944 
fxus66 kotx 182125 
afdotx 


Area forecast discussion 
National Weather Service Spokane Washington 
225 PM PDT Friday Jul 18 2008 


Synopsis... 
a dry weather pattern will dominate the inland northwest the rest 
of the week with a weak system moving across southern British 
Columbia this evening. Daytime temperatures will remain near seasonal 
averages on Saturday before warming to slightly above normal on Sunday. 


&& 


Discussion... 
tonight...water vapor satellite indicates an upper level closed 
low at 2 PM centered along the British Columbia/Alberta border 
approximately 250 miles north of the Idaho/British Columbia border. Thunderstorm 
activity has been increasing near this low where a 500mb cold pool 
of -20c exists. Models in good agreement that this low will track 
southeast this evening remaining well north of our area...but 
marginal instability will clip far NE Washington/north Idaho with 500mb temperatures 
near -16c where isolated showers are possible. Trends in satellite 
imagery suggest that lightning should stay north of the Idaho/British Columbia 
border so took out mention of thunder in the forecast. All other 
areas across eastern Washington will be too far south from this 
low where dry conditions will persist. 


For Saturday through Monday...models in good agreement with a dry 
west to southwest flow. Models indicate a very weak short wave to 
track through southeast Washington into Shoshone County Sat night and 
Sunday morning. Moisture looks very limited with just a few middle 
level clouds in these areas with the wave passage. Concerning 
temperatures...mixing 850mb temperatures down to the surface yields maximum 
temperatures near to slightly above normal through this forecast 
period. Jw 


Monday night through Friday...high degree of uncertainty remains 
in this forecast interval...especially with the trajectories of an 
incoming synoptic scale closed low as well as the strength and 
intensity of the split flow affecting the Pacific northwest. 
Dprog/dt of the last nine runs of the GFS have the placement of 
the low in various locations on 00z Wednesday...ranging from as 
far west as central California and western Oregon to having some 
remnants of it as far east as central Idaho...the 12z run on the 
16th which is the oldest run has the Idaho placement. Northern 
branch of the flow over the area shows differing placement and 
intensity as well with current model runs showing it to be more of 
a contributor to sensible weather on Tuesday and Tuesday night 
than older runs have as current runs pass a dry cold front through 
the region Tuesday. Therefore will have some cooling reflected in 
the maximum temperature forecast Tuesday relative to Sunday and 
Monday. Earlier reasoning for Tuesday afternoon/evening 
thunderstorm mention was for possible shortwave ejection taking a 
path around the periphery of the incoming small synoptic scale low 
then up northward through eastern Oregon into eastern Washington 
and northern Idaho...however with the northern branch cold front 
entrance near the same time there is a possibility of two distinct 
storm tracks for Tuesday afternoon and Tuesday evening...one that 
would take thunderstorms from eastern Oregon and southern 
Washington and propagate them to the northeast at 20-30 miles per hour. The 
second is along the cold front passage...which would translate 
with the west to east movement of the cold front...and probably 
affect locations near the British Columbia border. It needs to 
be emphasized that there is low confidence in timing any of this 
at the moment so will do no further harm to the forecast by leaving 
Tuesday after/evening forecast relatively unchanged. Wednesday through 
Friday should be less complex with the hope that all of the 
features of note discussed up to Tuesday will have passed through 
most of the region and be far enough east to not be a contributor to 
ongoing weather. Latest GFS show ridging that amplifies some with 
time but an intensifying large Gulf of Alaska low is placed in 
such a way as to keep the 500mb ridge axis over part of western 
Montana Wednesday...central Montana Thursday...and eastern Montana 
Friday. The 850mb thermal ridge axis remains in close enough 
proximity to eastern Washington and northern Idaho to continue to 
keep warm/hot temperatures with a small warming trend Wednesday 
through Friday. /Pelatti 


&& 


Aviation... 


Volcanic ash significant meteorological information issued at 18z today highlights what is 
a diffused layer of a combination of volcanic ash and sulfur 
dioxide suspended at an elevation between thirty thousand and 
thirty five thousand feet above locations from Portland Oregon to 
Great Falls Montana. Some pilot weather reports report smell and throat 
irritation possibly associated with flying through it. Otherwise 
VFR conditions expected through 00z Sunday. 


&& 


Preliminary point temps/pops... 
Spokane 54 84 55 89 57 89 / 0 0 0 0 0 0 
Coeur D'Alene 52 83 49 88 51 88 / 0 0 0 0 0 0 
Pullman 45 83 47 89 49 89 / 0 0 0 0 0 0 
Lewiston 56 91 59 96 61 96 / 0 0 0 0 0 0 
Colville 49 87 51 93 53 93 / 10 0 0 0 0 0 
Sandpoint 47 80 49 85 51 85 / 10 0 0 0 0 0 
Kellogg 53 81 53 87 55 87 / 0 0 0 0 0 0 
Moses Lake 56 90 55 94 57 94 / 0 0 0 0 0 0 
Wenatchee 60 88 61 91 64 91 / 0 0 0 0 0 0 
Omak 53 90 53 93 55 92 / 0 0 0 0 0 0 


&& 


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


$$