709
FXUS66 KPDT 020516
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1016 PM PDT Wed May 1 2024

.EVENING UPDATE...Satellite imagery shows a low tracking southeast
into the PacNW late this evening. A noteworthy change since the
afternoon forecast, 00Z CAMs are now indicating a band of
significant precipitation (0.25-0.75") on the north side of the
low as it tracks across central Oregon overnight. Precipitation
type remains a challenge with this system as surface temperatures
in guidance appear marginal for snow accumulation, and NBM
calibrated probabilities continue to advertise very low
probabilities of snowfall for the lower elevations of central
Oregon (<10% chance of 0.1") and modest probabilities (20-50%) of
advisory-level snow (>5") for the Oregon Cascades. However, 12Z
ECMWF EFI values of 0.7-0.8 with positive shift of tails are
highlighting the area for climatologically unusual snowfall, and
a good portion of ensemble members are highlighting light
snowfall as low as 2000 ft under the aforementioned band. Have
opted to issue a winter weather advisory for the Oregon Cascades
as 00Z HREF probabilities of exceeding 5" of snow are a widespread
40-80% along the east slopes of the Cascades, including over the
passes (HWY-20 and HWY-26). HREF Probabilities are low for
snowfall in central Oregon`s population centers (20-50% chance of
0.1"), but not insignificant for 1" (10-40% chance). Confidence is
too low to issue any highlights for central and north-central OR
and the Ochoco-John Day Highlands. Plunkett/86

&&

AVIATION...06Z TAFs...A weather system from the Pacific will
bring increasing chances of MVFR or lower conditions (CIGS and/or
VSBYs) overnight to KBDN/KRDM (80-95% chance of MVFR, 30-40%
chance of IFR, and <20% chance of LIFR) as ample cloud cover and
precipitation spreads over Oregon. Precipitation type will be a
challenge with this system, leading to lower-than-average
confidence in CIGs/VSBYs as precipitation may (20-40% chance) turn
over to snow or a rain/snow mix by late night and early morning
at KRDM/KBDN. Elsewhere, VFR conditions are very likely (85%
chance or greater). Winds of less than 10 kts are forecast.
Plunkett/86

PREV DISCUSSION... /issued 251 PM PDT Wed May 1 2024/

SHORT TERM...Tonight through Friday night...The forecast area is
currently under a northwest flow aloft and observing a break in
precipitation after last night`s shortwave trough. The air mass
remains marginally unstable over most of the region. SPC Mesoscale
Analysis shows SBCAPES between 100-500 J/kg and 7-8C lapse rates
between H7-H5, but a mid level cap is keeping clouds primarily as
towering cumulus. WSR-88D is picking up on a few weak returns over
Wallowa County that could be isolated shower activity. Winds are
a little breezy from the WSW sustained at 10-20 mph across the
Lower Columbia Basin.

Water vapor loops show a wave developing west of Vancouver Island,
and this is the next system to bring precipitation to our southern
forecast area late tonight through Thursday morning. The track of
the system and the left exit region of the jet will bring the best
potential over the southern half of Oregon, and the forecast will
show 60-80% PoPs south and west of Madras and south of John Day
(100% along OR Cascade crest). Looking at the HREF members, there
are differences on the placement of the main band of precipitation
which has made for some challenges in tonight`s forecast. This
will be a quasi-stationary front resulting in steady precipitation
for about 6-8 hours. Snow levels will be around 4000 feet and
current wet bulb temperatures are in the 30s, therefore many areas
will observe a snow or snow/rain mix that will affect some of the
highways such as HWY 26 and 97. Using the 10:1 snow amounts of
the HREF, the prob for 1" or more is around 70% and the prob for
2" or more ranges 50-70%. The high probabilities of snow
accumulation at 3" or more are shown in the higher terrain.
Confidence is high that Mt. Bachelor will have moderate to heavy
snow with this front. There will be a sharp line of areas
observing precipitation to areas with no precipitation, and
eastern WA and most of northeast OR will have dry conditions
tonight. It will be another chilly night as temperatures fall into
the 30s to around 40. No Freeze Warnings are in effect, although
overnight lows will fall into the mid-30s in the Kittitas and
Yakima Valleys.

The band of snow and rain will decrease in coverage Thursday as
drier air in a NNW flow spreads across the region. The front will
bring showers (30-50% chance) over the eastern mountains/valleys
along with a slight chance (15%) of thunderstorms as it tracks
eastward. CAMs show around 150 J/kg SBCAPEs and 15-20 kt of shear
between 0-6 km so storms, if any, will likely be weak and
diurnally driven (confidence 60%).

High pressure aloft will bring mostly dry conditions Thursday
night and Friday morning. This will be a weak transitory ridge
quickly forced eastward due to a deepening low offshore. Models
are in good agreement with the offshore low that will bring
widespread precipitation to western WA/OR Friday and will
gradually spread east of the Cascade Range Friday night. A
southerly flow will increase snow levels to around 6500 feet
Friday night. GFS is hinting of a weak AR over central OR in the
IVT progs, although the ECMWF is less impressive. This still looks
like a wet system to start the weekend, and Friday night looks
particularly wet for central and north central OR and south
central WA. Wister/85

LONG TERM...Saturday through Wednesday...An upper level low and
associated trough will move inland over northern
California/southern Oregon on Saturday and move generally eastward
through Sunday. Some weak ridging will build in later Sunday into
Monday, then the flow will become more zonal and another weaker
system will move across the Pacific Northwest later Monday into
Tuesday. Finally, the flow becomes more northwest and drier as we
head into Wednesday.

