195
FXUS66 KPDT 060518
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1018 PM PDT Sun May 5 2024

Updated Aviation Discussion

.AVIATION...06z TAFs...Conditions are expected to improve to VFR
across all sites overnight as rain tapers off and winds start to
decrease. Some gradual clearing will take place across some sites,
with cigs becoming sct-bkn at the mid-levels, before clouds build
back in during the day Monday and winds increase once again across
all sites. Scattered showers are expected to develop in the
afternoon as well, however confidence is low as to how much of an
impact these showers will have on conditions, and whether or not
showers impact sites at all, especially outside of central Oregon
and PDT/ALW. Winds expected to be W/NW with gusts as high as 30
kts for sites such as PDT and DLS. Shower chances will die down by
late afternoon tomorrow, and winds will finally slacken for good
by nightfall as well. Evans/74

&&

.PREV DISCUSSION... /issued 817 PM PDT Sun May 5 2024/

EVENING UPDATE...The main band of rain over the forecast area is
beginning to dissipate and shift eastward into the mountains this
evening as a deep low pressure system moves out of the region.
Orographic precip and windy conditions will persist as the
forecast area remains under the influence of an elongated,
negatively tilted trough centered over the Gulf of Alaska, but
most of our population centers should expect dry conditions and
gradually subsiding winds overnight. Broad NW flow aloft will then
dominate overhead during the day Monday, allowing for more high
mountain snow over the Blues and Oregon Cascades, where Winter
Weather Advisories are in effect. Convective showers may also
develop tomorrow afternoon across the Basin and foothills of the
Blues with the trough axis overhead, but shower coverage is
expected to be scattered in nature.

The ongoing Wind Advisory across the Oregon Basin and adjacent
areas will be allowed to expire at 11 PM tonight, and will likely
cancel the WWA for the Southern Blues early as precip is already
subsiding and is expected to cease altogether before its
expiration time at 2 AM. Otherwise all other headlines remain in
effect. Changes to the forecast were minimal, and were focused
mainly on updating winds and PoPs based on latest HREF and ongoing
trends. Evans/74

PREV DISCUSSION... /issued 436 PM PDT Sun May 5 2024/

SHORT TERM...Tonight through Tuesday night...Active weather
across the forecast area continues this afternoon as the
deformation band over the Basin and Blue Mountain foothills
gradually weakens and shifts back east as the upper-level low over
southeast Oregon begins to progress southeastward into the Great
Basin. Flow aloft will remain westerly to northwesterly,
supportive of upslope precipitation for much of the eastern
mountains and Cascade crest. Have issued Winter Weather
Advisories for the Cascade crest 09Z Monday until 06Z Tuesday for
areas above 4500 feet. While afternoon temperatures will not be
supportive of significant accumulation on Monday, overnight
periods will see more in the way of accumulation with 4-8" of
snow, locally higher, forecast for areas 4500-5000 ft. NBM
probabilities are supportive of an advisory; 24-hr probabilities
sit at 45-60% chance of 6" or greater, and 80-95% chance of 4" or
greater through 12Z Tuesday. Over the southern Blues and Ochoco-
John Day Highlands, precipitation associated with the deformation
band will taper off this evening and overnight with existing
Winter Weather Advisories slated to drop off by 06-09Z Monday.
Expect an additional 1-4" of snow, heaviest above 4500 feet. With
regard to the breezy to windy westerly winds, 12Z HREF guidance
suggests winds will drop below advisory criteria later this
evening into tonight so have opted to keep them as is.

Monday will see low chances (25% or less) of thunderstorms across
the eastern mountains and foothills. Forecast mid-level lapse
rates of 7-7.5 C/km and instability of less than 500 J/kg of
surface-based CAPE will support shallow convection with a low-
end lightning threat. Winds will remain breezy to windy, but
confidence in widespread advisory-level winds or wind gusts was
too low (generally <40% chance) to issue any new advisories.

