817
FXUS66 KPDT 210410
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
910 PM PDT Mon May 20 2024

.EVENING UPDATE...Quiet weather is expected overnight as the
longwave trough responsible for showery weather over the past
couple days exits to the east. Looking ahead, guidance is still
in excellent agreement that the next system, an upper-level
closed low currently located along the BC coast, will track
southeast into the PacNW by morning. However, 00Z guidance has
delayed onset of precipitation by several hours and the evening
update reflects this trend. Overall messaging remains the same:
expect widespread precipitation to spread east of the Cascades
tomorrow afternoon and evening with the heaviest amounts over the
Cascade crest, Blue Mountains and their foothills. Areas
immediately to the lee of the Cascade crest in south-central WA
down into north-central OR and central OR will see a significant
rain shadow and lighter precipitation amounts. Plunkett/86

&&

.PREV DISCUSSION... /issued 434 PM PDT Mon May 20 2024/

SHORT TERM...Tonight through Wednesday night...Instability
showers and a rogue thunderstorm have been observed this
afternoon in Wallowa County where SPC`s Mesoscale Analysis
indicates 500 J/kg of MUCAPE and mid-level lapse rates around 7C
between H7-H5. The rest of the forecast area is under partly
cloudy skies, and the extent of fair weather cumulus on satellite
is quite interesting. Skies will clear tonight in a drier NNW flow
aloft. It will be another chilly night with lows in the 30s and
40s, but the southwest surface winds for most of the evening will
keep temperatures from falling below freezing in the lowlands
currently in the growing season.

The dry conditions will not last long, as the next system spreads
precipitation over the forecast area Tuesday afternoon and evening
with numerous showers on Wednesday. This system is coming in as
an upper level trough over BC moving south and developing into a
closed low over WA and northern OR. This will be aided by a 100kt
jet at 250mb with the left exit region of the jet aimed over
northeast OR. Although rainfall amounts will not be substantial,
there will be a steady light to moderate rain over the Blue Mtns,
Grande Ronde Valley, and foothills Tuesday night through
Wednesday. The most likely rainfall amounts during this time will
be 0.25-0.5", and the NBM has a 40% chance that 0.75" will be
observed over the northern Blue Mtns (20% for 1" or more). The
ECMWF EFI that always does a good job indicating climatologically
high QPF has this area at 0.8 out of 1.0 category. Snow levels
will begin around 6500-7000 feet then will lower to around 5000
feet on Wednesday. By the time snow levels lower to 5000 feet,
precipitation will decrease, therefore snow accumulations will not
impact any of the populated areas. Only the Eagle Caps and the
Elkhorns will have any noteworthy snowfall of around 3-6" at the
highest peaks. Breezy to windy conditions will return on Wednesday
with gusts 25-40 mph, and there are only few areas that have a
chance (30%) of gusts stronger than 45 mph-- the Kittitas Valley,
Maryhill area, and portions of north central Oregon.

The deep upper low will travel into central ID Wednesday night
with precipitation tapering off or ending from the west. Lapse
rates will steepen on Wednesday as H5 temperatures lower to -28C,
therefore there will likely be graupel within the showers and
there is a slight chance (20%) of thunderstorms. Patterns like
this often result in cold core funnel clouds, so I cannot rule
that out as well. Scattered wrap around showers will continue in
Wallowa County Wednesday night. Wister/85

LONG TERM...Thursday through Monday...

Key Messages:

1. Breezy winds Friday and Saturday.

2. Rain showers return late in the week.

3. Above normal high temperatures late in the holiday weekend.

The extended period is characterized by an upper level trough
dropping into the Pacific Northwest from the British Columbia
coast late Friday into Saturday. This feature, coupled with a
passing cold front late Friday, will allow for a pressure gradient
to develop along the Cascades to promote elevated winds across
the east slopes of the Cascades, Simcoe Highlands, and the
Columbia Basin as gusts of 25 to 35 mph will be possible both
Friday and Saturday. Confidence in these wind gusts is moderate
(50-60%) as model guidance is not in full agreement in regards to
the upper level trough`s location. The ECMWF keeps the trough
slightly more north than the GFS, which would further tighten the
pressure gradient and produce wind gusts of 30 to 40 mph versus 20
to 30 mph with the GFS solution. The NBM suggests a 50% to 80%
chance of wind gusts of 39 mph or greater on Friday and a 55% to
85% chance of wind gusts of 39 mph or greater on Saturday over the
aforementioned areas, with the highest chances residing across
the Simcoe Highlands and the Kittitas Valley. Winds are expected
to slacken Sunday and Monday as an upper level ridge builds over
the region.

