265
FXUS66 KPDT 041810
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1110 AM PDT Sat May 4 2024

.AVIATION...18Z TAFs...A deep upper-level closed low, currently
centered offshore, will continue to influence the weather at all
sites through the TAF period as it tracks southeast across SW
Oregon and NW California. Confidence is high (80-90% chance) in
at least temporary MVFR conditions at BDN/RDM due to reduced VSBYs
and/or CIGs through the period. Elsewhere, chances of MVFR
conditions ramp up this evening into Sunday morning as rain
becomes more widespread. Due to the location of a surface low in
eastern Oregon, wind direction and magnitude will vary across TAF
sites through the morning before switching to southwesterly to
westerly at 10-20 kts sustained with gusts of 15-35 kts by late
afternoon and evening for all sites. Plunkett/86

&&

.PREV DISCUSSION... /issued 849 AM PDT Sat May 4 2024/

SHORT TERM UPDATE...A deep low is centered about 140 miles west
of Coos Bay with an increasing moist southerly flow ahead of it. A
weak deformation band has set up ahead of the low with stratiform
light rain along the WA/OR border in the Lower Columbia Basin.
PoPs were increased in this area to reflect the more widespread
rain. Previous models, including the high resolution CAMS, did not
handle this deformation band well.

A wind advisory is in effect for the base of the Oregon Blue
Mountains east-southeast of Pendleton and Milton-Freewater. There
have been gusts 55-60 mph along Cabbage Hill and near Cayuse
which is on the low end of warning criteria. Current MSLP
difference between BKE and MEH is 2.7 mb and has decreased
slightly since 12Z when there was a 3.3 mb difference. Will keep
the advisory as is due to the gradual weakening of pressure
gradients. Wister/85

PREV DISCUSSION... /issued 505 AM PDT Sat May 4 2024/

SHORT TERM...Today through Monday...The deep closed upper level
low off the PacNW coast continues to move to the SE and will
move over SW Oregon late today then into the Great Basin overnight
into Sunday. Meanwhile, the cold front will be very slow to
progress eastward as the low passes south of the forecast area.
Will see increasing rain over the western portion of the forecast
area this morning with precipitation slowly spreading east into
the rest of the forecast in the afternoon and overnight. Models
continue show a deformation band forming on the north side of the
low where the cold front stalls across northern Oregon and
southern Washington overnight into Sunday. This will lead to some
significant rainfall amounts on the oder of .5 to 1.0 inches
across the Columbia Basin and 1 to 2 inches over portions of the
eastern mountains and along the east slopes of the southern
Washington Cascades. Central Oregon should see the least amount of
precipitation at .25 to .5 inches. The low begins to move off
into the Rockies late Sunday and Monday which will cut off the
wrap around moisture and allow a northwest flow to develop. This
will shift the focus of precipitation Monday mainly to the Cascade
crest and over the eastern mountains in the form of showers
including a slight chance (15%) of thunderstorms over the eastern
mountains in the afternoon and evening. River forecast continue to
show that this significant rain will cause rivers to rise but
with no flooding expected as rivers remain below bankfull.

Snow levels start out this morning around 6000 to 7000 feet but
begin to lower across central Oregon through the day and overnight
as the low center migrates into SW Oregon and then into the Great
Basin. Snow levels across central Oregon will lower to 2500 to
4000 feet overnight through Sunday morning before rising back to
around 4000 feet Sunday afternoon. This will allow for some minor
snow accumulations in the lower elevations but more significant
accumulations above 4000 feet. Snow advisories are in effect for
the East Slopes of the Oregon Cascades for 4 to 10 inches mainly
along the crest and 3 to 6 inches in the John Day Highlands.

South to southeast winds have increased across the region
overnight with winds of 10 to 25 mph and some gusts 30 to 40
mph. These winds will continue through the morning and then
decrease as a transition to westerly winds starts to develop this
afternoon and evening. These westerly winds will increase
overnight and continue through Sunday at speeds and gusts that
warrant the issuing of a wind advisory for some zones across the
Lower Columbia Basin mainly from the eastern Columbia River Gorge
to Pendleton.

LONG TERM...Tuesday through Saturday...Deterministic models and
ensembles are in good agreement particularly through Thursday then
there are some differences in the evolution/amplitude of an upper
ridge over the area for Friday and Saturday. On Tuesday a
shortwave trough will move SEWD across the region in NW flow
aloft. This will result in mountain showers and snow showers
(50-75% chance) with snow levels 3500-4500 feet. QPF amounts are
expected to be mostly less than .2 inches so significant impacts
from snow are not anticipated. The NBM has low probabilities of
TSTMS for Tuesday afternoon for the eastern Mountains (15-18%) but
forecast soundings show little CAPE and insufficient depth for
lightning so chose to keep TSTMS out of the forecast for Tuesday.

The winds will be a concern on Tuesday as well. The GFS is
forecasting the PDX-GEG surface pressure gradient to increase to
11-14 mb which is indicative of significant west winds across the
lower elevations. NBM 24 hour max gust probabilities of 45+ mph
across the lower elevations exceed 70% over a wide area Tuesday.

After Tuesday the weather will become quiet with the available
guidance indicating a warming and drying trend. On Wednesday high
temperatures will still be around 5 degrees below normal for this
time of year but by Thursday high temperatures will be near
normal. For Friday and Saturday the majority of the ensemble
clusters favor a building upper ridge though there is a lower
probability scenario (15-30% chance) for weak troughing over the
area on Friday and then flatter westerly flow on Saturday. This is
similar to the operational GFS and the GEFS ensemble and results
in slightly cooler temperatures than what the NBM and ECMWF
ensemble are forecasting. Given that the more likely scenario is
for the upper ridge went with the warmer NBM temperatures for now.
By Saturday the NBM probabilities of 85+ degree high temperatures
in the Columbia Basin and Yakima Valley are 40-70%. 78

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  71  40  50  40 /  30 100 100  70
ALW  73  42  52  44 /  30 100 100  80
PSC  74  47  57  47 /  60 100 100  40
YKM  63  44  60  39 /  90  80  80  10
HRI  72  42  55  43 /  70 100 100  40
ELN  59  43  56  40 /  70  60  60  10
RDM  51  32  47  34 /  80  70  60  10
LGD  66  38  47  38 /  30 100  90  90
GCD  62  34  45  35 /  90 100 100  70
DLS  58  45  56  46 /  90  80  60  20

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 2 AM to 11 PM PDT Sunday for ORZ041-044-507-
     508-510.

     Winter Weather Advisory from 5 PM this afternoon to 11 PM PDT
     Sunday for ORZ506.

     Wind Advisory until 2 PM PDT this afternoon for ORZ507.

     Winter Weather Advisory until 11 AM PDT Sunday for ORZ509.

WA...Wind Advisory from 2 AM to 11 PM PDT Sunday for WAZ024-521.

&&

$$

SHORT TERM UPDATE...85
SHORT TERM...91
LONG TERM....78
AVIATION...86