152
FXUS66 KPDT 012151
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
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.


&&

.AVIATION...18Z TAFS (previous discussion)...VFR conditions for
all sites during this period before another system arrives
tonight. Confidence is moderate for KRDM/KBDN decreasing to MVFR
due to rain bringing low VSBYs and CIGs Thursday morning(50%), but
lowers for snow (30-40%). Winds will generally be light, but
KDLS/KPDT/KALW could have winds gusting around 17-22kts into this
evening before decreasing. Feaster/97


&&

.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 /  70  40   0  10
LGD  32  58  32  65 /  10  40  10   0
GCD  34  56  33  65 /  40  60  20   0
DLS  42  67  42  70 /  30  20   0  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
&&

$$

SHORT TERM...85
LONG TERM....77
AVIATION...97