137
FXUS66 KPDT 301718
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1018 AM PDT Thu May 30 2024

Updated Aviation Discussion

.AVIATION...18Z TAFs...VFR conditions to prevail at all sites
through the period. Smoke impacts have lifted from sites RDM/BDN
this morning, though low confidence (15%) if smoke impacts will
return again late tonight. Winds will be light, less than 12kts,
at most sites through the period. The exceptions will be at site
DLS/RDM/BDN where winds of 12-15kts with gusts 20-25kts will be
possible this afternoon. Lawhorn/82

&&

.PREV DISCUSSION... /issued 419 AM PDT Thu May 30 2024/


SHORT TERM...Today through Saturday...

Bottom Line Up Front: (BLUF)

1.  Mountain rain dissipates early today.

2.  Dry and warm conditions to return tonight.

3.  Mountain showers return with breezy conditions Saturday.

Models are in excellent agreement with the upper level trough
shifting out of the region today leaving only the Puget Sound
Convergence slightly in place over the Cascades. This will allow for
lingering mountain showers which will dissipate early this morning
with less than 10% probabilities of 0.01 inches of rain.
Temperatures today will remain mild with over 65% of the raw
ensembles showing the foothills, central and north central OR to be
in the upper 60s to low 70s and the Basin and adjacent valleys in
the low to mid 70s. Winds will loosen up across the majority of the
region with the exception of the the Kittitas Valley seeing
sustained winds to 25% with 80% of the ensembles in agreement.

Models remain in excellent agreement with the leading edge of the
upper level ridge slipping in behind the trough. This will put the
area under dry and warming conditions through Friday night. EFI
shows temperatures at or slightly above climatological normal with
over 50% of the raw ensembles showing the majority of the area to be
in the mid to upper 70s with isolated locations in the Basin seeing
temperatures in the 80s. With the upper level high in place,
northwesterly flow will dominate through Friday which will in turn
settle the winds across the region with high confidence (80%) winds
will remain below 10 mph.

Lastly, the models remain in excellent agreement in showing another
upper level shortwave making its way into the PacNW Friday night.
Day 3 ensembles clusters show little variances with the system. Main
difference with the models are with the timing of the expected
precipitation. Clusters are more in favor of the GFS solution which
will bring the precipitation to the Cascades with raw ensembles
showing a 20-40% chance of 0.01 inches of rain. Guidance is also
showing yet another increase in the winds with surface pressure
gradients tightening along the Cascades. However, winds will be
around 25 mph with 60-80% of the ensembles in agreement. Ensembles
also shows the majority of the winds to be along the Simcoe
Highlands and through the Kittitas Valley. EFI shows temperatures to
remain at or slightly above normal with over 58% of the raw
ensembles showing the majority of the area to remian in the upper
70s to low 80s. Bennese/90

LONG TERM...Sunday through Thursday...

Key Messages:

1. Breezy afternoon winds, peaking on Monday.

2. Widespread rainfall expected Sunday and Monday.

3. Afternoon thunderstorms possible Sunday and Monday.

The extended period is characterized by an upper level trough and
associated frontal system that approaches the coast Sunday before
driving through the Pacific Northwest on Monday. An upper level
ridge then builds on Tuesday in the wake of the departing trough,
allowing for drying and warming Tuesday onward. The main weather
concerns in the period reside with the passing system Sunday and
Monday, which will bring breezy winds, afternoon thunderstorms, and
ample moisture to the area. Also of note, high temperatures will be
reaching above normal values beginning Wednesday, and approaching
low 90s for lower elevations of the Basin late in the week.

