Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 221227
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
424 AM PST Wed Feb 22 2017

.SYNOPSIS...
The widespread wet weather of the past couple days will begin to
subside as a cooler and unstable air mass takes up residence
across the region later today. This air mass will deliver periodic
showers to the area, with most of them occurring over the
mountains. Temperatures will remain cooler than normal for this
time of year with highs generally in the mid 30s to mid 40s and
nighttime lows in the teens to middle 20s.

&&

.DISCUSSION...
Today and tonight...The widespread wet weather of the past few
days will finally come to an end during this period as any
remnants of the atmospheric river continue to shift southward with
the advance of a cold upper level trough. As of 2am, it appeared
most of the moisture from the atmospheric river was fixed from the
SE corner of Washington and toward the Silver Valley. This
moisture has combined with weak to moderate isentropic ascent in
this region and was responsible for the increase in light
precipitation during the past couple hours. The best ascent and
moisture availability was focused over locations south of Pullman
and onto the Camas Prairie. The models are fairly consistent on
weakening this ascent as the upper level trough begins to shift
toward the north Cascades by late morning. Before that happens its
possible some moderate snow could develop on the Camas Prairie and
perhaps into the Clearwaters. A winter weather advisory was issued
for this area last evening and we will continue with it for now.
Snow levels in this area will generally be 2000` or lower. By
afternoon the focus will shift from the stratiform precipitation
over this region to more of a showery regime as the upper level
trough shifts into NC Washington. 500 mb temperatures plummet to
-37c or colder which will combine with any daytime heating
resulting in some fairly good instability. The NAM is forecasting
lifted index values of zero or slightly lower across the northern
third of Washington by afternoon and this will likely lead to a
diurnal increase in shower coverage. Additional showers are also
expected across the ID Panhandle, especially south of I90. The NAM
sounding show relatively deep convection with cloud top
temperatures likely of -20c or colder. While this can support the
remote chance of thunderstorms, we suspect the cloud bottom
temperatures will be a little too cold to support a good charge
separation in the cloud. We suspect the setup will lead toward
more graupel showers than anything else. For tonight, the upper
level trough will continue to drop southward with good
destabilization occurring across the entire region. Despite this
trend we suspect the chances of showers will wane as the lower
portion of the atmosphere cools just as fast if not faster than it
does aloft. While a few showers will be possible over the
mountains, it should be dry most locations overnight. Temperatures
could be tricky. As the trough moves in there is a decent chance
the widespread stratus currently over eastern Washington should
break. The clearing skies (if they occur) will lead to rapid
cooling and temps will dip to their coolest levels in a few days.
We expect lows to be in the teens to mid 20s. This could lead to
some rapid fog development, however a transition to light north
winds in the boundary layer would suggest otherwise. Expect to see
lows in the mid teens to mid 20s for most locations, however there
is a high amount of uncertainty with this forecast.

On another note, we still have an areal flood watch out for a
portion of SE Washington and NC Idaho. Odds of any flooding at this
point look fairly small other than some minor field flooding due to
the precipitation of the past couple days combined with any snow
melt. Additional precipitation today will be rather light and will
largely consist of snow. Despite the small risk of any flooding, we
will continue with the watch since it will be easier to gather
ground truth (if any) during the day. fx

Thursday through Tuesday: The Inland NW transitions to a cooler,
showery west-northwest flow. A long-wave trough sets up over the
western U.S. and the jet stream and primary storm track largely
steer the more robust systems and deeper moisture away from the
region. Yet smaller scale features and the flow will still provide
some risk for showers, largely in the form of snow, with the best
risk around and just downwind of the mountains and across the
eastern Columbia Basin. Precipitation amounts look rather light,
compared to more recent systems. Yet details will need fine-
tuning. The night/early morning will have a patchy fog threat.
Otherwise expect a mix of clouds and sun. A notable feature of
the period will be temperatures, as they will be below seasonal
norms by some 6-11 degrees /J. Cote`

&&

.AVIATION...
12Z TAFs: Our confidence for most of the forecast sites is low
today. For this morning, we expect IFR conditions to persist at MWH
due to stubborn fog. This should break sometime between 18z-21z.
This is the easiest of the IFR sites. GEG-SFF-COE are very difficult
in comparison. We cant see how extensive the low clouds around here,
while radar is showing some very light precipitation echoes in this
area. This could consist either of snow flurries or light drizzle.
The problem is temps will remain right around 32 degrees through the
morning and periods of freezing drizzle can`t be entirely ruled out
of the equation. This threat should wane by midday with slowly
improving conditions thereafter. For LWS and PUW, the radar echoes
are a little more substantial and could deliver light snow
especially for PUW. Cigs there are already IFR and any snow will
only add a visibility to that category. As low as the confidence is
for the day, it will be even worse tonight. The widespread low
clouds and IFR conditions are expected to break for all locations
sometime today, but whether they reform after sunset is
questionable.  If the low clouds reform the best chances will occur
over the eastern sites, as drier northerly winds develop over central
Washington overnight. fx

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        36  22  35  19  33  19 /  20  10  10  10  20  10
Coeur d`Alene  37  21  36  17  35  16 /  30  20  10  10  30  10
Pullman        38  22  36  21  36  19 /  10  10  20  10  20  10
Lewiston       43  28  41  26  41  25 /  20  10  20  10  10  10
Colville       39  20  37  18  36  19 /  20  20  10  10  20  10
Sandpoint      37  21  36  19  33  17 /  20  20  10  20  20  10
Kellogg        36  22  34  20  33  18 /  30  10  30  20  30  10
Moses Lake     40  20  36  20  35  21 /   0   0   0  10  10  10
Wenatchee      39  23  35  20  33  21 /   0   0  10  10  20  20
Omak           38  19  35  19  32  20 /  10  10  10  10  20  20

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Flood Watch through this afternoon for Central Panhandle
     Mountains-Coeur d`Alene Area-Idaho Palouse.

     Winter Weather Advisory until 11 AM PST this morning for Central
     Panhandle Mountains-Lewis and Southern Nez Perce Counties-
     Lewiston Area.

WA...Flood Watch through this afternoon for Spokane Area-Washington
     Palouse.

&&

$$



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