595
FXUS66 KPDT 202119
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
219 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...18Z TAFS (previous discussion)...VFR conditions
currently at all sites, which will stay the course through the
period. Breezy winds are occurring at KDLS, with high confidence
(90%) of gusts between 25 and 30 kts will extend through midnight.
Winds will also increase for KBDN/KRDM/KYKM late this afternoon
and extend through the evening with gusts of 20 to 25 kts likely.
Winds of 10 kts or less will occur at all other sites. Increasing
cloud cover across all sites and lowered ceilings for KDLS toward
the end of the period as a system approaches the region. 75


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  41  66  45  57 /   0  40  90  60
ALW  46  69  49  60 /   0  40  90  80
PSC  48  71  51  65 /   0  30  70  50
YKM  42  66  42  62 /   0  40  30  20
HRI  46  69  48  64 /   0  40  70  40
ELN  45  63  42  59 /   0  40  40  30
RDM  33  65  42  54 /   0  30  60  20
LGD  36  66  44  53 /   0  20  90  80
GCD  35  66  44  54 /   0  20  90  70
DLS  46  63  49  61 /   0  50  60  20

&&

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

$$

SHORT TERM...85
LONG TERM....75
AVIATION...75