A transient upper level ridge will quickly pass through the area on
Sunday ahead of the approaching trough, which will tighten isobars
and produce increasing winds into Monday. The cold front, associated
with the upper level system, will allow a surface pressure gradient
to develop along the Cascades on Monday. This gradient is forecast
to hover around 10-13 mb between Portland and Spokane as advertised
by the GFS, which does meet the normal advisory criteria of 12 mb.
Elevated wind gusts of between 25-35 mph will be rather widespread,
but will be closer to between 35-50 mph over the Simcoe Highlands,
Kittitas Valley, northern Blue Mountain foothills, and along the
Lower Columbia Basin of Oregon. Further confidence in these winds is
gleamed from the NBM, which showcases an 80-90% chance over the
Simcoe Highlands, a 75-85% chance across the Kittitas Valley, a 60-
75% chance along the Lower Columbia Basin of Oregon, and a 40-50%
chance through the northern Blue Mountain foothills of experiencing
gusts of 47 mph or greater. Thus, there is moderate to high (70-90%)
confidence in advisory-level winds Monday afternoon over portions of
the area. Ensembles are also in agreement with rather tight isobars
on Monday, with members hinting at the gradient tightening even
further than the current forecast (34% versus 23%). Also, the timing
of arrival may also continue to trend slightly later as indicated by
33% of members.

Rain will begin along the Cascades Sunday afternoon as a warm front
nudges into the area ahead of the cold front, which will lead to
showers spilling into the Basin and over the Blues Sunday evening
into early Monday morning. Rain will continue through Monday morning
before slowly tapering off through the afternoon and early evening.
Showers will still linger across the Cascades and Blue Mountains
into Tuesday before only slight chances (15-25%) remain over the
Washington Cascades on Wednesday associated with upslope flow. Rain
amounts still look to peak between 5PM Sunday and 5PM Monday, but
may begin to nudge earlier as indicated by ensemble members
mentioned in the previous section. Rain amounts of 0.05-0.15 of an
inch are anticipated over lower elevations of the Basin and over
Central Oregon, 0.10-0.20 of an inch across the lower Columbia Basin
of Oregon, 0.25-0.50 of an inch along the northern Blue Mountain
foothills and the John Day-Ochoco Basin, and 0.40-0.80 of an inch at
elevation over the northern Blue Mountains and Elkhorns. Confidence
in these rain amounts are moderate to high (55-75%) as the NBM
suggests a 35-45% chance over lower elevations of the Basin, Lower
Columbia Basin of Oregon, and Central Oregon, a 40-50% chance over
the Yakima/Kittitas Valleys, and a 65-85% chance across the northern
Blue Mountains/foothills and Elkhorns of 0.25 of an inch of rain or
more. Confidence is further enhanced as a Atmospheric River (AR)
looks to be the culprit of moisture supply to this system as the
probability of Integrated Vapor Transport (IVT) greater than 250
kg/m*s is 90-100% Monday for inland regions east of the Cascades as
advertised by the GEFS. The ECMWF and GFS are also in agreement with
the amount of IVT between 600-700 kg/m*s on Monday, further
enhancing confidence in widespread rainfall and readily available
moisture Sunday evening through Monday evening.

The final concern associated with this passing system will be of the
potential for thunderstorm development Monday afternoon and evening
across the Blue Mountains, John Day-Ochoco Basin, and Wallowa
County. The passing cold front and terrain will help to enhance
lift, as surface based CAPE of 100-200 J/kg is forecast to be
present via the GFS and ECMWF. Low level shear of 35-45kts is also
expected, with the highest amounts located over eastern Wallowa
County. Low level lapse rates also look to peak later in the
afternoon/evening at between 7-8C/km. Thus, developing storms are
expected to stay discrete and sub-severe, but this will need to be
monitored as the event nears. 75


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  67  41  75  50 /   0   0   0   0
ALW  70  45  78  53 /   0   0   0   0
PSC  74  47  80  56 /   0   0   0   0
YKM  73  40  78  51 /   0   0   0   0
HRI  73  44  80  53 /   0   0   0   0
ELN  68  41  76  53 /   0   0   0   0
RDM  69  38  78  46 /   0   0   0   0
LGD  64  38  73  47 /   0   0   0   0
GCD  67  39  77  48 /   0   0   0   0
DLS  73  47  81  57 /   0   0   0   0

&&

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

$$

SHORT TERM...90
LONG TERM....75
AVIATION...82