756
FXUS66 KPDT 031640
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
940 AM PDT Fri May 3 2024

.UPDATE...Current radar and infrared satellite imagery showing dry
and clear conditions. However, there are still some lingering
stratus that continues to dissipate over the Southern Blue
Mountains and the John-Day Basin. Significant cloud cover is
pushing inland ahead of the next robust weather system that will
be passing through the Pacific Northwest over the weekend. Clouds
will be increasing today as rain chances slowly extend into the
east slopes later this evening. This system will bring with it
ample moisture and breezy to windy conditions, with widespread
precipitation expected Saturday afternoon through Sunday
afternoon. A Winter Weather Advisory has been issued along the
east slopes of the Oregon Cascades above 4000 feet beginning 5 AM
Saturday as up to 10 inches of snow will be possible. There is
also a potential for significant, advisory level snowfall across
the John-Day/Ochoco Highlands and the Blue Mountains at
elevations above 3500 feet Saturday evening through Sunday
afternoon, but this will need to be further analyzed due to
uncertainties in system track and strength.

The secondary concern over the weekend resides with breezy to
windy conditions Saturday evening through Sunday as a significant
pressure gradient sets up along the Cascades in the wake of the
passing cold front Saturday morning/afternoon. This will lead to
wind gusts over the Simcoe Highlands early Sunday morning reaching
between 35-45 mph, with these winds returning during the late
afternoon and evening hours. These elevated winds will also extend
across the northern Blue Mountain foothills, with gusts slowly
intensifying and approaching advisory criteria Sunday night. 75

&&

.PREV DISCUSSION... /issued 452 AM PDT Fri May 3 2024/

SHORT TERM...Today through Sunday...A transitory ridge of high
pressure will be passing over the region today. This will provide a
mild Spring day with temperatures getting into the low 70s around
the Basin and 60s in the mountains. High clouds will begin to
increase and spread across the forecast area late today and
overnight as the next weather system approaches. This closed upper
level low pressure system is dropping SE out of the Gulf of Alaska
with the center of the low taking a path into SW Oregon Saturday
afternoon then continuing into the Great Basin on Sunday. This will
bring the leading edge of a cold front into the Cascades tonight but
its eastward progress overnight and into Saturday will be slowed as
the low center moves into SW Oregon. Eventually the cold front turns
into a deformation band of precipitation Saturday night into Sunday
as moisture wraps around the low back across northern Oregon and
southern Washington. 48 hour QPF amounts ending 12Z Monday continue
to show a very high probability that lower elevations could see .5 -
1.0 inches of precipitation while the eastern mountains and Cascade
crest could see 1 - 2 inches of precipitation. This amount of
precipitation is expected to cause streams and rivers to rise and
peak early next week but remain below flood stages.

Snow levels will initially be high Friday night and early Saturday
around 6000 to 7000 feet. They will begin to lower Saturday
afternoon and night as the low passes south of the forecast area.
Central Oregon will see snow levels down to 3500 to 4500 feet
Saturday night but remain 4500 to 6000 feet across the rest of the
forecast area. This could produce 4 to 10 inches of snow along the
Oregon Cascade crest impacting travel over the passes so a Winter
Weather Advisory has been issued Saturday morning through Sunday
morning.

Other concern will be some increasing S-SE winds overnight and
Saturday. This will produce some gusty winds over the higher terrain
of eastern Oregon and cause some downsloping off of the Blue
Mountains along the Foothills. A wind shift back to windy westerly
winds is expected to take place late Saturday and continue through
Sunday.

LONG TERM...Monday through Thursday...Another, weaker weather
system will move across the Pacific Northwest on Monday and bring
another round of precipitation, mainly to the mountains.  However,
there will be chances even in the lower elevations Monday afternoon.
 QPF amounts will be fairly light in most areas. In the mountains
QPF will be a bit higher, ranging from 1/4 to 1/2 inch along the
crest of the Cascades to around 1/4 inch in the Blues and Wallowas.
Most other areas will see only a few hundredths.

Precipitation chances will end everywhere except over the mountains
by Tuesday morning.  Some additional shortwave energy could enhance
precipitation chances again Tuesday afternoon, mainly over the
eastern Oregon mountains, but overall QPF is expected to be very
light...only a few hundredths in most locations.

Snow levels start out at 4000 feet or higher on Monday, but drop to
between 2500 and 3000 feet by Tuesday morning.  So, certainly the
precipitation will mix with and change to snow in the higher
elevations of the Cascades and Blues, and a few inches are possible
along the crests with little elsewhere.  The ECMWF EFI focuses on
precipitation and snow over the Blue Mountains and nearby foothills
with and area of 0.6 to 0.8 on Monday.

