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Area forecast discussion...updated 
National Weather Service Pendleton or 
332 PM PST Sat Nov 21 2009 


Updated aviation discussion 


Short term...tonight through Monday...thoughts have changed little 
from the previous overnight shift. Models remained very consistent 
with the progression of the system across the area. With a couple of 
pronounced issues dealt with today. 


First, decided to follow the surface low as shown by the NAM with 
the 18z GFS now lending the closest support of recent runs. This 
gives a very tight pressure gradient across most of the area as the 
surface low tracks across the Washington Cascades and the lower 
Columbia Basin before lifting northeast later tomorrow. These 
gradients, combined with a very stable lower level air mass, will 
give a prolonged period of strong winds aloft down toward the 
surface. Will issue numerous wind advisories and high wind warnings 
as a result. 


The second issue deals with snowfall. Snow level will be somewhat 
variable with a brief period of warm air advection as warm diffluent 
air crosses the Cascades tonight. The Washington Cascades will see 
significant accumulations mainly tonight with the major passes 
impacted. Do think mainly western areas will bear the brunt after 
the cold front passes later tonight and Sunday. The Blue Mountains 
will start seeing accumulations later tonight at the higher 
elevations and passes but snow levels will drop and accumulation 
rates will increase mainly tomorrow after the cold front passes and 
a northwesterly flow develops. Both areas will see significant 
blowing snow but dont have the confidence to go with a Blizzard 
Warning, but it bears watching however. 


Other mountain areas will see snow and maybe some blowing as well, 
but should stay below advisory amounts. Precipitation should begin 
tapering off across the west by tomorrow afternoon and gradually 
taper off across the east tomorrow evening. /Jbonk 




Long term...Monday night through Saturday...a weak system will 
break up Monday night it rides over a building upper level ridge. 
The building ridge will move slowly east over the Pacific northwest 
Tuesday. The departing system will combine will upslope flow in the 
mountains to keep a slight chance of showers in northeast Oregon 
Tuesday. Also...slopover precipitation may occur near the Cascades. 
Subsidence associated with the ridge will provide light winds and 
clearing skies with the strong surface inversions Tuesday night. In 
the lower Columbia Basin patchy fog and freezing fog will form at 
night under the inversions and keep daytime temperatures Wednesday 
below normal. At higher elevations the surface inversions will break 
during the day and warmer air aloft associated with the ridge will 
reach the surface to produce large diurnal temperature spreads. 
Wednesday night through Friday there are dramatic differences 
between the GFS and European model (ecmwf) models even though both have southwest 
flow at 500 mb. The 0z European model (ecmwf) has a more progressive pattern with a 
front at an angle to the 500 mb flow that will move into eastern 
Washington and Oregon and form a deformation zone over the Blue 
Mountains that drops half an inch of rain Thursday. The 12z GFS has 
the front parallel to the 500 mb flow so it remains offshore until 
Friday. The GFS ensemble runs show 500 mb flow running to both 
extremes with no clustering. Decided to make little change to 
existing forecast with a chance of probability of precipitation most places and likely probability of precipitation 
over the mountains. Think European model (ecmwf) is overblown with amount of 
precipitation unless the deformation band does set up...but did 
trend toward it by keeping at least a chance of probability of precipitation across the 
area. When the 12z run of the European model (ecmwf) arrived it was slower with no 
deformation band...but it still supported the faster arrival of the 
front during the day Thursday. Both models had precipitation Friday 
with the GFS pushing the front through and the European model (ecmwf) moving the 
upper level trough through. With low confidence due to model 
differences kept a low chance probability of precipitation across the area Saturday. 
Coonfield 


&& 


Aviation...0z tafs...VFR conditions will prevail in most areas 
through 04z with broken middle level ceilings. Another Pacific weather 
system will move into the area tonight and early Sunday morning with 
rain and mountain snow. This will cause areas of MVFR conditions. 
IFR with possibly even LIFR conditions in local areas with rain and 
snow...especially near any mountain ranges or over the central 
Oregon plateaus near taf site krdm. Winds of 10 to 20 kts with gusts 
to 30 kts from the southwest will be common in all areas today. 
However the winds will increase overnight tonight as pressure 
gradients tighten up ahead of the next weather system. During that 
time winds are expected to increase to at least 15 to 25 kts with 
gusts to 40 kts or possibly even higher into Sunday. 76 




Preliminary point temps/pops... 
PDT 41 47 31 43 / 80 70 10 10 
alw 39 48 32 43 / 90 80 10 10 
psc 37 51 30 44 / 90 50 10 10 
ykm 32 45 29 44 / 90 40 10 10 
hri 38 49 29 45 / 80 50 10 10 
eln 34 39 26 39 / 90 70 10 20 
rdm 35 45 24 47 / 80 40 10 10 
lgd 36 42 23 40 / 80 80 20 10 
gcd 34 41 21 44 / 70 60 10 10 
dls 37 49 32 47 / 80 40 20 10 


&& 


PDT watches/warnings/advisories... 
or...High Wind Warning from 7 PM this evening to 10 am PST Sunday for 
orz049-050. 


Winter Storm Warning from 10 PM this evening to 7 PM PST Sunday 
for orz502. 


High Wind Warning from 1 am to 4 PM PST Sunday for orz042. 


Wind Advisory from 10 PM this evening to 1 PM PST Sunday for 
orz043. 


Wind Advisory from 10 PM this evening to 7 PM PST Sunday for 
orz501-504. 


Winter Weather Advisory from 10 PM this evening to 7 PM PST 
Sunday for orz503. 


Wind Advisory from 4 am to 4 PM PST Sunday for orz041-044. 


Washington...Winter Weather Advisory from 7 PM this evening to 4 PM PST 
Sunday for waz501-502. 


Winter Storm Warning from 10 PM this evening to 7 PM PST Sunday 
for waz030. 


Wind Advisory from 10 PM this evening to 7 PM PST Sunday for 
waz029. 


Wind Advisory from 4 am to 4 PM PST Sunday for waz024-028. 


&& 


Threat index 
Sunday : red 
Monday : green 
Tuesday : green 


Green: none or limited need for watches, warnings, or advisories. 
Yellow: a few watches, warnings, or advisories possible or in effect. 
Red: numerous watches, warnings, or advisories possible or in effect. 


For additional weather information, check our web site at... 
www.Weather.Gov/Pendleton 


$$ 


87/76/76