Weather

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Area forecast discussion...updated aviation discussion 
National Weather Service Spokane Washington 
440 PM PDT Thursday Aug 21 2008 


Synopsis... 
a cold upper low will move east of the area as high pressure moves 
into the area allowing temperatures to warm through Sunday. A 
Pacific cold front moves into the region Sunday night for cooler 
temperatures and a chance of rain. 


&& 


Discussion... 
tonight through Sunday...convection from the cold core upper low 
is already moving out of eastern Washington and into the Panhandle. This 
trend will continue as the low moves east so there won't be much 
if any shower activity in eastern Washington this evening. Convergent 
bands from the Cascades are firing off some showers over the 
Palouse and west of Spokane so I can't completely remove the 
threat of showers for there this evening. After that skies should 
clear quickly as the 850mb winds go calm. All of this points to a 
very chilly night tonight with local frost possible. A check of 
the record lows for Friday morning shows that we're still about 5 
degrees above setting any records. But the threat of frost is 
Worth a mention in the severe weather potential statement product. Given all of the rain showers 
today the boundary layer is fairly moist. Thus fog is a distinct 
possibility in the northern/eastern valleys and this may help to 
keep temperatures too warm for frost. 


On Friday a high pressure ridge quickly replaces today's low. This 
will create a fairly rapid warming trend. Temperatures on Friday 
will be 5-10 degrees warmer than today with as much as 15 degrees 
of warming in the central Panhandle. The ridge continues to 
amplify over our area on Saturday for another 5-10 degrees of warming. 


The next Pacific short wave currently out near the dateline moves 
into the eastern Pacific and deepens the long wave trough. As it 
does so it picks up the 1.75" precipitable water plume out in the 
Pacific and points it at the Pacific northwest. The well-defined low-level 
front is forecast to move onto the Washington coast Sunday and near the 
Cascades by Sunday evening. Dprog/dt of the GFS and European model (ecmwf) show 
that the models have sped up their solutions a bit over the past 
few runs. Thus I increase the threat of rain for the Cascades on 
Sunday. This faster timing also pushes the ridge to our east more 
quickly. This should keep temperatures down a bit from previous 
expectations so I knocked a few degrees off the Sunday high 
temperature forecast. Rj 


Sunday night through Thursday...a strengthening jet south of the 
Aleutians will help dig a trough across the eastern Pacific and 
that feature will be advancing inland at the start of this period. 
Model disagreement and run-to-run inconsistency develop regarding 
how quick to move the trough axis east of the County Warning Area. Loose agreement 
indicates the feature moving east somewhere between Tuesday and 
Wednesday. Sunday night through Monday...the inland northwest starts in a 
relatively moist southwesterly flow (with precipitable waters  pushing over 1.0 inch or near 
200 percent of normal). A cold front pushes across the Cascades 
Sunday night and into Idaho Monday...backed up by the upper trough. 
The frontal/synoptic forcing with these two features increases 
through Sunday night throughout the County Warning Area...then the focus shifts 
toward the eastern County Warning Area Monday. Look for increasing clouds and 
increasing rain chances throughout the County Warning Area...especially late Sunday 
night and early Monday. By Monday afternoon an increasingly westerly 
flow will move the best precipitation chances to the Idaho Panhandle and 
backed up against the Cascades. Instability is not very impressive 
with the system...save for some marginal cape/Li values over the 
northeast County Warning Area. For now the risk thunder will be left out. 


Monday night/Tuesday...the inland northwest remains under the 
influence of the upper trough and another middle-level short-wave 
passes through. This will keep the risk for precipitation alive...though 
by this time the atmosphere has begun to dry out...with precipitable waters  
reducing to under 0.50 inches (or under 80 percent of normal). So 
the risk for precipitation will remain only in the slight to chance 
category. 


Tuesday night through Thursday...the inland northwest transitions to a west-northwest 
flow with weak short-wave ridging. Some minor disturbances slip 
across southern British Columbia through the period...the furthest south of 
which comes across on Wednesday/Thursday. Look for occasional 
middle/high clouds through the period...with a slight risk for showers 
over the mountain zones (especially the north and East Mountain zones). 


Temperatures will undergo a cooling trend early this period...as 850 
temperatures drop into the lower teens and 1000/500 mb thicknesses drop 
from about 570 dam on Sunday to 550-555 dam Monday. This supports 
temperatures some 5-10 degree below average through Wednesday. Toward Thursday 
guidance indicated some recovery in 850 temperatures and thicknesses... 
supporting temperatures trending closer to average. /Jcote 


&& 


Aviation... 
predominantely VFR ceilings/visible at taf sites through 00z Saturday...with 
the less dominant potential for local MVFR/IFR ceilings. Widely scattered 
-shra/isolated -tsra over eastern taf sites will dissipate toward 
02-04z...with clouds and winds on the decrease. With the clearing 
skies and reduced winds...a risk for fog will be found over the 
northeast valleys...some of which may be in the vicinity of 
kgeg-ksff-kcoe. As such...local MVFR/IFR visible/ceilings are possible from 
09-15z. Otherwise expect dry conditions with few clouds. /Jcote 




&& 


Preliminary point temps/pops... 
Spokane 45 75 49 84 56 85 / 10 0 0 0 0 0 
Coeur D'Alene 42 74 47 85 50 85 / 10 0 0 0 0 0 
Pullman 40 76 43 86 50 85 / 20 0 0 0 0 0 
Lewiston 50 81 55 93 61 93 / 20 0 0 0 0 0 
Colville 40 82 44 85 47 88 / 10 0 0 0 0 0 
Sandpoint 39 73 42 83 44 84 / 20 0 0 0 0 0 
Kellogg 45 74 50 85 55 86 / 30 10 0 0 0 0 
Moses Lake 47 80 49 88 54 91 / 0 0 0 0 0 0 
Wenatchee 50 82 53 88 59 85 / 10 0 0 0 0 10 
Omak 45 84 49 90 55 86 / 10 0 0 0 0 10 


&& 


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


$$