Weather

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fxus66 kotx 211216 
afdotx 


Area forecast discussion 
National Weather Service Spokane Washington 
413 am PST Friday Nov 21 2008 


Synopsis... 
another cold front will move through the inland northwest 
tonight. Precipitation amounts will generally be light with this 
quick moving frontal system. Strong high pressure will bring dry 
and seasonably cool weather Sunday and Monday...and the weather 
pattern will continue to be rather quiet through most of next 
week. 




&& 


Updated aviation discussion 


Discussion... 
issued 249 am 
today and tonight...early morning water vapor imagery shows the 
upper trough axis (with yesterday's front) has fractured and 
moved into central Montana. Post frontal...orographic snow 
showers remain over the mountains of the Idaho Panhandle...but 
these showers will diminish quickly. The NAM forecasts the 500mb 
cold pool (-25c to -28c) quickly exiting the Idaho Panhandle 
between 12z-15z...which should bring an end to the convective snow 
this morning. Gusty winds over the Palouse and Columbia Basin 
should also diminish quickly this morning. Surface pressure 
tendencies show significant pressure rises over eastern 
Washington. The RUC 850mb wind analysis also shows the low level 
wind field weakening considerably. Gusts as high as 41kts were 
measured by the Pullman ASOS early this morning. By 15z...winds 
should be in the 10-15kt range and begin to back to a more 
southerly direction this morning...then become east by late in the 
day. 


The next storm system shows up on the morning water vapor as a 
well defined baroclinic Leaf near 140w. The 00z runs of the NAM 
and GFS were in remarkably good agreement with the timing of the 
cold front tonight. The 06z GFS has trended a couple hours slower 
and looking at satellite trends this later model run is 
preferred. This frontal system will not produce as much 
precipitation as yesterday's storm. The vorticity center is 
expected to track further north...and the best q-vector 
convergence is forecast to be over southern British Columbia. So 
even though snow levels over the Waterville Plateau into the 
Okanogan Highlands will be low enough for snow in the low 
elevations...precipitation amounts will not likely be enough to 
produce measurable snow. With help from orographic lift...the 
crest of the Cascades and the mountains along the Canadian border 
can expect an inch or two tonight into tomorrow morning...mainly 
above 3000 feet. /Gkoch 


Saturday through Monday...yet another very deep impingement of 
stratospheric air into the middle and even lower levels of the 
atmosphere is evident over the Pacific this morning as water vapor 
imagery shows extremely strong drying encroaching on 140w as of 
10z. Pv fields from the 00z models' 12 hour forecasts indicate 
some lack of resolution of the tropopause undulation/fold over the 
Pacific with the bias of the models generally toward the weak/fast 
side when compared with water vapor imagery. The net result is a 
slow down in the forward progress of the aforementioned wave 
across the northwestern portion of the country as it gradually 
shears out and weakens once it pushes on shore. Both the European model (ecmwf) and 
UKMET showed at least some semblance of a strung out tropopause 
fold signature in their pv fields whereas the GFS indicated a more 
bimodal pv distribution separating the field prematurely into two 
distinct vortices over the Pacific...so the forecast this morning 
favors the European model (ecmwf) and UKMET consensus. 


In proceeding with the European model (ecmwf)/UKMET consensus...the main vortex 
shifts across the area between 00z and 12z Saturday with the best 
synoptic dynamics already emerging into the Front Range of the 
Canadian rockies by Saturday morning. However...the trailing low 
and middle-level front around the southern periphery of the system lag 
quite a bit behind the main system. As a result...frontogenesis in 
the middle and low levels coupled with a strung out frontal absolute 
vorticity field and net vertical upward increasing positive vorticity advection along the 
frontal zone will result in more stratiform rainfall still 
affecting a portion of the eastern County Warning Area through Saturday morning. 
However...as the front clears the area Saturday afternoon...probability of precipitation 
will gradually come down and precipitation will quickly transition 
into the Post-frontal showery upslope variety into the Idaho 
Panhandle mountains. 


Behind the front...colder and drier air gradually filters in. The 
drier air looks a bit more dominant on the 00z runs than it did at 
this time yesterday with no model really indicating significant 
areas of boundary layer saturation representative of fog. 
However...with decoupling and strong radiational cooling under the 
incoming synoptic scale ridge axis on all models...low-lying areas 
will certainly stand a chance at some fog development Saturday 
night into Sunday morning. 


The next system to approach the area will likely hold off until at 
least Monday night if not Tuesday. However...with this system the 
models are having significant difficulty with stream splitting and 
the handling of the focusing of development on the northern or 
southern stream. As a result...the forecast was left largely 
unchanged beyond Monday. /Fries 


&& 


Aviation... 
low stratus will flirt with kgeg...ksff...kpuw...and kcoe between 
12z and 18z. Gusty surface winds have sufficiently mixed the 
boundary layer overnight to keep most of the stratus away from these 
airports...but as the pressure gradient relaxes this 
morning...stratus over the Panhandle mountains may build westward. 
Since kcoe is closest to the mountains...this terminal will have the 
best chance of experiencing IFR ceilings this morning. The other 
sites may experience some ragged temporary stratus...but IFR will 
not likely be the prevailing condition. The next cold front will 
enter eastern Washington after 07z and bring the chance for light 
precipitation. /Gkoch 




&& 


Preliminary point temps/pops... 
Spokane 42 31 43 25 43 25 / 0 40 10 0 0 0 
Coeur D'Alene 44 33 43 27 43 26 / 10 50 40 10 0 0 
Pullman 46 36 45 28 46 28 / 0 30 30 10 0 0 
Lewiston 51 38 50 30 51 30 / 0 10 20 10 0 0 
Colville 39 31 41 23 39 24 / 10 40 10 0 0 0 
Sandpoint 43 32 41 27 42 24 / 20 50 30 10 0 0 
Kellogg 40 34 39 29 40 27 / 40 40 40 20 0 0 
Moses Lake 46 31 48 26 45 26 / 0 20 0 0 0 0 
Wenatchee 42 35 46 28 42 27 / 0 20 0 0 0 0 
Omak 41 32 44 24 42 24 / 0 30 0 0 0 0 


&& 


Otx watches/warnings/advisories... 
Idaho...none. 
Washington...none. 
&& 


$$ 

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