831
FXUS66 KPDT 040444
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
944 PM PDT Fri May 3 2024

.Updated for Aviation...


.AVIATION...
06Z TAFS...Through the morning, VFR conditions are expected
everywhere. Then, BDN and RDM are expected to become MVFR as RA
moves in (High Confidence 60-80%). At DLS, confidence has risen
since the previous TAF package in MVFR conditions, but later in
the afternoon, (30-50%). There is even a low chance of MVFR at PDT
by evening (20-30%). All other locations are expected to remain
VFR.

The other issue will be winds, which are expected to become gusty
in the 20 to 25 kt range at all sites except YKM and PSC. Psc may
gust to around 15 kts.

&&

.PREV DISCUSSION... /issued 816 PM PDT Fri May 3 2024/

UPDATE...
Latest radar imagery showed showers mainly west of the Cascades,
but the leading edge just starting to move into some areas east of
the Cascades. Expect continued slow eastward motion during the
overnight hours. The forecast looks generally on track and made
minor updates to POPS and updated overnight lows, especially over
central Oregon, where cloud cover and precipitation should keep
lows warmer than the forecast was indicating.

PREV DISCUSSION... /issued 434 PM PDT Fri May 3 2024/

Updated for Aviation...

AVIATION...
00Z TAFS...VFR conditions are generally expected through the
period, except for BDN and RDM, and possibly DLS. An upper low
will move toward the coast tonight and inland over northern
California Saturday night. Rain will develop in advance of this
low as early as tonight for locations such as DLS, RDM and BDN and
continue through much of the day on Saturday. RA is expected to
develop later Saturday elsewhere. Latest guidance suggests likely
MVFR conditions at BDN and RDM on Saturday (60-80% confidence)
and a lower chance at DLS (20-30%) confidence. At this time, will
keep DLS TAF VFR, but may need to add some MVFR conditions in
later issuances.

Winds will also increase at most sites on Saturday and could gust
as high as 25 kts.

PREV DISCUSSION... /issued 329 PM PDT Fri May 3 2024/

SHORT TERM...Today through Sunday Night...

Key Messages:

1. Significant rain and mountain snow through the weekend.

2. Some area rivers nearing or exceeding action (bankfull) stage.

3. Breezy winds late Saturday through Sunday evening.

Current radar and visible satellite imagery showing light to
moderate returns beginning to approach the west slopes of the
Oregon Cascades as mid-to high level clouds extend into the
Basin. This is in response to a robust upper level trough that is
dropping from the the Gulf of Alaska and closing in on the
Pacific Northwest coast. The low and associated cold front will
pass through our area on Saturday as wrap-around moisture will
keep widespread precipitation chances in the forecast through
Sunday afternoon. This cold front will allow for snow levels to
quickly drop 6000-7000 feet early Saturday morning to 2500-4000
feet early Sunday morning across Central Oregon. These dropping
snow levels coupled with ample moisture will lead to significant,
advisory level snowfall to occur across the east slopes of the
Oregon Cascades and through the John Day-Ochoco Highlands. This
has warranted to issuance of Winter Weather Advisories beginning
at 5 AM Saturday across the east slopes of the Oregon Cascades and
starting at 5 PM along the John Day-Ochoco Highlands above 4000
feet. 4 to 10 inches of snow is expected across the east slopes
above 4000 feet and 3 to 7 inches of snow is likely across the
John Day-Ochoco Highlands above 4000 feet. There is also a
potential for significant snowfall across the southern Blue
Mountains with 2-5 inches possible, mainly above 4000 feet. These
amounts across the southern Blue Mountains are just below
advisory criteria, but will need to be closely monitored.
Confidence in these snow amounts are moderate to high (70-80%) as
the NBM suggests a 65-85% chance of 6 inches or more snowfall
across the east slopes of the Oregon Cascades Saturday afternoon
through Sunday afternoon, a 70-90% chance of advisory level snow
amounts (3 inches or more) across the John Day-Ochoco Highlands
Saturday evening through Sunday evening, and a 40-60% chance of
advisory level snow amounts (5 inches or more) along the southern
Blue Mountains Saturday evening through Sunday evening.

Substantial low and mid-level moisture is associated with this
system as rain accumulations of 0.50 to 1.25 inches are expected
across lower elevations of the Basin Saturday morning through
Sunday night, with 1.5-2 inches likely across the Blue Mountains
and Cascades. The only areas that are expected to see 0.25-0.50 of
an inch include the Bend-Redmond area and the Kittitas Valley.
Confidence in these rain amounts are moderate (50-65%) as the NBM
suggests a 40-65% chance of 0.50 of an inch of rain across lower
elevations, with higher chances residing along the northern Blue
Mountain foothills and the John Day Basin. The NBM also highlights
a 50-60% chance of rainfall of 1 inch or greater over the
northern Blue Mountains and Elkhorns, with only a 10-30% chance
over the Oregon Cascades. This moisture will also translate to
rises across area rivers, with only the Naches River near
Cliffdell currently expected to reach action (bankfull) stage
briefly Saturday evening through Sunday afternoon. The John Day
River at Service Creek looks to stay just shy of action (bankfull)
stage Monday afternoon, but will continue to monitor as the
weekend event unfolds.

