334 
fxus66 kpdt 211701 
afdpdt 


Area forecast discussion 
National Weather Service Pendleton or 
901 am PST Sat Nov 21 2009 


Update...relatively few changes made to today's forecast with 
primary focus being directed on the approaching system affecting 
tonight and tomorrow. An early look at the 12z runs show rather 
good continuity from the NAM and GFS regarding position of the 
surface low so will need to work out the differences and relative 
impacts. As previously stated, the specific track of the low will 
determine the difference between advisory and warning amounts and 
wind speeds. Recently released precipitation estimates from 
nesdes indicate available moisture and rain rates are less that 
have been seen with the most recent system so may be leaning 
toward the overall drier NAM qpf, but this model still brings 
significant quantitative precipitation forecast to the northern Blue Mountains after a brief 
period of moderate precipitation mainly to the Washington portion of the 
County Warning Area. /Jbonk 


&& 


Previous discussion... /issued 300 am PST Sat Nov 21 2009/ 


Short term...today through Sunday...the back side of a very sharp 
and deep upper trough continues to move across the region early this 
morning. The energy digging down the back side of the trough is 
keeping a few showers...mostly snow...going over portions of the 
area...especially along and just east of the Cascades. As this 
trough exits the area this morning should see these showers wind 
down further...to mainly a few along the Cascade crest. A potent 
system is currently taking shape out along 150w this morning. This 
system will swing across the region late tonight and on Sunday. 
There is a chance that a surface low will form off the northwest 
Oregon coast late tonight and then move east across Washington state 
Sunday. There is some uncertainty as to the track of this surface 
low as some data points to it swinging more northeastward through 
western Washington into British Columbia. The track of this surface 
low will determine the weather conditions over our area Sunday. If 
the low moves into the interior northwest as some data suggests then 
we would see strong winds over central Oregon...the Blue Mountain 
foothills...the Grande Ronde Valley and Wallowa County. Also there 
would be significant precipitation over many areas as the system moves 
through. Also there would be westerly upslope flow behind the 
system. A quick cut off of precipitation in the Lee of the Cascades due to 
the west flow would limit snow along the Cascade East Slope zones. 
Meanwhile the upslope flow into the Blue Mountains would allow 
considerable Post system snow accumulations. The alternate solution 
reduces the wind risk across the area...and alters the focus of 
precipitation and snow accumulations away from The Blues and to the 
Washington Cascade east slopes. At this time will side with the 
solutions (nam and european) that advertise the surface low tracking 
east into the interior northwest. As such will issue wind and winter 
storm watches for the areas mentioned above that would be affected 
by the easterly track. Will only go with watches though due to the 
continued uncertainty. 90 


Long term...Sunday night through next Saturday...a weather system 
will be departing the forecast area Sunday night while an upper 
ridge builds over the region. This will push most of the moisture 
with any new systems that try to move into the Pacific northwest up 
into SW Canada and extreme northwest Washington. As a result our forecast 
area will see mostly dry conditions with just isolated to scattered 
showers in the mountains. This scenario will persist through 
Thursday. There will be patchy fog development in the usual low 
areas of the lower Columbia Basin and especially along the larger 
rivers such as the Columbia...Yakima and snake rivers. A Pacific 
trough will then move into the region again on Friday and Friday 
night bringing another chance of rain and mountain snow. There is 
considerable model differences that far out in the extended period 
so will not make any changes to the current forecast which is 
already representative for that system. 88 


Aviation...12z tafs...VFR conditions will prevail in most areas 
through today with scattered to broken middle level ceilings. Another 
Pacific weather system will move into the area late tonight and 
early Sunday morning with rain and mountain snow increasing. This 
will cause areas of MVFR to IFR conditions with possibly even LIFR 
or vlifr conditions in local areas with snow or fog...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 20 to 30 kts with gusts to 40 kts or possibly 
even higher into Sunday. 88 


&& 


Preliminary point temps/pops... 
PDT 48 37 47 31 / 10 80 70 10 
alw 49 38 48 32 / 10 80 70 10 
psc 52 36 51 30 / 10 80 50 10 
ykm 49 32 48 29 / 10 80 40 10 
hri 50 35 49 29 / 10 80 50 10 
eln 46 32 45 26 / 20 80 50 10 
rdm 44 34 45 24 / 20 70 40 10 
lgd 43 36 43 22 / 20 80 80 20 
gcd 42 33 42 21 / 10 70 60 10 
dls 50 37 49 32 / 20 80 40 20 


&& 


PDT watches/warnings/advisories... 
or...high wind watch from late tonight through Sunday afternoon for 
orz501-504. 


Winter Storm Watch from late tonight through Sunday afternoon 
for orz502. 


High wind watch from this evening through Sunday morning for 
orz042-043. 


High wind watch from 7 PM PST this evening through Sunday 
morning for orz049-050. 


Washington...high wind watch from late tonight through Sunday afternoon for 
waz029. 


Winter Storm Watch from late tonight through Sunday afternoon 
for waz030. 


&& 


Threat index 
today : red 
Sunday : red 
Monday : 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 

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