910
FXUS66 KPDT 030718
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1218 AM PDT Sun Jun 2 2024

Updated Aviation Discussion

.AVIATION...Mostly VFR with occasional MVFR conditions expected
overnight as precipitation rolls across the region. Primary
impacts would be lowered CIGs just in the MVFR category, or
reduced visibilities in showers. Otherwise, VFR conditions
expected, continuing through the rest of the period. CIGs will
remain between around 3-5k overnight, but should become BKN around
10k after the bulk of the precipitation has exited by late this
morning. Winds remain breezy overnight, but become significantly
gustier by the mid to late morning hours. By 18Z, expect
widespread gusts of 30-40 knots, highest DLS/PDT/PSC, with some
gusts potentially even reaching 45-50 knots during the strongest
winds. Goatley/87


&&

.PREV DISCUSSION... /issued 815 PM PDT Sun Jun 2 2024/

UPDATE...
For the evening update, made some adjustments to pops based on
latest radar trends and model guidance. These trends suggest a
lull in precipitation over much of the Basin for at least a few
hours before more rain moves in. Therefore have lowered pops in
these areas. Otherwise, with the cloudy sky and moist airmass,
temperatures are not expected to fall much, so in many locations
increased overnight lows. However, the overall forecast rationale
remains the same.

PREV DISCUSSION... /issued 241 PM PDT Sun Jun 2 2024/

SHORT TERM...Tonight through Tuesday...Satellite and radar
imagery this afternoon are showing much of the western half of
OR/WA under light to moderate rainfall, while radar returns are
beginning to fill in across the eastern halves of the states.
Surface observations are showing some of this precip making it to
the ground along the Cascade east slopes and into portions of
Kittitas/Yakima valleys.

The precipitation spreading across the PacNW this afternoon is
associated with a pair of frontal boundaries that are coupled with
an upper level trough that is currently off the coast of central
BC. The leading warm front of this system will continue to spread
bands of rainfall across the intermountain PacNW through tonight,
with the cold front following behind late tonight through tomorrow
morning with rain showers. These frontal systems are being
supported by a moderate Atmospheric River, in which ECMWF and GFS
ensemble means show a Integrated Vapor Transport (IVT) of between
500-750 kg/m*s, which translates to PWATS between 1 to 1.25 inches
across the PacNW. The AR will be aimed at the PacNW through
tomorrow late tomorrow morning, resulting in peak rainfall
amounts occurring through this time period. Through tomorrow
morning, the Cascade crest and upper slopes, the Northern Blues,
and the Wallowas can anticipate rainfall accumulations between
1.25 to 2 inches; 0.5-1.25 inches across portions of the Southern
Blues, Simcoe Highlands, Wallowa county and the WA Blue mountain
foothills; 0.25 to 0.5 inches OR Blue Mountain foothills, Grande
Ronde valley, and Ochoco-John Day Highlands; 0.1 to 0.25 across
the eastern half of the Columbia Basin and Horse Heaven Hills; and
up to 0.1 inches across central and north central OR, western
half of the Columbia Basin, and the Yakima/Kittitas valleys. As
the trough drops into the PacNW and the cold front exits to the
east tomorrow, shower activity will mainly be confined along the
Cascade crest, with mainly light showers across the eastern
mountains. There will also be isolated thunderstorm chances across
the Strawberries, Blues, and Wallowas with a heavy downpour or
two possible in the late morning through the afternoon tomorrow.

Windy conditions will also be present through Monday as the cold
front moves across the intermountain PacNW. The frontal passage
and the upper trough approach will tighten the cross-Cascade
pressure gradients, producing windy conditions through the gaps
and into the lower elevations. Wind gusts between 45-55 mph and
sustained winds between 25-35 mph will impact the Basin, eastern
Gorge, Simcoe Highlands, north central OR, Blue Mtn foothills, and
the Yakima/Kittitas valleys beginning mid morning and persisting
into the evening. Wind advisories have been issued for these
areas. Otherwise, sustained winds across the remaining lower
elevation areas will increase to 20-25mph and gusts up to 40mph
tomorrow.

Winds and shower activity will decrease across the forecast area
Monday evening as the upper trough exits into ID and western MT.
However, a moist westerly flow aloft and a shortwave passage
Tuesday will continue to produce light rain showers across the
Cascade crest. Locally breezy to breezy winds will redevelop
Tuesday as the shortwave tightens cross-Cascade pressure gradient
once more, however winds will only increase to 15-25mph and gusts
up to 40mph in the lower elevations.

