679
FXUS66 KPDT 041205
AFDPDT

Area Forecast Discussion...UPDATED AVIATION.
National Weather Service Pendleton OR
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

&&

.AVIATION...12Z TAFS...Increasing aviation concerns today that
continue tonight between deteriorating flight categories, light
gusts, and possible snow at central OR terminals tonight. Latest
satellite shows an extensive cloud shield with clouds moving north
across the area and obs denoting mainly high-end VFR ceilings
(greater than 7 kft). This system will prompt precipitation to
spread gradually north and east over the next 12-18 hrs as moisture
gets ushered north and low levels and moisture in the column
increases from increased forcing for ascent over time.

Chances start ramping up (to and >80%) through 18Z across the
western-most terminals before ramping up (>80%) across the KPSC,
KPDT, and KALW terminals around 0Z-6Z, the former having the longest
wait. High confidence (>80%) in MVFR conditions developing with
precip and worsening flight categories, initially over central OR
and the KDLS terminal today then KYKM, KPSC, KPDT, and KALW tonight
with a strong consensus seen in the forecast envelope composed of hi-
res deterministic guidance runs and past runs. Of note, winds will
be gusty but confidence is high (80%) in peak gusts staying around
20-25 kts, except at KPDT when gusts are expected to increase late
tonight to 30 kts. Lastly, snow mixing in with rain will be a
possibility at KBDN and KRDM, however, confidence in predominately
snow as the p-type is currently low (less than 30%) through 12Z
tonight.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  71  40  50  40 /  30 100 100  70
ALW  73  42  52  44 /  20 100 100  80
PSC  74  47  57  47 /  20 100 100  40
YKM  63  44  60  39 /  80  80  80  10
HRI  72  42  55  43 /  40 100 100  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 /  80  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.

     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...91
LONG TERM....78
AVIATION...80