Tuesday afternoon, flow aloft will turn northwesterly then
northerly as the upper-level low over the High Plains retrogrades
and an upper-level ridge builds offshore. Will see some showers
over the eastern mountains, and perhaps along the Cascade crest,
but confidence in any thunder is very low (<10% chance).
Plunkett/86

LONG TERM...Wednesday through Sunday...The long term period will
see a ridge building out in the eastern Pacific early on and
remaining in control of our weather through next weekend. This
will give us a period of mostly sunny and dry weather with the
main forecasting challenge being just how warm temperatures will
get by next Saturday and Sunday. The Extreme Forecast Index
indicates little in the way of unusual weather aside from above
normal temperatures next weekend.

Models start out in good agreement Wednesday with a ridge along
the west coast and a large upper low centered over South Dakota.
Circulation around the two features will send a north to
northeasterly flow into our area. Skies will be mostly clear aside
from some lingering clouds over the eastern mountains.
Temperatures will be warming 4 to 8 degrees from Tuesday with
highs in the 60s with a few lower 70s in the Washington Columbia
Basin and in the 50s in the mountains. Winds will be generally
light though pressure gradients between the ridge and the low to
our east will create 15 to 20 mph northwest winds in the Kittitas
Valley.

On Thursday, models show the upper low splitting with most of the
energy moving to the Great Lakes while a remnant low dives
southwestward into Utah or eastern Nevada. This in turn allows the
upper ridge to build into British Columbia and northern Washington.
Model ensemble members have some differences as to the exact
locations of the low and the ridge axis, but they make little
difference for our area. With the ridge overhead, dry and sunny
weather continues with temperatures warming to the 70s in the
Columbia Basin and in the 60s elsewhere. Pressure gradients
between the ridge and the low will develop breezy northeast
winds of 10 to 20 mph in the afternoon.

Friday sees the upper low weakening further and model clusters
show that most of the ECMWF and Canadian members favor a more
southerly position over southern California and Arizona while GFS
favors the low being over northern Nevada and California. While we
continue to have sunny and dry weather, the GFS solution is
several degrees cooler than the other two models. The NBM favors
the warmer temperatures and have temperatures in the mid 70s to
lower 80s with mid 60s to lower 70s in the mountains.

Saturday and Sunday sees the upper low dissipating and moving east
though 64 percent of the ECMWF and 20 percent of the other models
keep a lingering circulation over northern Arizona and central
California on Saturday. All of the models keep a broadened ridge
over the west with the axis overhead on Saturday. On Sunday, just
a broad flatter ridge remains with 77 percent keeping the ridge
axis directly over Oregon and Washington while the other 23
percent have the ridge centered to our east over Montana. Dry and
fair weather is expected with temperatures continuing to warm
though with more differences between the model ensemble members.
The best compromise yields temperatures 10 to 15 degrees above
normal both days with highs in the 80s in the lower elevations and
in the 70s in the mountains. However, ensemble probabilities show
a 25 to 50 percent chance of hitting 90 in the Columbia Basin and
Yakima Valley while central and north central Oregon have a 40 to
70 percent chance of hitting 80 degrees each day. Perry/83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  38  56  38  55 /  90  80  40  20
ALW  43  59  40  58 / 100  80  40  30
PSC  46  64  44  65 /  60  40  10  10
YKM  37  62  37  63 /  10  30   0   0
HRI  42  62  42  62 /  60  50  10  10
ELN  39  58  37  57 /  20  30  10   0
RDM  32  52  30  50 /  20  70  40   0
LGD  37  54  35  49 / 100  90  70  50
GCD  33  51  32  46 /  90  90  90  50
DLS  45  58  42  59 /  50  70  20   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 11 PM PDT this evening for ORZ041-044-507-
     508-510.

     Winter Weather Advisory until 5 AM PDT Tuesday for ORZ502.

     Winter Weather Advisory until 2 AM PDT Monday for ORZ503.

     Winter Weather Advisory until 11 PM PDT this evening for ORZ506.

     Winter Weather Advisory from 2 AM to 11 PM PDT Monday for ORZ509.

WA...Wind Advisory until 11 PM PDT this evening for WAZ024-026-521.

     Winter Weather Advisory until 5 AM PDT Tuesday for WAZ030.

&&

$$

SHORT TERM...74
LONG TERM....83
AVIATION...74