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Area forecast discussion...updated aviation discussion 
National Weather Service Spokane Washington 
420 PM PDT sun Sep 7 2008 


Synopsis... 
the inland northwest will be under the influence of high pressure 
through Monday. The next low pressure system expected to affect the 
region arrives late Tuesday...bringing an increase in clouds and 
locally windy conditions. 


&& 


Discussion... 
tonight through Wednesday...water vapor imagery shows an upper level 
trough advancing on the Pacific Ridge this afternoon acting to nudge 
the ridge axis to the east. As the ridge moves east the jet stream 
that has been in place over the northeast corner of the forecast 
area will also slide east ending the threat of showers over the 
northeast mountains at least temporarily. Rising heights will result 
in dry conditions with a warming trend for daytime temperatures. 
Readings will warm to slightly above seasonal normals. The clearing 
skies and light winds will result in good radiational cooling with 
min temperatures a bit cooler than sundays readings. Morning 
temperatures will drop below 35f for a good portion of the northeast 
valleys with a few locations reaching the freezing mark mainly near 
Priest Lake in Idaho. It does not appear that there will be enough 
areal coverage to meet freeze warning criteria. After a quite 
period for Monday and Tuesday a shortwave trough will swing through 
the region late Tuesday with the greatest impact across the northern 
and eastern zones. As the upper trough drops through the inland 
northwest a strong vorticity maximum will swing through the base of the 
trough clipping the northeast portion of the forecast area. This 
will bring the best chance of showers to the northeast zones 
overnight Tuesday. Have maintained a slight chance of thunder across 
the northeast corner of the forecast area for Tuesday afternoon and 
evening. Instability is not overly impressive but with a strong vorticity 
maximum and a -20c 500mb cold pool sliding overhead cannot rule out the 
possibility of thunder. Once the flow turns northerly behind the 
trough winds will be a concern. As the northerly flow is channeled 
down the Okanogan Valley Tuesday evening breezy to windy conditions 
will develop. On Wednesday a shortwave ridge will build over the 
region for a dry but cooler day in northwest flow aloft. 
Temperatures will be near seasonal normals. /Kelch 


Wednesday night through Thursday...a short-wave ridge translates 
across the inland northwest. Larger-scale subsidence with rising 500 mb 
heights this period should easily squelch any minor ripple that 
comes down in the northerly flow. With a largely dry atmospheric 
column...the forecast calls for no precipitation with mainly a few high 
clouds skirting by the northern County Warning Area. 


Thursday night through Saturday...a Gulf of Alaska trough moves into 
western Canada and flattens the ridge over the inland northwest. As the trough 
moves inland some lift (weak positive vorticity advection/omega) ripples across the region. 
This is especially Friday and Saturday across the north to 
northeast County Warning Area. Some comparatively deeper moisture skims across the 
north and eastern County Warning Area during this period as well. Tack on the 
westerly flow...this warrants at least a slight chance for showers 
over the northern mountains Friday. Chances then expand to the Idaho 
Panhandle mountains on Saturday. The rest of the County Warning Area will see just a 
few more clouds but an otherwise dry forecast...and some breezy 
conditions by Friday. 


Saturday night and Sunday...the next trough digs into the western Gulf 
of Alaska and a ridge amplifies downstream near the Pacific coast. 
While guidance generally agrees on this evolution...there is some 
disagreement as to how amplified the pattern becomes. But a 
general west-northwest flow is expected. Look for passing high clouds and 
some afternoon cumulus across the north and eastern mountains in 
shallow instability...and generally dry conditions. 


Temperatures will remain near or slightly above normal much of 
this period. Expect the milder temperatures by Thursday (as 
compared to wednesday) with the building ridge and 850 temperatures 
pushing into the middle/upper teens. A slight dip in temperatures is 
expected toward Saturday with the squelched ridge. /Jcote 


&& 


Aviation... 
VFR conditions at all taf sites through 00z Tuesday...as high 
pressure builds over the inland northwest. Sunday afternoon 
cumulus will decrease...leaving mainly passing high clouds 
into Monday. 
/Jcote 








&& 


Preliminary point temps/pops... 
Spokane 47 77 49 79 48 73 / 0 0 0 0 20 0 
Coeur D'Alene 45 76 47 78 47 72 / 0 0 0 0 20 0 
Pullman 37 78 40 79 46 74 / 0 0 0 0 20 0 
Lewiston 53 84 55 84 54 80 / 0 0 0 0 10 0 
Colville 42 78 44 80 44 75 / 0 0 0 10 30 0 
Sandpoint 41 72 43 74 44 69 / 0 0 0 20 30 10 
Kellogg 45 74 47 74 46 69 / 0 0 0 0 30 10 
Moses Lake 51 83 53 85 51 80 / 0 0 0 0 0 0 
Wenatchee 54 82 56 84 55 79 / 0 0 0 0 0 0 
Omak 49 83 51 84 49 80 / 0 0 0 0 0 0 


&& 


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


$$