The incoming system will also attribute to breezy winds that will
begin tonight ahead of the approaching cold front. Winds are
expected to increase to 30-40 mph across the base of the northern
Blue Mountain foothills beginning shortly after midnight tonight
and extend through much of the morning on Saturday. The cold front
will pass late Saturday morning to allow these winds to slacken
through the afternoon as a pressure gradient begins to set up
along the Cascades to increase winds through the Simcoe Highlands,
eastern Gorge, and the northern Blue Mountain foothills beginning
Saturday evening and extending through the day on Sunday. Winds
are expected to peak between midnight on Sunday through noon
Sunday as advertised by the GFS, NAM, and SREF indicating a
11-12.5mb pressure difference between Portland and Spokane. These
values are just shy of advisory criteria as gusts of 35-45 mph
will be possible. Winds will be slowly decreasing through the late
afternoon and evening, but gusts of 30-40 mph will still be
possible until early Monday morning. Confidence in these wind
gusts are high (80-90%) as the NBM suggests an 85-95% chance of
wind gusts of 47 mph or greater across the northern Blue Mountain
foothills and the Kittitas Valley. This may warrant the need for
wind highlights to be issued, so stay tuned. 75

LONG TERM...Monday through Friday...Elongated troughing will bring
about a cool weather pattern for much of the work week, before a
high pressure ridge moves in and warms up temps toward seasonal
averages and perhaps even above normal values by the start of next
weekend.

Ensembles are in pretty good agreement on the synoptic pattern next
week, starting with a deep low centered over the Gulf of Alaska that
partially envelops the PacNW with cold air. Guidance suggests that
this low will essentially split into two systems: a low that remains
over northern Canada and another low that gradually shifts eastward
into the northern Plains. During this splitting process, the parent
Alaskan low will advect in moisture into the forecast area on
Monday, however QPF guidance early on indicates that this will
primarily be a mountain/high elevation event. NBM probabilistic
guidance depicts only about a 20% chance of much of the Basin seeing
at least 0.05 inches, with chances increasing across our foothill
zones (40-60%) and central Oregon (45-55%). The warm air advection
from this weekend`s system might shield the forecast area from lower
snow levels, with Monday`s levels in the 4000-5000 ft range, but
cannot completely rule out our Cascade Passes from seeing some light
accumulating snow. That being said, confidence is on the lower end
(30-40%) due to how borderline snow levels are in the NBM for
Monday.

Once the low splits, our forecast area will find itself on the
western flank of the departing low to the east, resulting in some
cold air advection via northerly flow aloft. Temps look to remain
below average Tuesday into Wednesday by as much as 15 degrees,
however temperature ensembles do not currently suggest a freeze risk
for the Basin. PoPs fall off on Tuesday as drier air prevails
overhead with PWATs in the 0.2-0.3 range, leaving primarily light,
orographically driven precip over the mountains thanks to N/NW flow
aloft.

For Wednesday onward, the forecast looks to be dry and warmer.
Ensembles are in pretty strong agreement that a ridge will build in
by the end of next work week, allowing temps to soar well into the
70s across our population centers. Should high pressure persist, as
8-10 day ensemble clustering suggests, highs in the lower Basin
could even reach into the low 80s by next weekend. Confidence is
high (70%) in this warm and dry forecast panning out based on what
ensembles are showing. Evans/74

AVIATION...18Z TAFS...VFR conditions for today with currently high
clouds at most sites, except for KPDT/KALW having clear skies. Cloud
coverage will increase starting this afternoon into tonight/Saturday
morning. KPDT might have winds gusting up to 24kts tonight as the
low off OR coast pushes a cold front over PacNW. This will also
bring rain showers into KDLS/KRDM/KBDN/KYKM Saturday morning as
well. Winds will be less than 10kts, but will increase for KPDT/KALW
this evening and overnight. Feaster/97

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  50  68  37  47 /  10  40  90  80
ALW  56  72  41  50 /   0  30  90  80
PSC  53  72  46  56 /  10  30  90  70
YKM  49  63  44  62 /  10  80  70  40
HRI  52  70  41  54 /  10  40  90  80
ELN  44  61  42  57 /  10  80  50  40
RDM  45  49  31  47 /  50  80  50  50
LGD  49  65  37  47 /  10  20  90  80
GCD  46  61  34  44 /  10  80  90  90
DLS  49  56  45  55 /  30  80  70  50

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 5 PM Saturday to 11 PM PDT Sunday
     for ORZ506.

     Winter Weather Advisory from 5 AM Saturday to 11 AM PDT Sunday
     for ORZ509.

WA...None.
&&

$$

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