursday night through Saturday...A complex interplay between an
initially very moist air mass and a strong arctic push will bring
a very active weather regime to the region through this
period...featuring wet and windy but surprisingly mild late
November conditions Thursday night through Friday night...with a
stark drying trend with temperatures beginning to plunge like a
rock on Saturday.

Two features of note on Pacific satellite are the deep closed low
spinning off the a very moist air mass...and an
incipient arctic origin trough barely perceptible on satellite
over the Yukon dropping into northern BC this afternoon. The
latest GFS and ECMWF models are in better agreement today and
honing in on a solution that involves the digging arctic trough
picking up and enhancing the Pacific moisture over the forecast
area into a long running semi-orographic precipitation pattern
followed by a strong push of very dry arctic air down the Okanogan
Valley and eventually exchanging the current moist maritime air
mass over the region with a bone dry and cold air mass.

Thursday night will include a moist orographic pattern with
strong westerly flow across the mountains promoting a decent rain
shadow over the basin...with snow levels in this mild maritime
air mass maintaining between 5000 and 6000 feet. On Friday a deep
surface low pressure will form to the north and deepen
aggressively under the dynamic jet region ahead of the incoming
trough as it descends through the forecast area. This will allow
rain and high mountain snow to become more widespread and include
most of the Columbia Basin during the day Friday. Winds will
become very gusty during Friday afternoon especially over the
exposed basin terrain...and a Wind Advisory may be necessary with
model soundings suggesting 45 to 50 mph gust potential in the
mixed layer. This will also promote continued mild temperatures.

Friday night the arctic cold front will follow the now exiting
surface low with an impressively strong northerly gradient setting
up down the Okanogan Valley and eventually encompassing the deep
basin by Saturday. This is another potential Wind Advisory or
even a High Wind Warning pattern that will need to be monitored
and refined on subsequent shifts. By Saturday afternoon a stark
clearing trend from north to south will become apparent as very
dry air invades the region. As snow levels quickly drop
precipitation will be quickly ending so no winter storm highlights
are expected. Temperatures on Saturday may start out at their
high temperatures and either remain steady or plunge during the
day. /Fugazzi

Saturday Night through Sunday Night: A trough will exit the
region Saturday night with the best chance of snow extending from
Pullman south toward the Blue Mountains and Camas Prairie. The
flow will turn north to northeasterly which will bring in cold
Canadian air. Min temperatures will be about 7-14 degrees below
average for this time of the year. Temps in the single digits to
low teens will be common. Below zero is not out of the question
for the Methow and Okanogan Valleys. Sunday and Sunday Night the
models agree on showing a broad ridge in place with very dry
conditions. Have taken out all previous mention of snow and
decreased sky cover. There is the potential for valley fog, but am
not confident quite yet as to where it would form. Will have to
wait to see how much drying we get behind the trough.

Monday through Wednesday: Both the EC and GFS show another trough
moving through the area on Monday. There are some discrepancies
between the two...mainly being the GFS takes the main energy and
slides it along the Canadian border where the EC digs it further
south into our area. So we have some discrepancies as to where the
best chance of rain and snow will set up. Right now am trending
more towards the wetter/slightly warmer GFS and have chance of
precipitation across extreme eastern WA and all of the ID
Panhandle. This is quite a bit different from the previous
forecast that had a chance of precip just about everywhere. After
the trough passes Monday another broad ridge sets up across the
Inland Northwest. The best chance of precip will remain across
southeast WA and the LC Valley/Camas Prairie areas. Temperatures
will start to moderate a bit, but will still be below average for
this time of the year. /Nisbet


00Z TAFS: A very moist boundary layer will promote widespread IFR
and LIFR ceilings and visibilities over the Columbia Basin through
06Z...affecting all TAF sites except KPUW and KLWS. After 06Z a
developing surface low over British Columbia will create a
southerly gradient breeze which may allow some improvement in
visibilities and marginal ceiling improvement...however since this
will be developing at night and thus involving a nocturnal
inversion locking in the low level moisture...confidence is only
moderate for this improvement. Confidence is higher for
significant improvement during the mid morning hours on Thursday
when diurnal mixing will play a larger role in breaking up the fog
and stratus.


Spokane        39  49  41  46  27  30 /  30  30  20  70  60  10
Coeur d`Alene  36  47  41  45  29  31 /  30  40  40  80  70  20
Pullman        44  52  43  49  32  34 /  20  30  30  90  90  50
Lewiston       43  55  46  53  38  40 /  10  10  20  60  70  60
Colville       36  45  40  43  22  24 /  60  50  50  70  30  10
Sandpoint      33  45  38  42  26  26 /  70  60  70  80  80  20
Kellogg        37  44  39  41  30  31 /  60  80  60  90 100  50
Moses Lake     38  52  41  51  27  31 /  10  10  20  30  30  10
Wenatchee      39  47  41  48  29  30 /  10  10  30  30  20  10
Omak           34  43  37  41  16  20 /  30  20  20  20  20  10


WA...Dense Fog Advisory until 10 PM PST this evening for Northeast
     Mountains-Spokane Area-Upper Columbia Basin-Waterville


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