Also on Monday, there is at least some chance of thunderstorms
mainly over the eastern Oregon mountains, as this next systems moves
through.  CAPE values are expected to be in the 100 to 300 J/KG
range, though the best CAPES look to be further southeast.  LI
values look to be between 0 and -2 degrees C.

By Tuesday evening into Wednesday and for the remainder of the
period, a ridge will build over the Pacific Northwest.  This should
bring drier weather and warmer temperatures.  A few shwoers over the
eastern mountains can`t be ruled out on Thursday, but confidence is
low (<30%) at this time.

Winds will start out quite gusty as a result of the pressure
gradient between the departing trough in the Great Basin and higher
pressure to the west.  WInds will gust 25 to 35 mph and possibly as
high as 40 mph in portions of the Columbia Basin, Blue Mountain
Foothills, Columbia Gorge.  Winds will decrease during the day but
remain gusty and become more widespread as the next system
approaches. Gusty winds look to continue into Tuesday, with speeds
25 to 35 mph, before decreasing into Wednesday.

The ECMWF EFI keys in on the Columbia Basin and Foothills of the
Blue Mountains for winds on Monday, with values ranging from 0.6 to
0.8, with a core of 0.8 to 0.9 over the Blue Mountains.

The NBM probabilities of wind gusts >= 39 mph on Monday are 70 to
90% across much of the Basin and Blue Mountain Foothills Simcoe
Highlands and Cascade Gaps.

On Tuesday, the NBM probabilities of wind gusts >=39 mph are 80%
plus across much of the Basin, Gorge, Blue Mountain Foothills ,
Simcoe Highlands, and Kittitas Valley.  The probabilities of wind
gusts >=47 mph are 50 to 70% across portions of the Columbia Basin,
Foothills of the Blue Mountains, Simcoe Highlands and 60-90% across
the Kittitas Valley.

High temperatures on Monday will start of 10 to 15 degrees below
normal and will mainly range from the 50s in central Oregon to the
60s across the Columbia Basin.   By Wednesday, high temperatures
will begin to rise and by Thursday, highs will be close to normal in
most areas and above normal in the Basin.

The ECMWF EFI shows this trend as well for high temperatures.  On
Monday, there is a large area of -0.7 to -0.8 across most of Oregon,
and -0.6 to -0.7 across much of southeastern Washington.  On
Tuesday, the area moves a bit eastward.  There is a large area of -
0.7 to -0.8 across much of the region and even an area of -0.8 to -
0.9 over the Oregon Blue Mountains.  By Wednesday, most of the
anomalous temperatures are gone in the EFI.

AVIATION...12Z TAFS...VFR conditions and little concerns area-wide
before elevated aviation concerns begin right at the end of the TAF
period. Latest nighttime microphysics RGB imagery shows large clear
skies, except for a few swaths of thing high clouds crossing the
Cascades and areas of low stratus in the higher terrain of central
OR and the central OR mountains.

The next weather system offshore in the eastern North Pacific will
be approaching the OR coast tonight with its attendant cold front
reaching the Cascades near the end of the TAF period. There will be
increasing mid and high clouds out of the system late morning-
afternoon before low-end VFR ceilings (less than 7 kft) materialize
west to east early tonight as moisture gets transported from south
to north in tandem with increasing large scale forcing for ascent.
High confidence in low-end VFR ceilings late tonight, >70%, with
increasing potential for sub-VFR ceilings across central OR. Latest
LAMP ceiling probs have been growing the last three runs with
chances for MVFR now >30% at KBDN and KRDM after 09 UTC. Current
thinking is MVFR flight categories may need to be introduced if this
trend continues and timing of the system speeds up. Already high
chances for rain that coincide with this potential after 09 UTC
tonight (>70%). Otherwise, increasing southerly to easterly winds
this afternoon ahead of the system as it evolves south and east the
next 24-hrs, however, potential for frequent gusts up to 20 kts is
low (less than 50%) at all sites.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  70  50  67  41 /   0  20  50 100
ALW  73  54  71  44 /   0  10  40 100
PSC  73  56  72  45 /   0  10  40  90
YKM  69  49  65  42 /   0  30  80  70
HRI  75  54  70  43 /   0  20  50  90
ELN  67  44  61  42 /   0  30  80  70
RDM  65  41  50  32 /  10  70  90  70
LGD  65  46  63  39 /   0  10  50 100
GCD  64  45  61  35 /   0  10  90 100
DLS  72  49  57  46 /  10  60  90  90

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 5 AM Saturday to 11 AM PDT Sunday
     for ORZ509.

WA...None.
&&

$$

UPDATE...75
SHORT TERM...91
LONG TERM....77
AVIATION...77