Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 281158
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
358 AM PST WED JAN 28 2015

.SYNOPSIS...
Areas of low clouds and fog will persist across portions of
Inland Northwest as high pressure remains in place. A few weak
storm systems will affect the area into Saturday, bringing low
shower chances and cooler temperatures to the region. However the
ridge of high pressure will be the dominate feature, keeping the
threat of fog alive, before breaking down toward Sunday. The next
best threat of precipitation, including possible rain and snow,
arrives then and continues into at least early next week.


&&

.DISCUSSION...
Today through Thursday... Quiet weather to continue through this
period as upper level ridge retains its control on the weather.
The latest water vapor imagery shows a weak shortwave trough
dropping southward through the southern quarter of Washington.
Meanwhile a slightly more vigorous shortwave trough is moving
northward toward 40n/130w. This disturbance will continue ease
north-northeast through tonight and cross the Cascades on
Thursday. Whether or not this feature produces much weather is
questionable. Cross- sectional analysis shows the lifting from
this feature will be rather minimal...while all the moisture will
be mid and high- based. So the main weather through this period
will continue to revolve around the widespread blanket of low
clouds and fog shrouding much of the Inland Northwest this
morning.

The NAM solution is handling the coverage of the low cloud blanket
quite well and suggests it will likely persist through most of
the day. How much of this manifests itself as fog vs elevated
stratus is the big question and its a fairly low confidence
answer. We suspect todays fog coverage will be much less expansive
than yesterday. The main reason is the strong inversions of the
past several days are a thing of the past as the inversion top
temperatures cool. Temps at Mullan Pass are already down 16
degrees from yesterday at this time and with cooler air continuing
to work into the region behind shortwave number 1 we suspect that
we will see a decent destabilization through the boundary layer.
This generally isn`t a good thing for fog but given the model
sounding profiles there still will be enough of an inversion to
maintain stratus and low cloud bases. We will allow the current
fog advisories for the north Panhandle and Upper Columbia Basin
continue however would not be surprised if visibilities rise
through the morning allowing for an early termination. The other
question is one of drizzle. A couple model solutions are depicting
patchy drizzle through the morning...especially over the eastern
Columbia Basin, Spokane Area, and into the northern Palouse. We
also suspect the drizzle threat could spread into the central
Panhandle due to weak upslope flow into the mountains from low-
level westerly winds. By afternoon we begin to see a switch in the
winds in response to the offshore shortwave trough and a weak low
moving up the coast. This will turn the winds to more of a
southeast regime and should bring an end to the low clouds and fog
over the Palouse...Lewiston area...Blues...and maybe southern
Spokane County. Meanwhile these same winds could bank and
subsequently deepen the low clouds and fog near the Cascades and
over the northern Washington mountains. By late tonight and into
Thursday moisture from the offshore trough will cross the Cascades
and move over these areas. Whether or not this triggers some more
drizzle or light seeder-feeder precipitation is the question. The
ascent through the mid/upper level moisture is quite weak.
Confidence in what happens is quite low at least in terms of light
precipitation. Potential impacts would be low with only trace
amounts of precipitation possible. fx

Thursday night through Saturday: Mostly dry conditions, with a
continued low cloud and fog threat marks the end of the week. A
ridge of high pressure remains the dominant force in the weather,
with a subsidence inversion of varying strength holding some
moisture in the boundary layer. This will serve to promote the low
clouds and fog, especially during the night and early morning
hours. Some models depict some drizzle coming out of that stratus.
There is not much forcing except a slight upglide in the weak
westerly wind near the aforementioned inversion. This may be
enough to squeeze some of that drizzle out. Confidence in where is
lower. For Thursday night/Friday morning I have the main threat in
the lee of the Cascades and sheltered northeast valleys. Friday
night into Saturday I shift the threat toward the higher Palouse.
Two reasons: (1) a weak shortwave passes and (2) the lower level
flow turns westerly. The shortwave will also bring a general snow
shower threat, possibly mixed with some rain, across the higher
terrain of far NE WA and the NE and Cntrl ID Panhandle. The
precipitation amounts look light, if anything measures, and
appears to begin to wane from the north late Saturday morning and
afternoon as drier air slips in, with PWATs dropping slightly.
That drop is short-lived as a more organized system begins to
approach the region Saturday night, with the leading warm front.
By then look for thickening middle and high clouds. A slight
threat of showers will come toward the Cascades and northern
mountains too, but the overall precipitation potential remains
low. /J. Cote`

Sunday through Tuesday: The region transitions to a wetter and at
least more seasonably colder pattern. An organized frontal wave
makes its way in with a subtropical moisture tap Sunday into
Monday. Strengthening isentropic ascent with the leading warm
front and that deepening moisture will lead to increasing
precipitation chances into Sunday afternoon into Sunday night.
Average PWATS rise to between 0.40 and 0.70 inches. Going into
Monday precipitation chances will remain high across the Cascades,
eastern third of WA and ID. However chances will begin to wane in
the lee of the Cascades Monday as the westerly, downsloping flow
increases. Monday night into Tuesday models continue to disagree
on the features that come in, but the broader pattern is the west-
northwest flow. This, in conjunction with PWATS remaining at least
near or slightly above normal for this time of year (0.30-0.40
inches), will keep a threat of precipitation alive. Chances will
be highest within the Cascades and the Idaho Panhandle to Blue
Mountains and lowest in the lee of the Cascades. As for snow
levels they are expected to average between 2-4kft, but will be
locally closer to 1kft during the morning and night hours along
and north of I-90. So this means a possible snow or rain/snow mix
even down to some of the valleys again. As for temperatures, they
will drop closer to average for this time of year. /J. Cote`

Beyond: something we will keep an eye going into the middle of
next (Wednesday or Thursday) is the potential for an arctic
intrusion. At this point confidence is very low that it will
occur. Not enough model runs have consistently indicated this
solution. At this point model consistency favors the arctic air
remaining east of the Continental Divide and at least maintain the
seasonal temperatures. However we will watch model trends. /J.
Cote`


&&

.AVIATION...
12Z TAFS: Another TAF period full of fog and stratus looks
promising with lower level inversions still in place. Abundant moisture
in the low levels has led to periods of drizzle around the GEG
area. Fog persists through the AM hours before inversions weaken
with a brief break down of the ridge allowing for much of the fog
to lift into a stratus level albeit most locations will remain in
IFR conditions. With an abundance of moisture remaining in the
lower levels the stratus is expected to persist through the
afternoon and evening. Confidence degrades as we move into
Tonight as to whether the stratus lowers leading to more fog or
if it remains elevated. This will really depend on if any
clearing occurs today. /Fliehman

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        41  32  42  29  39  30 /   0   0   0   0  10  10
Coeur d`Alene  41  33  42  29  41  29 /  10   0  10  10  10  10
Pullman        49  36  46  32  44  32 /  10   0   0   0  10  10
Lewiston       54  38  50  35  46  34 /  10  10   0   0  10  10
Colville       38  31  39  26  36  26 /   0  10  10   0  10  10
Sandpoint      40  29  40  27  38  27 /  10  10  10  10  10  10
Kellogg        42  32  43  30  42  28 /  10  10  10  10  10  20
Moses Lake     41  33  43  30  39  31 /   0   0   0   0  10  10
Wenatchee      41  33  43  32  39  32 /   0  10  10   0   0  10
Omak           37  29  38  28  36  28 /   0  10  10   0  10  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Dense Fog Advisory until Noon PST today for Northern Panhandle.

WA...Dense Fog Advisory until Noon PST today for Upper Columbia Basin.

&&

$$




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