290 
FXUS66 KPDT 110447
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
847 PM PST Sun Jan 10 2021

.UPDATE...
Overall, it is a quiet night. Areas of fog have developed once 
again, especially along the Oregon Blue Mountain Foothills, which
has prompted the issuance of a dense fog advisory. Otherwise, 
just made some adjustments to temps where needed. Winds are light.

&&

.AVIATION...
06Z TAFS...Once again, conditions are variable over the region.
RDM and BDN are VFR and should stay that way. DLS and PSC are VFR
but is expected to decrease to MVFR overnight. IFR is not out of
the realm of possibility in the morning.

YKM has been MVFR wand will likely stay that way for most if not
all the period.  

ALW is currently VFR. It is expected to decrease to MVFR overnight
and could go as low as IFR or even LIFR by morning before
improving to MVFR.

PDT has been LIFR with both very low CIGS and VSBYS. Once again,
it looks like they may stay that way for a good portion of the
period. There may be some improvement during the late afternoon,
early evening before a return to low conditions once again.  

Winds will be light through the period, mainly 5 kts or less.



&&

.PREV DISCUSSION... /issued 306 PM PST Sun Jan 10 2021/ 

SHORT TERM...Tonight through Tuesday...Upper level ridge over 
the region has kept conditions fairly dry this afternoon, while 
low stratus has been clearing and allowing some sunshine across 
the Columbia Basin. Stratus will continue to clear through the 
afternoon, but will redevelop along with patchy fog and freezing 
fog across the Basin starting this evening, and persisting into 
tomorrow morning.

Starting Monday, a series of weather systems will move through the
region while a deep moisture plume currently visible over the
Pacific will enhance precipitation amounts, mainly over the
Cascades. Early Monday, a weak cold front will swing across the
forecast area with increasing mid to high level clouds and mainly
light snow in the WA Cascades. Surface gradients will start to
tighten with this frontal passage, leading to breezy conditions
along the Blue Mountain foothills and in the southern Grande Ronde
valley. West southwest flow aloft behind this weak system will 
help to advect a warmer, moist airmass over the region Monday 
while raising snow levels to 5-7k feet by Monday night for areas
west of the Blue mountains.

Late Monday, a surface low and upper level shortwave swinging 
around a parent low in the Gulf of Alaska of will approach the 
forecast area. Arrival of the moisture plume in the PNW will 
coincide with this system, and westerly upslope flow will aid in 
heavier precipitation amounts across the Cascades. While snow 
levels will be 5-7k feet Monday night into Tuesday, expect 
snowfall to be confined to the crests of the Cascades, while 
moderate to heavy rainfall will be possible below the crests of 
the Cascade east slopes. Frontal boundary associated with surface
low will move across the forecast area early Tuesday with light 
to moderate rainfall expected. Westerly upslope flow will keep 
mostly rain showers going in the mountains through Tuesday while 
the lower elevations will see a brief break in the precipitation. 
Late Tuesday, another upper level shortwave and surface low 
pressure system will approach the forecast area with another round
of moderate to heavy rainfall below the Cascade crests, and 
chance of rain everywhere else. Overall, models are indicating 
that portions of the Cascade east slopes will receive anywhere 
from 2-4 inches of liquid precip while portions of the Blue 
mountains can expect 1-2 inches of liquid precip. This amount of 
rainfall and possible snow melt will likely result in rising 
rivers and streams Tuesday through Thursday.

Breezy to windy conditions will develop across the lower 
elevations with each passing system Monday and Tuesday, with peak
winds expected both Tuesday morning and Tuesday night in the lower
elevations. Lawhorn/82

LONG TERM...Wednesday through Sunday...Atmospheric river event 
will subside by the end of Wednesday, models starting to converge 
a bit more as far as the extent and quantity of precip produced by
this event. Both the GEFS and Euro ensembles are producing more 
aggressive values for IVT transport, with the ECMWF QPF ensembles 
responding accordingly with more precip in the Cascades, with 
several inches possible, compared to previous days' runs. Now the 
Euro is wetter than the GEFS as far as rain in the Basin and 
central OR, with members producing several tenths of an inch while
GEFS shows most areas barely receiving a tenth of an inch. 
Canadian ensembles appear to be approaching the Euro solutions as 
well. Wintry precip not expected to be an issue, with NBM snow 
levels forecast to be above 4500 ft for most areas, but the amount
of rain falling on snow in the mountains could create hydro 
concerns, with rivers and streams along the eastern slopes needing
to be monitored during the duration of this event.

Beyond Wednesday, high pressure ridge takes over and leaves the
area dry on Thursday. Both deterministic GFS and Euro then flatten
the ridge a bit and usher in SW flow over the PacNW, leaving a
slight chance for precip mainly in the Cascades on Friday. Next
best chance for area-wide precip looks to be Saturday as an
embedded shortwave within NW flow passes through the area. QPF
ensembles are a bit divided on this feature, however, with the
Euro much wetter than the GEFS. Snow levels will fluctuate a bit
after the AR event as the flow aloft shifts, ranging from
3000-4000 ft in the Cascades during Friday's precip, to 2000-3000
ft on Saturday as upper-level winds shift more northerly. High
temps will start at around the upper 50s in the lower elevation
midweek, leveling off to the mid to upper 40s by next weekend.

AVIATION...00z TAFs...Conditions have improved to VFR for most
sites, however fog and low cigs expected to redevelop this evening
as high pressure continues to dominate the area. Expecting most 
severe fog to develop in PDT/ALW/PSC, with YKM/DLS seeing low 
stratus, however brief periods of LIFR fog possible for these 
sites. BND/RDM expected to be VFR through the period with sct-bkn 
cigs of 10 kft and higher. Winds will be light and variable, with 
cigs seeing improvement Monday afternoon.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  31  46  36  51 /   0  10  30  60 
ALW  33  47  38  50 /   0  10  30  70 
PSC  33  45  37  51 /   0  10  40  50 
YKM  34  42  34  47 /  10  30  60  80 
HRI  31  44  37  52 /   0  10  30  50 
ELN  35  40  33  43 /  10  30  70  80 
RDM  33  47  38  52 /   0  10  30  60 
LGD  30  37  29  39 /   0  10  20  70 
GCD  29  43  32  45 /   0   0  20  60 
DLS  37  44  37  50 /   0  40  70  80 

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Dense Fog Advisory until 10 AM PST Monday for ORZ507.

WA...None.
&&

$$

SHORT TERM...77
LONG TERM....74
AVIATION...77