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000
FXUS66 KPDT 280543
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1042 PM PDT Sat Apr 27 2024

.Updated Aviation Discussion.



.UPDATE...

Key Messages:

1. Winter Weather Advisory along the Cascades Monday morning.

2. Breezy afternoon conditions Sunday and Monday.

A robust upper level shortwave will spin off the parent low
located along the British Columbia coast, and will drive across
the area late Sunday through Monday morning. This will bring a
brief period of widespread precipitation across the area Monday
morning, with limited rain accumulations (less than 0.05 of an
inch) expected at lower elevations of the Basin and Central
Oregon. The primary impact with this incoming shortwave will be
moderate snowfall at pass elevations and higher, with
accumulations of 5 to 8 inches possible. Thus, a Winter Weather
Advisory has been issued across the Washington Cascades above 3500
feet and the Oregon Cascades above 4000 feet from 11 PM Sunday
through 11 AM Monday. Confidence in these snow amounts is moderate
(50-60%) as the NBM showcases a 40-70% chance of 5 inches of snow
or greater over Snoqualmie, White, and Santiam passes, with the
HREF suggesting a 30-70% chance. The best chance for 5 inches or
above will occur over White pass (50-70%), then Santiam pass
(30-65%), and finally Snoqualmie pass (40%).

The upper level low pressure will be approaching the British
Columbia coast tonight into Sunday morning as a weak cold front
passes to redevelop a pressure gradient across the Cascades,
leading to breezy winds that will peak through the afternoon and
evening. Highest winds are anticipated to occur along the Simcoe
Highlands as gusts of 30-40 mph will be possible out of the west.
Wind gusts across the Basin are expected to reach between 25-35
mph on Sunday. The shortwave poised to pass through the area on
Monday, which is expected to bring advisory-level snow amounts
across higher elevations of the Cascades, will also attribute to
even windier conditions through the Simcoe Highlands, northern
Blue Mountain foothills, and the Kittitas/Yakima Valleys through
the afternoon and evening. Winds will approach advisory criteria
in these areas as gusts of 40-47 mph will be possible. Moderate to
high confidence (70-90%) resides with Sunday and Monday`s wind
values as the NBM suggests a 85-95% chance of wind gusts of 39 mph
or greater Sunday afternoon/evening over the Simcoe Highlands,
with a 60-80% chance of gusts of 47 mph or greater on Monday
afternoon/evening. The NBM also presents a 60-80% chance of 39 mph
gusts along the northern Blue Mountain foothills on Sunday
afternoon/evening, and a 60-80% chance of 47 mph gusts on Monday
afternoon/evening. This may warrant the need for wind highlights
Monday afternoon/evening, so stay tuned as we continue to analyze
this early week event. 75


&&

.AVIATION...06Z TAFS...VFR conditions currently at all sites, which
will stay the course through the period. Breezy winds are continuing
at KPSC as gusts of 20-25kts are expected to persist overnight.
Elsewhere, winds will stay 10kts or below. Ceilings have dropped to
SCT/BKN060 for all sites, with the exception of BKN250 at KYKM/KPSC
that will continue through Sunday morning. A weak cold front will be
passing through the area late Sunday morning, attributing to
increasing wind gusts of 20-25kts that extend through the afternoon
before peaking in the evening between 25-30kts. 75


&&


PREV DISCUSSION... /issued 309 PM PDT Sat Apr 27 2024/

SHORT TERM...Today through Monday...Current satellite shows
stratus clouds beginning to slink over the Cascades. Radar shows a
few areas with signals with the current observations showing a
light amount of rain has fallen in isolated areas along the
eastern slopes of the Cascades with only 0.01 inches in the last
hour. Winds have also kicked up a bit bringing a few areas of
patchy blowing dust just northeast of the Tri-Cities along HWY 395
as called in by a spotter and confirmed on cameras. A few
observations in the area show sustained winds near 25 mph and
gusts near 35 mph. These breezy winds will continue through the
afternoon so be mindful of blowing dust when driving near Tri-
Cites.

Models are in firm agreement with the upper level low that
brought the rain showers across the region yesterday to be down
over the Four Corners while the next upper level low slides down
the coast of B.C.. This has caused the flow over the region to
turn to a more zonal flow or westerly and increasing the winds
through the Cascade Gaps. 80-90% of the raw ensembles show that
the winds will continue through the late afternoon with sustained
speeds of 20-25 mph and gusts to 35 mph. Blowing dust has been
spotted along HWY395 and will continue through the afternoon as
well. EFI continues to show the region under near normal seasonal
temperatures with over 90% of the ensembles showing the Gorge,
foothills of the Blues, Basin and adjacent valleys seeing highs in
the mid to upper 60s, mid elevations in the mid to upper 50s and
the higher terrains in the mid 40s.

