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Area Forecast Discussion (AFD)

Issued by Pendleton, OR (PDT)

FXUS66 KPDT 191134 AAA

Area Forecast Discussion
National Weather Service Pendleton OR
340 AM PST Tue Feb 19 2019

Updated Aviation Discussion

.SHORT TERM...Today through Wednesday night...Big temperatures 
differences are across the forecast area this morning with most of 
south central WA under high clouds and in the single digits and 
teens, including the Kittitas and Yakima valleys.  Meanwhile the 
remainder of the forecast area is mostly cloudy with stratus clouds 
and temperatures in the teens and 20s.  WSR-88D has picked up on a 
few weak returns over the Blues, and light snow (mainly flurries) 
have been observed.  Clouds will increase today and snow will 
develop as the day progresses.  Snow will develop along the Cascades 
by late this morning, and the Blues and Wallowas will have some 
scattered orographic snow showers during this time. After 1 PM, all 
of the forecast area will have at least a slight chance of snow.  A 
shortwave trough off the B.C. coast this afternoon will deepen as it 
tracks across the Pacific NW tonight.  This is a rather vigorous low 
pressure system aided by a strong 130+ knot jet along the backside of 
the trough.  Winter storm warnings and winter weather advisories 
have been posted for a large portion of the forecast area.  

There have been several forecast challenges with the approaching 
system.  The biggest challenge has been precipitation type.  Warm 
air advection will occur ahead of the trough, and snow levels will 
rise to only around 1000-1500 feet tonight for the western 
half of the forecast area. Elevations at and below 1500 feet may 
observe light freezing rain or sleet tonight mixed with snow, and 
this was added to the forecast.  I will also mention the mixed 
precipitation in the Winter Storm Warning for the Blue Mountain 
Foothills. Surface temperatures along the foothills will be in the 
mid 30s this afternoon and evening before lowering to around 30 
degrees overnight, therefore it may be difficult for the Blue Mtn 
Foothills to receive 3-6 inches of snow.  Will keep the warning but 
lower amounts to 3-5 inches.  Ice accumulations in the Yakima Valley 
and the Lower Columbia Basin should be light and not enough coverage 
for any highlights, but a slight chance to chance will be mentioned 
in the forecast.  Confidence is high that the northern Blue 
Mountains will have substantial amounts of snow accumulation and 
around a foot of snow from Meacham north to the Bluewood Ski Resort. 
 The Grande Ronde Valley is under a Winter Weather Advisory.  This 
looks reasonable but amounts could be close to 5 inches that would 
meet warning criteria.  Based on the position of the low over 
northeast Oregon Wednesday and Wednesday night, Wallowa County may 
observe snow accumulations meeting warning (i.e. 5+ inches) and will 
upgrade the advisory to a warning for this zone.  Winter weather 
advisories for snow have also been issued for the southern Blues, 
the John Day Basin and nearby mountains, and the east slopes of the 
WA/OR Cascades.  Wister 
.LONG TERM...Thursday through next Wednesday...The region and 
forecast area will remain under a cold and unsettled weather 
pattern. On Thursday there will be some lingering snow showers over 
the area, mainly over the eastern Mountains and Foothills of the 
Blues. However, cannot rule out a few snow showers over the Lower 
Columbia Basin as well. Additional snow amounts will be very light. 
Then on Friday there will be a break in the weather with an upper 
transient ridge moving across the area with dry conditions. The next 
weather system will then move into the region and CWA on Friday 
night and Saturday. Temperatures and thicknesses will be marginal to 
support snow in the lower elevations and will mention a chance of 
rain and snow, while the mountains, especially the Blue Mountains, 
will receive more snow. It is too soon for any highlights at this 
time for Friday night and/or Saturday but QPF amounts do suggest 
that winter weather highlights may be needed eventually. Colder air 
will drop down from the north on Saturday and Saturday night 
changing any mixed precipitation to all snow showers, but the cold 
air will not be significant. However, it will still be below normal 
with highs mostly in the 30s in the lower elevations with 20s in the 
mountains. This is about 10 degrees below normal for late February. 
There will be another subtle break in the weather on Sunday night 
and Monday before the next weather system brings more precipitation 
to the forecast area on Tuesday with another push of colder air 
dropping down from the north into the CWA. This colder air will 
ensure all precipitation to be snow. QPF amounts do suggest the 
possible need for more winter weather highlights again for next 
Tuesday with the push of this colder air from the north and 
significant moisture. There will also be considerable upslope snow 
into the Blue Mountains and Foothills for Tuesday into Wednesday 
with the cold front stalling over north central Oregon on Wednesday. 
Could see significant upslope snow showers with significant 
accumulations in the central and northeast mountains as well as some 
snow showers over the Blue Mountain Foothills. However, model 
differences by this time will be considerably different from each 
other and will take a compromise between the models and keep it 
colder than normal, but not a significant arctic push of cold air. 
The last few instances when the models were indicating an arctic 
push of cold air, it did not materialize and temperatures remained 
warmer than what the models predicted a couple days in advance. 
Winds will be mostly light through the extended period. 88


.AVIATION...12Z TAFs...MVFR CIGS at PDT this morning while other 
terminal airports are currently VFR.  ALW may also have brief 
periods of MVFR CIGS this morning.  Otherwise, clouds will thicken 
as the day progresses and snow (mixed with rain, sleet, or freezing 
rain at elevations AOB 1500 feet) will develop across the region, 
mainly after 21Z. All TAF sites have the potential for MVFR or less 
conditions in rain and snow.  PDT and ALW will have the best 
potential for accumulating snow tonight and IFR/LIFR conditions 
possible. A PROB30 for -FZRA was also included at PSC after 06Z. 
Winds will generally be 10 knots or less but DLS may have slightly 
higher gusts.  Wister


PDT  34  28  31  22 /  50  90  90  20 
ALW  34  29  32  22 /  50  90  90  20 
PSC  34  28  33  23 /  20  50  60  10 
YKM  33  24  34  22 /  30  30  30  20 
HRI  36  31  34  23 /  30  60  60  20 
ELN  35  21  34  18 /  30  40  30  20 
RDM  39  26  36  20 /  20  60  70  50 
LGD  33  27  33  22 /  50  90  90  30 
GCD  33  27  34  20 /  30  80  80  30 
DLS  38  32  40  26 /  40  60  60  30 


OR...Winter Weather Advisory from 1 PM this afternoon to 4 PM PST 
     Wednesday for ORZ509.

     Winter Storm Warning from 4 PM this afternoon to 4 AM PST 
     Thursday for ORZ050.

     Winter Storm Warning from 1 PM this afternoon to 10 PM PST 
     Wednesday for ORZ502-507.

     Winter Weather Advisory from 1 PM this afternoon to 10 PM PST 
     Wednesday for ORZ503.

     Winter Weather Advisory from 4 PM this afternoon to 10 PM PST 
     Wednesday for ORZ049.

     Winter Weather Advisory from 4 PM this afternoon to 4 PM PST 
     Wednesday for ORZ505-506.

WA...Winter Storm Warning from 1 PM this afternoon to 10 PM PST 
     Wednesday for WAZ030.

     Winter Storm Warning from 1 PM this afternoon to 4 PM PST 
     Wednesday for WAZ029.

     Winter Weather Advisory from 1 PM this afternoon to 1 PM PST 
     Wednesday for WAZ520.