791 
FXUS66 KOTX 182348
AFDOTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
345 PM PST Mon Feb 18 2013

.SYNOPSIS...The weather pattern will become increasingly active
over the Pacific Northwest this week. Tonight into Tuesday, an
upper level disturbance will bring a chance for accumulating snow
to portions of southeast and east central Washington, as well as
the Idaho Panhandle. Wednesday should be a relatively quiet, but
cool day. Look for increasing clouds and the threat for some
light snow on Thursday. A big storm will potentially bring strong
winds to the region on Friday into Saturday along with heavy
mountain snows.


&&

.DISCUSSION...

...LOCALIZED SNOW ACCUMULATIONS TONIGHT AND TOMORROW IN SOUTHEAST
WASHINGTON AND SOUTHERN IDAHO PANHANDLE...AND A THREAT OF WINTRY
CONDITIONS RETURNING FRIDAY INTO EARLY SATURDAY...

Tonight: An upper level trough will near the WA coast while a low
level occluded front will push east of the Cascades. Cloud cover
will continue to thicken and lower this evening while the best chances
of precipitation will lie across the northern mountains as a weak
upper level shortwave presses east along the Canadian border. Dry
low levels will give way to areas of virga across the Columbia
Basin this evening as seen by the increase in radar returns but
lack of ground reports. But overnight as the front pushes across
eastern Washington and easterly winds lighten up, anticipate a
band of light snow to develop with minor accumulations of under an
inch spanning from Blue mountains, Palouse, Spokane/Coeur d'Alene
area into northern Idaho.

Tuesday and Tuesday night: The upper level trough will continue to
deepen as energy dives south into California. This will slow the
eastward progression of the frontal boundary and stall it out near
southeast Washington and southern Idaho panhandle. Meanwhile, the
models show a weak upper level shortwave shooting from the parent
low and across northeast Oregon to southeast Washington. The
combination of the low level deformation band left by the decaying
front and this upper level lift has the potential to lead to
intense bands of snow that will persist through the day on
Tuesday. Then by Tuesday night, the upper level trough axis shifts
over the region enhancing the lift while low level wind flow shift 
northwest and enhance the upslope flow. Confidence is lowered by
the placement of the these snow bands and specifics amounts. But after
blending the NAM and GFS qpf amounts, have indicated the most
likely area will stretch from the Palouse region into the central
Idaho panhandle where 1 to 3 inches of snow are possible and south
from the Blue mountains to the Camas prairie where 2 to 4+ inches
are possible. Snow chances will decrease rapidly to the north and
west in the wake of the upper trough axis. Temperatures will
remain cooler than normal. /rfox. 

Wednesday and Thursday: Wednesday has the potential to be a
relatively quiet weather day during an increasingly active stretch
of weather. There is good agreement between the NAM, GFS and
Canadian models that eastern Washington will be between systems on
Wednesday with a weakening trough axis over western Montana and a
series of storm systems over the northern Pacific. By Thursday,
moisture will be on the increase. The medium range models are in
fair agreement that a low amplitude shortwave will move through
the Inland Northwest Thursday. This upper level system will not
pack the same punch as storms expected Friday and into the
weekend. Snow levels will be low enough for snow to the valley
floors on Thursday and Thursday night, but at this time,
accumulations appear to be light. /GKoch

Friday through Saturday night: Models remain in good agreement
with the depiction of a robust, wintry system moving into the
region this period. Energy now coming off the Asian continent
tracks into the Gulf of Alaska later this week, deepening toward
the Pacific Northwest as a 140kt+ jet digs down its backside. The
parent low occludes near the B.C. coast Friday morning. While a
weaker disturbance moves east from the region early Friday, the
warm front and accompanying subtropical moisture plume leading the
strong system comes in from the west. Models still disagree over
the precise timing. The GFS envelopes the region with modest
isentropic lift and moisture earlier Friday, while the ECMWF holds
it back until late Friday and offers a shorter period of
isentropic lift, before the cold/occluded front passes Friday
evening into Friday night. The forecast leans toward the slower
ECMWF, which has had better continuity and carries some agreement
from the GEM. Overall this means precipitation chances rising
through the day Friday, peaking Friday evening with the incoming
cold front and supporting mid-level trough, diffluence and jet
dynamics, before waning overnight.