The first low pressure system will head mainly to the south,
however wrap around moisture on the north side of the low is
expected to be abundant. Model guidance has come into better
agreement this model cycle after have quite a bit of discrepancies
in previous runs. The ensemble clusters generally have an
agreement between about 53% to about 57% through Tuesday. By
Wednesday, there is much less agreement, mainly due to the
position of the upper low over the northern plains and how it
handles upstream energy (or lack thereof). Agreement is no more
than about 40% at this time, and this is day 7, and normal model
variability has to be taken into account at this time scale.

Latest QPF values are over an inch in most of the mountain
locations and even over 0.75 inches in many of the lower
elevation locations. The only place that seems to miss out on the
rain is central Oregon, though they still get over 0.25 inches.
This is a significant increase, especially across many of the
lower elevation areas.

Snow levels will start out over 5000 feet everywhere, and in most
areas over 6000 feet on Saturday, but by Sunday morning, will
decrease to between 3000 feet in central Oregon to about 4500 feet
elsewhere. So a mix of rain and snow or even all snow is possible
Sunday morning, though accumulations are expected to be light,
except at the highest elevations where there could be a few
inches. During the day, snow levels will rise again to between
4500 and 5500 feet.

Precipitation should begin to wind down, especially across the
lower elevation areas, Sunday into early Monday. However,
chances will increase again later Monday as the next system moves
through. Finally, by later Tuesday into Wednesday, precipitation
chances will decrease most everywhere.

The ECMWF EFI shows QPF values of 0.8 to 0.9 over central Oregon
on Saturday, with the highest values west of the Cascades. For
snow, there is a small area 0f 0.7 to 0.8 over the Oregon Cascades,
high shift of tails values of 1 to 2 extend northeastward into
the higher terrain of the southern Blue Mountains. These values
indicate higher uncertainty due to the low and trough and at least
some low potential for snow. Any snow would be anomalous in May.
On Sunday, the EFI QPF anomalies shift north and eastward to
northeastern Oregon and southeastern Washington, where values are
0.7 to 0.8, with a shift of tails of 2, centered over the Columbia
Basin. The snow is generally 0.6 to 0.7 over the northern Blue
mountains and a shift of tails of 2. So the ECMWF while indicating
some target areas is also indicating some continued uncertainty
in the amounts.

Winds will also be gusty, especially later Saturday into Sunday,
mainly across the Columbia basin, Gorge and Foothills of the Blue
Mountains, Yakima Valley and Kittitas Valley. Breezy winds are
expected to continue into Monday, and could become more widespread
across the region. Winds will generally gust 25 to 35 mph. The
ECMWF EFI is not keying in on anything regarding winds on
Saturday, however it does focus in on winds on Sunday with a large
area of 0.7 to 0.8 and a small 0.8 to 0.9 across the Oregon
Columbia Basin and Blue Mountain Foothills. This area shifts east
and north on Monday into Washington, over the Blue Mountains and
nearby foothills and Columbia Basin with a large area of 0.6 to
0.7 and the focus of 0.7 to 0.8.

The NBM probabilities of winds >= 39 mph on Sunday across the Blue
Mountain Foothills, Columbia Basin, Gorge, Simcoe Highlands Yakima
Valley and Kittitas Valley are as high as 70 to 90%. However, the
same NBM probabilities for winds >=47 mph in the same locations
are only as high as about 40 to 60% across a much smaller area.

Finally, temperatures are expected to be below normal through the
period due to the presence of the trough initially. They will warm
a bit by midweek and will end up closer to normal, but still
below, especially in central Oregon.

On Saturday, highs will range from the 50s in central Oregon to
the 60s across the Columbia Basin. In central Oregon, these high
temperatures would be about 15 degrees below normal, where
elsewhere, they may only be about 3 to 5 degrees below normal.
Sunday will be the coolest day. Highs will be near 50 in central
Oregon, to the upper 50s in the Basin (and could be cooler).
Again, this is about 15 degrees below normal in central Oregon and
about 10 or so degrees cooler elsewhere. By Monday, temperatures
will begin to warm, but will still be below normal, especially in
central Oregon.

The ECMWF EFI also keys in on the anomalous high temperatures
beginning Sunday, mainly from central Oregon west, with values of
-0.7 to -0.8. The area spreads eastward on Monday, but the values
 are less. Most of the state of Oregon is covered on Monday in the
 - 0.6 to -0.7 range. On Tuesday, virtually all of Oregon and
 most of Washington is covered in -0.6 to -0.7 anomalies for
 highs, with a large portion of Oregon in -0.7 to -0.8, with
 similar results Wednesday, though a bit further east.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  35  62  36  70 /   0  20   0   0
ALW  37  66  40  73 /   0  10   0   0
PSC  39  69  42  75 /   0   0   0   0
YKM  34  67  37  70 /   0   0   0   0
HRI  40  68  39  75 /  10  10   0   0
ELN  33  66  36  67 /   0   0   0   0
RDM  34  55  33  64 /  90  70   0  10
LGD  32  58  32  65 /  10  40  10   0
GCD  34  56  33  65 /  50  70  20   0
DLS  42  67  42  70 /  50  30   0  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 11 AM PDT Thursday for ORZ509.

WA...None.
&&

$$

EVENING UPDATE...86
SHORT TERM...85
LONG TERM....77
AVIATION...86