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Area forecast discussion...updated 
National Weather Service Spokane Washington 
341 PM PST Sat Nov 21 2009 


Synopsis... 
a stronger and colder storm system will bring the potential for 
moderate to heavy snowfall over much of the region tonight and 
Sunday. A series of storm systems will continue to pass through 
the inland northwest every 24 to 36 hours through at least the 
middle of next week. 




&& 


Discussion... 
..light to moderate snowfall expected tonight through Sunday 
across the inland northwest... 


Tonight through Sunday...afternoon water vapor showing a strong 
Pacific storm charging toward the Washington coast with a classic 
baroclinic Leaf signature indicative of the organized deepening 
surface cyclone and associated conveyor belts. Models have come into 
better agreement regarding the incoming storm system which now 
track the surface low over the Kittitas Valley...along the I-90 
corridor...before tracking to the northeast across the northern 
Idaho Panhandle into southeastern British Columbia. The low will 
bottom out this evening while crossing onto the Washington coast 
just below 990mb...then gradually fill and weaken overnight into 
tomorrow. The heaviest precipitation overnight will focus along 
the Cascades...Waterville Plateau...Okanogan region...as the low 
nears the coast tonight and increasing easterly flow enhances lift 
under a blanket of deep isentropic ascent. As the low jumps the 
Cascades after 12z Sunday...the precipitation shield will begin to 
shift toward far eastern Washington and Idaho. Radar imagery will 
likely illustrate a well defined deformation band along the 
northern periphery of the low which is where the 6 hour liquid 
amts will near three-tenths of an inch then as the low ejects to 
northeast midday tomorrow...continued strong upslope into the Idaho 
Panhandle will result in the second chance for moderate 
precipitation rates...mainly focusing into the Panhandle 
mountains...Idaho Palouse...and Coeur D Alene area. Snowfall totals 
will generally run 2-5 inches in the valleys with 6-10" in the 
mountains. With the highest totals across the central Idaho Panhandle 
as the second wave of precipitation forms in the wake of the low. 
This is where winter weather warnings were posted and valleys can 
expect 3-6" with 8-10" in the mountains. 


Current observations from the east slopes...Okanogan Highlands...and 
Wenatchee area suggest temperatures have warmed into the middle to 
upper 40's below 2000' with dewpoints in the upper 20's to lower 
30's. Calculated wetbulb cooling suggest temperatures will remain 
in the middle to upper 30's with the onset of precipitation so all 
winter highlights in the aforementioned zones were focused mainly 
above 2000'. 


Winds will be breezy through the period with the highest gusts 
expected Sunday afternoon under the cold advection regime. Models 
continue to hint 850mb winds of 35-50kts down the east slopes and 
this will need to be monitored closely. These breezy winds will 
also cause areas of blowing snow at times but with the snow being 
of a wetter nature...expect this will be localized. /Sb 


Sunday night through Tuesday night...model guidance is in good 
agreement that the period will begin on a quieting note as 
sundays upper level trough shifts into central Montana. The trough 
will be slowly replaced by a well-defined upper level ridge 
resulting in rapid stabilization over the entire region. Thus the 
Post-frontal showers over the Idaho Panhandle due to moist upslope 
SW flow will decrease rapidly. Model cross sections show the 
potential instability decreasing from around 550 mbs at 00z to 
less than 700 mbs by 12z. Meanwhile relative humidity values 
decrease rapidly above 700 mbs as well. This suggests that most of 
the Post-frontal shower activity over the central Panhandle will 
decrease after 06z with only a few persistent showers . 