As for snow levels and precipitation-type, some models are warmer
today than this time yesterday. However this largely comes from
the more amplified GFS. There is about a 2000 foot difference in
snow levels between the ECMWF and GFS by Friday afternoon. This
degrades confidence in the precise impacts a little. Yet when it
comes to surface temperatures models are more consistent in
showing highs in the mid to upper 30s across a large part of the
CWA, with some 40s to lower 50s in the Basin/Wenatchee area and
L-C Valley. Tack on the potential PWATs around 0.45-0.55 inches
where it is difficult to expect just snow starting around this
level, suggests favoring higher snow levels. This lends a forecast
with rain or mix of rain and snow below 2000-3000 feet, changing
to snow through the evening. This means impacts during the day and
at least the early commute time should be limited. All snow is
expected generally above 3000 feet. With this snow, the biggest
possible impacts at this time should be around the mountain zones
and Camas Prairie, and secondarily over the Palouse/C'dA area and
thirdly over the Spokane area and areas into the Basin along and
north of Highway 2. The potential remains for moderate to heavy
accumulations above 3000 feet starting around midday to early
afternoon Friday, while light to moderate accumulations will
threaten the other areas outlined above toward evening.

The other aspect of the storm system will be winds. As a sharp
shortwave and cold front slices through the region, mixing with
higher levels will bring some breezy to windy conditions. They are
expected to begin to increase through the late morning to
afternoon hours Friday as the front approaches, but the strongest
winds are expected Friday evening into Saturday morning. Some
guidance is showing winds around the around the 900-850mb level
(500-2500 feet AGL) between 30 and 50kts. How effectively this is
mixed down to the surface remains to be seen during. Guidance
shows mixing heights not really reaching well into this lay until
after the precipitation has wound down later Friday evening and by
then this region of winds has shifted out. Mountains, on the other
hand, will still have the opportunity be directly impacted being
more exposed. Blowing snow may not be a large impact outside of
the mountains given that the snow looks to be more on the wet-side
at this time.

As the system continues to move east on late Friday night into
Saturday a secondary low develops over northwest to north-central
Montana. An inverted surface trough deepens and trails west
through north Idaho and southwest into eastern Washington, until
shortwave riding briefly builds in Saturday night. Moisture wraps
around the exiting low into the aforementioned inverted trough,
with conditionally unstable lapse rates around 7 C/km and pockets
of CAPE with the mid-level trough. Combined with embedded
vorticity lobes dropping through in the northwesterly flow, this
will keep precipitation chances high across through the eastern
third of Washington, as well as near the Cascades. Snow looks like
the predominant weather type, with some rain or a rain/snow mix in
the afternoon. Look for continued breezy to windy conditions,
especially in the morning, before gradients and mixing slackens
through the afternoon and into Saturday night./J. Cote'

Sunday and Monday...The active pattern continues, with yet another
wet weather system to affect the Inland Northwest early next week.
The problem is with the timing. The features on this system look
similar to the Friday/Saturday system, with perhaps a little less
moisture. The track is similar, meaning that a good chance exists
for precipitation at many locations, save for parts of the
Columbia basin. However, with the extended models differing on
timing that is wide as the state of Washington, confidence to focus
one particular period for higher precipitation chances was not
high. The one area that precipitation chances were increased
slightly was over the Idaho panhandle late Sunday into Monday. If
the faster GFS verifies, the panhandle will be in the post-frontal
up-slope convective region (it is -37 degrees Celsius at 500 mb).
But if the ECMWF verifies, the brunt of the storm will be
affecting that region during that time frame. Lots of time to look
at this and fine tune the details. ty

&&

.AVIATION...
00Z TAFS: Low pressure digging off the coast will eject a weak upper 
level disturbance into eastern Washington and north Idaho tonight. A 
tough and low confidence set of TAFs especially for Spokane, Coeur 
D'Alene, Lewiston and Pullman. Steep lapse rates to almost 20 
thousand feet will contribute to convective (or showery) bands of 
snow tonight and potentially into tomorrow. The NAM and GFS forecast 
bands of light to moderate snow showers spreading over southeast 
Washington and north Idaho beginning around 09z. Localized pavement 
accumulations will be possible and could impact the aforementioned 
airports. But given the hit and miss nature of convective snow 
showers, it is tough to get too specific with snow amounts, 
visibilities or ceilings. Frequent amendments may be needed from 
early Tuesday morning onward. /GKoch

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        28  38  25  38  25  38 /  60  50  10  10   0  40 
Coeur d'Alene  28  38  26  37  27  38 /  60  50  40  10   0  40 
Pullman        30  40  28  39  27  41 /  60  60  60  20   0  30 
Lewiston       32  45  32  45  30  45 /  50  60  60  10  10  20 
Colville       26  41  26  41  24  40 /  60  20  10  10  10  50 
Sandpoint      27  38  24  36  28  37 /  60  50  30  20  10  50 
Kellogg        28  36  28  35  29  35 /  30  50  60  40  10  50 
Moses Lake     26  42  24  44  26  44 /  20  10   0  10   0  20 
Wenatchee      27  42  26  44  27  42 /  20  10  10   0   0  10 
Omak           24  37  21  41  21  37 /  60  10   0   0   0  20 

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.
&&

$$