While widespread impacts from the increased rainfall across the
region is not expected, the heavy rainfall and subsequent snow
melt along the Cascades will lead to rises through area rivers and
streams. With how dry it`s been, however, many of these
rivers/streams are forecast to stay will within bankfull...except
for the Naches River near Naches and Cliffdell where river
forecasts put river levels just over bankfull. Outside of main
stem rivers/streams, rapid rises may be possible on smaller
streams and creeks during the period of heaviest precipitation,
especially those originating out of the Wallowa mountains.
Lawhorn/82

LONG TERM...Wednesday through Sunday...Models are in good agreement
about having a ridge over the western CONUS through most of the long
term period though differences about the location of the ridge axis
and the strength of the ridge grow increasingly over time and these
differences make big differences to the forecast of our area. Model
ensemble member clusters start out Wednesday with unanimous agreement
in having the ridge axis along the Washington/Oregon/Idaho border
and just some minor differences as to the strength of the ridge. On
Thursday, 38 percent of the members (mainly from the ECMWF) keep the
ridge axis stationary from Wednesday while the rest of the model
ensemble members move the ridge axis east over Idaho and Montana
with a southwesterly flow developing over our area. On Friday, 52
percent of the members (again mostly the ECMWF) develop a negatively
tilted ridge with the ridge axis extending from northern Nevada to
the British Columbia coast while the other model clusters keep the
ridge axis over the Rockies and develop a stronger southwesterly
flow over our area. Saturday is very similar to Friday though the
ECMWF led clusters and the GFS/Canadian led clusters develop
differences as to whether the ridge will be strong or very strong.
The GFS/Canadian led clusters also have a trough about 800-1000
miles offshore approaching the area which maintains the
southwesterly flow aloft and nudges the ridge slightly further east.
By Sunday, differences become much larger. 66 percent of the model
clusters (ECMWF with some support for the GFS) have the trough
offshore weakening into a weak closed low with a ridge axis over
eastern Oregon and Washington while 23 percent have a stronger
trough still offshore with a strong ridge over the Rockies. The
remaining 11 percent have a deep trough at the coast with a weaker
ridge over the Rockies.

The Extreme Forecast Index shows no significant weather Wednesday
and just some signals for unusual heat (0.7 - 0.8) over the eastern
mountains and Oregon Cascades. Saturday and Sunday, the heat values
rise to 0.6 to 0.9 with the higher values mainly over Oregon. There
are also some shift of tails indicating that the uncertainty of the
ridge location and strength may make more extreme temperatures
possible. Additionally next weekend, CAPE values rise to 0.6 - 0.9
from the Oregon Cascades over the eastern mountains and this will
suggest afternoon thunderstorms on those days. Overall forecast
confidence starts out good and becomes below normal by next weekend.

Wednesday will be a dry and sunny day with highs rising 5 to 8
degrees to the mid 70s to lower 80s with mid 60s to mid 70s in the
mountains. Thursday will be similar with high temperatures rising 4
to 7 more degrees to the mid 70s to mid 80s. A few late afternoon
showers and thunderstorms will be possible over the Ochoco-John Day
Highlands near Seneca.

On Friday, temperatures rise another 4 to 7 degrees with highs in
the upper 80s to mid 90s in the lower elevations and in the mid 70s
to mid 80s in the mountains. The NBM has the Columbia Basin and
Columbia Gorge at a 70-85 percent chance of reaching 90 degrees, the
Yakima Valley at a 64 percent chance and the Blue Mountain Foothills
at a 40-50 percent chance. Late afternoon thunderstorms return to
the Ochoco-John Day Highlands with a little greater coverage and
extend into the John Day area.

Saturday warms another 2-4 degrees according to the NBM with highs
of 88-98 in the lower elevations and in the mid 70s to mid 80s in
the mountains. The NBM also shows the Columbia Basin with an 85-95
percent chance of reaching 90 degrees, the Blue Mountain Foothills a
65-75 percent chance and a 45-55 percent in central Oregon. There is
also a 30-45 percent chance of reaching 100 degrees from Hermiston
north to the Tri-Cities. However, the NBM also shows an increasing
spread of 5 to 8 degrees between the 25th and 75th high temperature
percentiles, reflecting the growing uncertainty in the forecast.
With the increasing heat, there will be increasing instability and a
slight chance to chance of afternoon showers over the mountains and
perhaps the Blue Mountain Foothills. Have limited mention of
thunderstorms to the central Oregon Cascades and the Ochoco-John day
Highlands northeast into Wallowa County.

Temperatures cool about 5 degrees on Sunday to the mid 80s to mid
90s with mid 70s to lower 80s in the mountains. The NBM spread
between the 25th to 75th percentiles increases to as much as 10
degrees showing the increasing uncertainty in the forecast. Once
again have a slight chance to chance of showers in the Cascades,
eastern mountains and Blue Mountain Foothills with a slight chance
of thunderstorms limited to the eastern mountains. Perry/83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  54  64  48  72 /  90  50  10  10
ALW  57  67  50  72 /  90  80  10  20
PSC  58  71  54  77 /  80  60  10  10
YKM  51  67  47  73 /  60  20  10  20
HRI  59  69  51  76 /  80  30  10   0
ELN  48  62  44  69 /  70  40  10  30
RDM  55  62  45  73 /  60  20  10   0
LGD  52  62  44  72 /  80  80  10  10
GCD  53  65  45  76 /  90  70   0   0
DLS  57  66  52  70 /  90  30  10  20

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 8 AM to 11 PM PDT Monday for ORZ041-044-507-
     508-510.

WA...Wind Advisory from 8 AM to 11 PM PDT Monday for WAZ024-026>029-
     521.

&&

$$

SHORT TERM...77
LONG TERM....83
AVIATION...87