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Area forecast discussion...updated aviation discussion 
National Weather Service Spokane Washington 
435 PM PDT Friday Aug 29 2008 


Synopsis... 
colder air will advance into the area through the weekend...with 
showers again returning to mainly mountain locations. Rather dry 
conditions for the valleys will prevail through next week...with 
temperatures remaining mainly below average. 




&& 


Discussion... 


Tonight through Sunday...water vapor imagery this afternoon shows 
the ridge that has kept the skies relatively clear through the day 
today is gradually beginning to lose the battle to an incoming 
upper trough off the northwest portion of Vancouver Island. The 
12z European model (ecmwf) and 12z NAM both had a very good handle on the depth and 
placement of the upper pv anomaly when compared with the location 
of and depth of the dry layer indicated on water vapor...so their 
solutions were rather favored through the first 48 hours or so of 
the forecast. The 12z GFS and UKMET were both a bit slow with the 
approach of the wave...and the overspread of quantitative precipitation forecast from the GFS when 
compared with other models was a bit troubling...likely a result 
of too much instability generated over the County Warning Area this evening with a 
cap overhead and tons of dry air above the boundary layer. 


Both the European model (ecmwf) and NAM contain the bulk of the quantitative precipitation forecast through tonight 
in British Columbia as a large field of q-vector convergence over 
a rather deep layer with a strong moisture fetch traverses through 
southwestern Canada as the short wave gradually deamplifies over 
the ridge that gives way over the inland northwest. At the same 
time this wave is moving through southern Canada...a second wave 
begins to dig down the back side of the mean long wave trough off 
shore...resulting in minimal height falls tonight over the 
County Warning Area...and a rather uninspiring frontal passage from the southern 
flank of the system that traverses British Columbia. Both the NAM 
and European model (ecmwf) indicate little in the way of moisture above the 
boundary layer with the frontal passage...and given the very weak 
instability fields...any showers that develop along the Canadian 
border will likely be contained to mountain areas and will be 
pretty shallow and short-lived. The presence of the right entrance 
region of the upper jet overhead would normally help for 
precipitation generation...however with the boundary layer and 
upslope southwesterly flow being the location of any generation of 
convective showers with a strong cap overhead and little in the 
way of synoptic moisture with the front...it seems upper 
divergence will have little impact on the evolution of the shallow 
convective field well below its influence. 


Upslope flow conditions will necessitate the continuance of low 
end probability of precipitation for most upslope mountain areas through the period as the 
second wave shifts southward around the trough base and into 
Oregon. The result is a lot of cooling in the middle-levels and the 
continuation of a rather moist boundary layer across the County Warning Area. 
However...synoptic moisture continues to be lacking...so any 
organized shower development does not seem very likely even with 
middle-level instability present. But the upper troughing 
overhead...temperatures will again fall much below normal through 
the weekend. /Fries 


Sunday night and Monday...trough axis begins to shift to the east 
as upper-level ridging builds into western Washington. Model guidance 
begins to slightly diverge at this time-frame with the GFS track 
and resulting moisture field east of the County Warning Area. The slower solutions 
consisting of the NAM...ECMWF...and Canadian are generally in line 
and this was the favored solution. This will allow middle-level 
moisture to wrap around the low into the Idaho Panhandle and far 
eastern Washington through midday Monday. However...with 
upper-level stabilization building in from the west I do not imagine 
the moisture will amount to much over the basin except fair 
afternoon cumulus. The other concern over the period will be gusty 
winds down the Okanogan Valley Sunday afternoon into the evening as 
a 1022mb surface high tracks into southern British Columbia setting 
up a 7-9mb north to south pressure gradient. Current forecast 
illustrates 16-25 miles per hour winds down the valley with gusts of 30 miles per hour. 
Overnight temperatures Sunday night will be tricky. An unseasonably 
cool airmass will remain over the region but cooling will be limited 
due to amount of cloud cover over the eastern zones and wind speeds 
in the west. High temperatures on Monday will be slightly warmer 
than Sunday with 850 mb temperatures rebounding a few degrees... 
especially along the east slopes and western basin. /Bodnar 


Monday night to Friday...the broad trough over the Great Basin 
starts to fill and shift northeast...while a ridge amplifies over 
the eastern Pacific. Weak ridging develops Monday night...but models 
diverge as to how quick to move the mentioned trough and its 
associated moisture east. The European model (ecmwf)/dgex/sref/Canadian are slower 
to move out the moisture wrapped up over the Idaho Panhandle. In 
contrast...by the start of this period the GFS has already pushed 
the deeper moisture toward southwest Montana and Wyoming. The gefs 
is a tad slower than the operational GFS. Given the dichotomy...I 
slowed the exit of the precipitation risk Monday night...holding some 
slight to low-end chance probability of precipitation over the Panhandle. Elsewhere...the 
short-wave ridging to the west will lead to generally dry conditions. 
However the next short-wave approaches and is expected to keep 
skies from clearing out completely. Tuesday to Friday...guidance 
brings two or three middle-level disturbances by. The west-northwest flow becomes 
north-northeast toward Thursday with the ridge/trough pattern amplifying. Look 
for passing middle and high clouds through the period. A risk for 
showers will be found across the mountain zones through Wednesday. 
As the flow becomes north-northeast the risk will wane to mainly the east/southeast mountain 
zones. Temperatures will remain below average this period...supported 
by 850 temperatures in the lower teens to near 10 degree c. Some moderation 
is indicated toward the end of the period. /Jcote 


&& 


Aviation... 
generally VFR conditions will prevail through 00z Sunday. A rather 
dry cold front will pass the area overnight...with periods of broken 
clouds in the 8-10 kft area as it does so. Winds will be rather 
brisk from the southwest at most locations through the afternoon and 
overnight as the front passes. Most locations will not see 
significant weather as the front passes...save for a few mountain 
showers across the Canadian border region. /Jcote 


&& 


Preliminary point temps/pops... 
Spokane 53 70 47 67 46 70 / 10 10 10 10 10 0 
Coeur D'Alene 52 70 45 65 45 69 / 10 10 10 20 20 20 
Pullman 51 70 43 65 43 69 / 0 10 10 10 20 20 
Lewiston 61 77 51 71 51 74 / 10 10 10 10 20 30 
Colville 47 72 41 69 44 74 / 20 10 0 10 20 0 
Sandpoint 47 68 39 64 44 69 / 20 10 10 20 20 20 
Kellogg 55 67 44 61 44 67 / 10 20 10 20 30 30 
Moses Lake 54 76 48 74 50 76 / 0 0 0 10 10 0 
Wenatchee 55 73 52 72 52 77 / 10 0 0 10 10 0 
Omak 51 75 45 74 48 77 / 10 0 0 10 10 0 


&& 


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


$$ 

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