Generally dry weather will then persist through most of Monday until 
another front pushes into western Washington late in the day. 
Moisture associated with this disturbance was currently located 
just west of 150w and was adequately handled by both the GFS and 
European model (ecmwf) as of 22z. Both model solutions bring the pre-frontal upper 
level ridge into north Idaho by 00z Tuesday with a swift moving 
shortwave trough passing through the southern third of British Columbia overnight. 
This system should be much less dynamic and impressive than the 
one expected tonight and Sunday. In fact...the models have been 
trending toward keeping the precipitation fairly light with most 
of it holding over the northern third of the County Warning Area. Given the track 
of the upper level trough...the primary flow in the 850-700 mb layer 
will remain out of the west so without the benefit of a strong 
warm front or isentropic ascent we would also expect to see a fair 
amount of downslope flow over the Cascades. While both the GFS and 
European model (ecmwf) do hint at a weak warm air advection pattern in the 850-700 
mb layer its not necessarily enough to overcome the downslope 
impacts of the Cascades. While this system wont have the benefit 
of strong dynamical forcing it will feature a high amount of 
moisture...as evidenced by precipitable water values around 
150-180 percent of normal. There will also be persistent weak 
isentropic ascent. Whats interesting in this scenario is the front 
loses much of its eastward momentum as yet another system pushes 
into the coast by late Tuesday. This next system amplifies yet 
another shortwave ridge over the area...which allows the Monday 
front to sit and gradually weaken over the region and shift slowly 
south through Tuesday . Given this scenario we would expect to see 
a light and rather prolonged stratiform precipitation event much 
of which will fall as snow. Given the weak forcing we prefer the 
drier GFS and European model (ecmwf) solutions over the rather wet NAM. At this 
point it appears winter highlights will not be necessary. Most of 
the precipitation is expected to fall north of i90. Fx 


Wednesday to Saturday...the medium-range starts with a ridge over 
the west...a warm front near the British Columbia border and a cold front near 130w. 
This leaves passing middle/higher clouds and a chance of rain and snow 
across the Cascades and Washington/British Columbia border mountains into Wednesday nt...with some 
early fog/stratus threat. Thereafter models diverge over cold front 
passage. The forecast leans to the GFS...with better consistency and 
Gem/dgex agreement. The European model (ecmwf) trended to GFS. All this translates 
to the cold front pushing to the Cascades Thursday am and east Thursday nt 
into Friday am...increasing precipitation chances. A threat for orographic 
rain/snow showers continues Friday before the next system approaches 
later Saturday. Slightly above normal temperatures Wednesday trend 
closer to normal by Saturday. /Jcote 


&& 


Aviation... 
a weak ridge aloft will promote VFR conditions at all taf sites 
through 03z tonight...with lingering snow showers locally 
obscuring mountains in the Idaho Panhandle and Cascade crest. 
The next Pacific storm system will spread IFR ceilings to the 
western reaches of the region after 04z...spreading into the 
eastern terminals after 10z. The incoming storm system will 
also tighten pressure gradients leading to increasing winds 
from kpuw-kgeg with gusts of 25-35kts. The precipitation 
will begin to wane from the west after 18z. 




&& 


Preliminary point temps/pops... 
Spokane 31 37 25 38 30 40 / 80 90 10 20 40 20 
Coeur D'Alene 30 38 27 39 30 40 / 50 90 20 20 50 20 
Pullman 31 37 27 40 29 44 / 70 90 10 10 30 20 
Lewiston 35 42 31 47 32 49 / 40 90 20 10 20 20 
Colville 32 40 27 39 29 40 / 80 100 10 50 60 10 
Sandpoint 30 37 27 34 27 36 / 30 90 20 50 60 40 
Kellogg 29 33 28 33 29 37 / 30 100 70 20 50 30 
Moses Lake 31 44 24 43 26 45 / 100 50 0 30 30 0 
Wenatchee 32 43 29 43 32 45 / 100 50 0 30 40 0 
Omak 31 43 22 41 29 45 / 100 80 0 70 80 0 


&& 


Otx watches/warnings/advisories... 
Idaho...Winter Weather Advisory from 4 am to 10 PM PST Sunday for 
northern Panhandle. 


Winter Storm Warning from 4 am Sunday to 1 am PST Monday for 
central Panhandle mountains. 


Winter Storm Warning from 4 am to 10 PM PST Sunday for Coeur 
D'Alene area...Idaho Palouse. 


Washington...Winter Weather Advisory from 4 am to 4 PM PST Sunday for 
northeast mountains...Spokane area...Washington Palouse. 


Winter Weather Advisory from 4 am to 10 PM PST Sunday for 
northeast Blue Mountains. 


Winter Weather Advisory from 10 PM this evening to 10 am PST 
Sunday for east slopes northern Cascades. 


Winter Weather Advisory from 10 PM this evening to noon PST 
Sunday for Okanogan Highlands. 


Winter Weather Advisory from 10 PM this evening to noon PST 
Sunday for Okanogan Valley...upper Columbia Basin... 
Waterville Plateau. 


&& 


$$ 

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