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FXUS66 KPDT 171607
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
807 AM PST Tue Dec 17 2019

.UPDATE...A ridge of high pressure over the area today will shift
eastward followed by a shortwave trough on Wednesday. Following 
the trough will be a fast and moist westerly flow that will bring 
significant precipitation to the Cascades. In the near term the 
main concern is patchy freezing fog. Still have the freezing fog 
advisories going in several places until noon and will evaluate 
in the coming hours to see if they can be let go. The wind 
advisory in the Grande Ronde Valley is already verifying so will 
keep it in place. Temperatures will remain cold today underneath 
the stratus deck in the Columbia Basin if it does not break up. 
Current forecast is on track and just made some minor adjustments 
for this forecast package. 78 

&&

.PREV DISCUSSION... /issued 325 AM PST Tue Dec 17 2019/ 

SHORT TERM...Today through Thursday night...A ridge of high 
pressure will remain over the area through tonight. This will lead 
to continued fog and stratus around the Lower Columbia Basin with 
cold air trapped beneath the stratus. On Wednesday the ridge will 
move to the east allowing a decaying upper level low to into the 
Pacific Northwest followed by a wetter warm front on Thursday. This 
will bring some light precipitation into the Cascades Wednesday and 
begin spilling over onto the east slopes in the afternoon with 
initial precipitation type as primarily snow. By evening and 
overnight this light precipitation will spread across the rest of 
the forecast area and be accompanied by some warmer air aloft 
resulting in a transition to some wintery mixed precipitation across 
north central Oregon and then spreading into the Lower Columbia 
Basin of Washington overnight and early Thursday. Snow levels over 
central and eastern Oregon will also be on the rise late Wednesday 
night and Thursday. Cold air trapped along the east slopes of the 
Washington Cascades will keep precip type as snow before changing 
over to a wintery mix on Thursday or Thursday night. Travel along I-
90 between Ellensburg and Snoqualmie Pass will be most impacted by 
this system with several inches of snow Wednesday night through 
Thursday followed by some wintery mixed precipitation. By Thursday 
night snow levels will be 3000 to 5000 feet with some travel over 
the mountains still being impacted by snowfall. 

LONG TERM...Friday through Tuesday...A warm front lifts through the 
area during the day Friday, shifting best rain chances to far 
northern portions of the forecast area as well as along the Cascade 
crests. This lull in activity will be short lived as the main system 
begins to move onshore by Saturday afternoon bringing additional 
scattered to widespread rain and mountain snow. By Monday the system 
will begin to weaken and break apart with only slight chances for 
precipitation, mainly across the higher terrain of central and 
northeast Oregon. Snow levels through Saturday expected to remain 
between 3500-5500 feet before dropping to near 1000-2500 feet by 
Monday as the system exits. At this time, the area looks to be dry 
heading into the Christmas holiday, but will need to keep an eye on 
a potential system off the coast. Temperatures through Saturday will 
be rather warm for this time of year with afternoon highs climbing 
into the low to mid 50s for central Oregon into the lower Columbia 
Basin. As the main front pushes east of the region Sunday through 
Tuesday, we will see high temperatures return to normal with mid 30s 
to lower 40s expected. Richards

AVIATION...12z Tafs. Dense fog/freezing fog will continue to impact 
both KPDT & KALW with lifr conditions expected and visibilities of 
1/4 SM. Patchy fog will also impact KYKM, but visibilities should 
generally remain above 1 1/2 SM. Fog will gradually lift into a low 
stratus deck this afternoon and evening before patchy dense freezing 
fog becomes possible again overnight, possibly affecting PDT, ALW, 
and YKM. KBDN/KRDM are expected to maintain vfr conditions through 
the period. Richards

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  33  27  33  32 /   0   0   0  50 
ALW  35  27  32  31 /   0   0   0  50 
PSC  33  28  32  30 /   0   0  10  50 
YKM  35  24  31  26 /   0   0  20  60 
HRI  32  28  32  30 /   0   0   0  50 
ELN  35  23  32  23 /   0  10  20  70 
RDM  38  25  42  31 /   0   0  20  60 
LGD  34  25  35  29 /   0   0   0  50 
GCD  33  26  37  30 /   0   0  10  50 
DLS  36  29  36  34 /   0   0  20  80 

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Freezing Fog Advisory until noon PST today for ORZ044-507.

     Wind Advisory until 4 AM PST Wednesday for ORZ049.

WA...Freezing Fog Advisory until noon PST today for WAZ026-027-029.

&&

$$

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