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fxus66 kotx 091118 
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Area forecast discussion 
National Weather Service Spokane Washington 
318 am PST Friday Jan 9 2009 


Synopsis... 
drier and cooler air has temporarily moved into the inland 
northwest. This airmass change will dramatically slow the runoff 
going into area rivers and streams through today. The weather 
pattern will remain cooler through the weekend...however a weak 
front will bring the threat of light to moderate snow to locations 
near the Canadian border and Saturday and then shift southward on 
Sunday. Strong ridging will then bring dry conditions to the area 
through most of next week. 






&& 


Discussion... 
today and tonight...quiet weather at last will be the norm over 
the inland northwest today. Early this morning...the water vapor loops 
were depicting a departing shortwave trough over SW Montana...with 
resultant unsettled northwest flow behind it. Earlier this morning...this 
triggered scattered showers over the southeast corner of the County Warning Area...with a 
nice orographic signature over the northern portions of the Camas 
Prairie. Lately...the Spokane radar has generally lost all 
echoes...as increasing warm air advection in the middle-atmosphere has 
put a lid on the potentially unstable airmass. This increasing 
stability developed as yet another zonal flow pattern develops 
over the inland northwest. More on this later though. For today...we 
suspect most locations will generally be dry while snow levels 
will be considerably lower than yesterday. This comes as good 
news for all the Hydro concerns over the area...as the runoff 
today from snowmelt will be rather negligible and limited mainly 
to valley locations. The lack of significant precipitation will 
also help matters. This will allow US to begin paring back some of 
the Hydro headlines. We will begin near the Cascades...where 
according to the latest hydrographs...all rivers have crested or 
will very shortly. Over central Idaho and southeast Washington...the situation 
is a little more complicated as much of the runoff from the past 
few days is still working down lower portions of the rivers. This 
process has also been held up in many cases by significant icing. 
Ice damming is still likely present on the St. Joe river downstream 
from Calder. For this reason we will continue with the Flash Flood 
Watch for downstream locations. Meanwhile the ice jams have 
seemingly broken on hangman creek and the Palouse river...both of 
which are beginning to fall. Expect many of the other unable gaged 
smaller rivers and streams still have some icing problem...so we 
will let all other Hydro headlines ride for now and allow the water 
slowly work its way out of the system. Now onto the weather. 


The latest GFS and NAM were both quite similar in bringing a 130kt 
westerly jet across southern British Columbia and Alberta later this afternoon 
and tonight. According to the latest AMSU/SSMI/GPS precipitable 
water imagery this jet was embedded with a tap of sub-tropical 
moisture streaming northeast from the western Hawaiian Island. 
Despite the tap into the sub-tropics...this plume was much 
narrower and held a fraction of the moisture that was embedded over 
the pacnw over the past several days. This moisture plume is 
expected to set up over extreme southern British Columbia and Alberta late this 
afternoon and into tonight. Model cross sections through the 
northern tier of zones suggest initially we will see increasing 
clouds near the Canadian border...however the precipitation will 
likely hold off until after 00z. Persistent westerly flow in the 
850-700 mb layer will result in significant rain shadowing over 
the Cascade. This suggests areas such as the Okanogan Valley and 
Highlands will largely be spared by this moisture whereas the 
north Idaho Panhandle and Cascade crest will receive most of the 
snow. Quantitative precipitation forecast totals through tonight are not impressive...with totals 
ranging from .07 to .15 inches over the northern selkirks and 
northern Cascade crest...with much lighter amounts elsewhere. This 
may result in 1 to 2 inches of snow for northern valley locations 
such as Metaline Falls and Bonners Ferry. Fx 


Saturday through Thursday...subtropical moisture tap sags into the 
west side of a general longwave ridge with an axis just off the 
coast Saturday and Saturday night. With no significant amount of 
cold air damming expected to be in place for much of The Lowlands 
with perhaps the exception of the far northern third of the 
forecast area a substantial rainshadow off the Cascades is likely 
as this system sags through. Locations along the periphery of the 
shadow...locations in the immediate vicinity of the Cascade 
crest...extreme northern Washington mountains and portions of the 
northern Idaho Panhandle will be better positioned to get 
measurable precipitation when this baroclinic band/subtropical 
moisture tap sags southeast through the forecast area Saturday and 
Saturday night. Sunday into Tuesday the longwave ridge axis 
shifts further west allowing at least two disturbance to slide 
into the area...first feature would be the baroclinic 
band/moisture tap from Saturday and Saturday night that moves back 
northward and through the forecast area as a warm frontal type 
feature...and after that another disturbance that slides down 
through southern British Columbia along the east side of the 
ridge in a northwest to southeast trajectory and over the forecast 
area Monday and Monday night...with the basin still not set up with 
cold enough air to allow any efficient isentropic upglide probability of precipitation 
again will remain fairly well focused over the earlier discussed 
periphery of the Cascades. Depending upon how close the surface 
low associated with the disturbance passage Monday is there could 
be some breezy conditions as the surface pressure gradient 
tightens across the County Warning Area Monday. Wednesday and 
Thursday the western US ridge amplifies and also Narrows...the 
result being the northwest to southeast oriented storm track on 
the east side of the ridge either gets pushed far enough east to 
allow shortwaves migrating through it to influence the forecast 
area or perhaps the path might actually just be void of shortwaves 
during that time...so there is the tendency to keep that part of 
the forecast the driest. Forecast temperatures do not show much of 
a trend through this time period except perhaps from about 
Wednesday on as GFS shows some colder air spilling over the 
Continental Divide from Canada and Montana...so a slight cooling 
trend...more apparent in overnight lows is depicted from then on. 
/Pelatti 




&& 


Aviation... 
clearing developing from the west and northwest Thursday night... 
with wind slowly decreasing will allow for some stratus development 
across the Palouse through Spokane/c'da area later overnight into 
Friday morning...with patchy fog. Look for IFR/local MVFR ceilings to 
develop toward 12-15z. Once the stratus develops it appears it will 
be slower to erode until middle and high clouds ahead of the next 
system approach later Friday (20z-03z). The western taf sites are 
expected to see predominately VFR conditions with the potential for 
vcfg early Friday. /Jcote 






&& 


Preliminary point temps/pops... 
Spokane 37 25 36 27 36 30 / 10 10 20 40 20 20 
Coeur D'Alene 37 27 36 27 35 30 / 10 10 30 60 30 20 
Pullman 38 27 40 33 41 32 / 0 0 10 30 20 20 
Lewiston 47 29 42 35 44 36 / 0 0 0 10 10 10 
Colville 36 23 32 25 33 27 / 10 20 30 50 30 10 
Sandpoint 39 27 32 28 34 29 / 10 30 40 60 30 30 
Kellogg 36 22 36 29 34 31 / 10 10 30 60 40 40 
Moses Lake 40 26 35 27 36 32 / 0 0 0 10 0 0 
Wenatchee 42 27 37 30 39 31 / 0 0 0 10 0 0 
Omak 38 25 31 25 35 27 / 0 10 10 10 10 0 


&& 


Otx watches/warnings/advisories... 
Idaho...Flash Flood Watch through Saturday morning for central Panhandle 
mountains...Idaho Palouse. 


Washington...none. 
&& 


$$ 

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