A departing system early in the period will keep lingering
precipitation occurring over Wallowa County through Thursday
afternoon before drying into the early evening hours. Elsewhere, a
transient upper level ridge will keep the Basin and Central Oregon
under mostly sunny skies and dry conditions. However, the
approaching trough that will bring elevated winds Friday and
Saturday will also bring rain showers along the Cascades late
Thursday before extending across much of the area overnight and
through Saturday morning. Guidance is in slightly better agreement
with the overall amount of rain, which is potentially a trace
along the Blue Mountain foothills and 0.10 to 0.20 of an inch at
elevation over the Cascades and Blues Friday and early Saturday.
There is a slight difference in rain arrival Friday evening, as
the GFS begins rain about 3 to 6 hour earlier than the ECWMF as
its track is further south. Confidence in these rain amounts is
moderate (40-50%) as the NBM pinpoints the best chances for rain
will be 5 PM Friday through 5 PM Saturday, with the northern Blue
Mountains incurring a 20% to 30% chance of 0.25 of an inch or more
rainfall and the foothills only receiving a 5% to 15% chance.
When dropping rain amounts to 0.10 of an inch or more, chances
bump up to 40% to 50% across the northern Blue Mountains and 20%
to 30% along the foothills. The trough exits to our east Saturday
evening to open the door to a developing upper level ridge,
bringing with it dry and warm conditions through the remainder of
the holiday weekend.

High temperatures will stay below normal through the week as
predominately northwest flow aloft keeps cooler air advecting into
the region. Over the latter half of the weekend, the departing
system gives way to a building upper level ridge that sets up
across the Pacific Northwest as an upper level low pressure
stalls in the Gulf of Alaska. This low pressure slowly drops south
into early next week, enhancing southwest flow aloft and
increasing high temperatures to above normal values by Sunday as
the trend continues into midweek as the system slowly approaches
the coast. Currently, high temperatures Tuesday and Wednesday are
forecast to be around 10 degrees above normal, but low confidence
(20-30%) resides with these values as model variability exists and
primarily associated with the location and strength of this upper
low pressure system.

Current confidence resides more with the ECWMF outcome, as 56% of
ensemble clusters on Friday and 68% of ensemble members on Saturday
align with more of a northern track of the trough. This hints at
the  expectation of higher wind gusts Friday and Saturday and a
later onset of showers late Friday. Thunderstorm chances over
northern eastern Wallowa County Friday and Saturday also are less
likely with the more ECMWF solution, which is indicated by only a
10% to 20% chance of Thunderstorms via the NBM. The clusters are
in more agreement when it comes to warming late in the period as a
result from enhanced southwest flow aloft, as 82% of clusters
show this flow initiating by Monday. 75

AVIATION...00Z TAFS...VFR conditions currently at all sites
through the period but with increasing and lowering clouds late
tonight through Tuesday and possible rain Tuesday afternoon at DLS
and YKM. Cigs will be lowering to 050-100ft AGL Tuesday
afternoon. Breezy winds (10-20kts) with gusts around 25kts through
midnight. Otherwise, winds will generally be less than 10 kts
overnight then increasing to 10-15kts Tuesday afternoon.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  41  67  45  57 /   0  30  90  60
ALW  46  70  49  60 /   0  30  90  80
PSC  48  71  51  65 /   0  30  70  50
YKM  41  67  42  62 /   0  50  30  20
HRI  46  70  48  64 /   0  40  70  40
ELN  45  63  42  59 /   0  60  40  30
RDM  32  65  42  54 /   0  30  60  20
LGD  35  67  44  53 /   0  10  90  80
GCD  33  67  44  54 /   0  10  90  70
DLS  46  64  49  61 /   0  60  60  20

&&

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

$$

EVENING UPDATE...86
SHORT TERM...85
LONG TERM....75
AVIATION...91