Sunday models show the pattern begins to shift the upper level low
continues to descend into the PacNW sending a shortwave with
precipitation out ahead of it. Guidance shows a disturbance will
bring enhanced precipitation over the Cascades as well as
ensembles showing cooler temperatures as well. NBM shows the snow
levels will begin to drop to below pass levels descending as low
as 2400 ft overnight Sunday into Monday. EFI shows temperatures to
be slightly below normal average along the WA Cascades but near
normal elsewhere. 80-90% of the ensembles have the lower
elevations cooling to the upper 50s low 60s, mid-elevations
cooling to the upper 40s to mid 50s and the higher terrains in the
low to mid 30s.With this, precipitation falling across the
Cascades will be that of snow. With the highest concentrations
above 3500ft in the WA Cascades and 4000ft in the OR Cascades. EFI
has picked up on this event and has it marked as an above average
seasonal snowfall event with values of 0.81 and a shift of tails
nearing 0-1. Not surprised as it is late April. Looking at the raw
ensembles and guidance, there is now higher confidence (60-80%)
that Snoqualmie and Santiam Pass could see roughly 5-7 inches
along the I-90 above 3500 ft and 6-8 inches HWY20, Santiam Pass so
a winter weather advisory has been issued for 11PM Sunday night
to 11AM Monday morning. Winds will also become a highlight Sunday
through Monday as the westerly flow deepens with the incoming
upper level low. Guidance shows that surface gradients begin to
tighten sharply across the Cascades leading to breezy to windy
conditions through the Cascades Gaps and along the foothills of
the Blues through portions of the Columbia Basin. Raw ensembles
show 80-90% probabilities of 30-35 mph or higher sustained winds
with gusts nearing 40 mph through those areas with locally higher
gusts. Bennese/90

LONG TERM...Tuesday through Saturday...An active pattern is
likely (>95% chance) Tuesday, followed by more uncertainty in
pattern details Wednesday through Saturday as ensemble guidance is
struggling to resolve the tracks of multiple waves with origins
in the vicinity of the Gulf of Alaska.

A compact upper-level closed low is anticipated to weaken into a
broad upper-level trough Tuesday as it tracks southeast from the
Gulf of Alaska into the PacNW. This is the second in a series of
two shortwave impulses, the first anticipated Monday. The main
weather concern with this trough will be low snow levels
facilitating a 30-50% chance of advisory-level snow for the
Cascade crest. Chances of accumulating snow are low (~5% chance)
for population centers in central OR, the Kittitas Valley, the
foothills of the Blue Mountains, and the Simcoe Highlands. Sub-
freezing morning low temperatures are possible (50-80% chance)
Tuesday morning for the Kittitas Valley. Probabilities of sub-
freezing temperatures are lower (20-50% chance) for the Yakima
Valley and the foothills of the northern Blue Mtns of OR.

Ensemble members depict the trough exiting Wednesday and favor a
brief upper-level ridge building into the PacNW from the Pacific.
Ensemble clusters vary in strength of the aforementioned ridge
with roughly 60% showing an amplified ridge while the remaining
20% suggest weak ridging.

Thursday, ensemble clusters are struggling to resolve the track of
another closed low from the Gulf of Alaska. Roughly 15% of members
depict a deep closed low tracking onshore into WA/OR with
widespread precipitation and cool temperatures, while a further
~23% show a weak troughing pattern over the PacNW with
precipitation focused more over the Cascades and WA. The remaining
members all advertise warmer temperatures as an upper-level ridge
axis moves either overhead (37% of members) or remains offshore.
The latter two solutions would lead to mostly dry conditions for
our forecast area.

Friday, clusters show a similar pattern (59% chance of the upper-
level ridge, 29% chance of a weaker ridge, and 11% chance of the
deep closed low). The trough would result in cool, unsettled
weather, while the ridge would be warm and drier.

Saturday, ensembles are favoring an upper-level ridge (53% of
members) and dry weather, while the remaining are advertising a
third system from the Gulf of Alaska with precipitation across the
PacNW. Plunkett/86

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  40  60  39  55 /  10  10  10  40
ALW  45  63  42  57 /  10  20  20  70
PSC  47  66  44  62 /   0   0  10  50
YKM  38  63  36  59 /   0  20  10  30
HRI  43  65  41  60 /  10  10  10  30
ELN  38  57  35  54 /  20  20  20  40
RDM  35  57  32  50 /  10  10  10  40
LGD  39  58  37  49 /  30  30  30  70
GCD  37  61  36  50 /  30  10  20  50
DLS  46  61  42  57 /  30  10  40  60

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 11 PM Sunday to 11 AM PDT Monday
     for ORZ509.

WA...Winter Weather Advisory from 11 PM Sunday to 11 AM PDT Monday
     for WAZ522.

&&

$$

SHORT TERM...90
LONG TERM....86
AVIATION...75