969
FXUS66 KPDT 190936
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
236 AM PDT Sun May 19 2024

.SHORT TERM...Today through Tuesday...We`re staring down a wet and
windy forecast as a train of upper-level lows threatens the forecast
area through the next week. With the northwesterly origin of these
systems, expect temps to remain at or below seasonal averages as
well, with some chilly overnight lows possible over the next couple
of nights.

Over the last couple of days, we`ve been under the influence of a
broad western Canadian low, whose southern flank has placed a jet
over the forecast area. Mountain showers and gusty winds have
resulted, as well as some weak bands of showers across the Basin,
but Sunday will mark the low`s departure, and much of the forecast
today looks drier and calmer wind-wise as a result. That being said,
we remain under a broad NW flow regime, meaning winds through the
Cascade Gaps will remain locally gusty, and showers may still
develop across our mountain zones, mainly through the Washington
Cascades and eastern mountains of Oregon. The airmass remains dry,
so PoPs are low (20-30%), but the environment does support weak
orographic shower development over the aforementioned areas.

As the flow behind the departing low turns more northerly, colder
air will advect in, making for a chilly Sunday night/Monday morning.
Overall looking like winds and cloud cover will be enough to
preclude subfreezing overnight lows across areas eligible for Freeze
Warnings, but temps may approach freezing overnight for the Grande
Ronde Valley and foothills of the Southern Blues in particular, as
far as these eligible zones are concerned. Monday also looks mostly
dry, but the trough axis of the departing low tilts toward our
forecast area just enough to invite instability across mainly the
eastern mountains. MUCAPE is supportive for storms, running around
400-600 J/Kg Monday afternoon, but moisture once again is a limiting
factor. PoPs remain on the lower end at around 20-40%, mainly for
Wallowa County and the eastern mountains of Oregon.

Finally, Tuesday marks the initial arrival of an upper-level low
almost due north from Canada. Precip chances look to envelop the
forecast area by early Tuesday evening as a result. Expecting mostly
rain showers, as the atmosphere will be cool given the northerly
approach of this system. Confidence is high (60-70%), however, that
the Tuesday into Wednesday period will be wet as a result of this
system. The arrival of this low will also welcome a return to windy
conditions, although guidance thus far does not suggest anything
headline worthy for now. Evans/74

.LONG TERM...An upper level low will move across the region
Wednesday bringing showers, mainly over the mountains and a chance
of thunderstorms. However, due to the proximity of the low, there
will even be rain chances over the lower elevations. By later
Wednesday into Wednesday night the low will have moved eastward
and out of the area and rain chances should be decreasing. Due to
the proximity and path of the low, there will be instability aloft
for thunderstorms, but being 4 days away, the exact details will
come into better focus as the day gets closer. Based on what is
shown in both the deterministic GFS and ECMWF, the NBM precip
fields look overdone on Thursday.

Thursday night should be dry as well in advance of the next low
moving southeast across the Pacific Northwest.  The GFS is stronger
and has a closed low, and also further south, while the ECMWF is
weaker, has more of an open trough and further north.  There will be
a trough and rain either way, but the exact position and strength of
this system will determine impacts as we go into the Memorial Day
weekend, and the models are showing considerable disagreement at
this time.

QPF with the Wednesday system looks decent, with around three-
quarters of an inch in the Blue Mountains and Wallowas and one-half
to three-quarters of an inch in along the crest of the Cascades.  In
the lower elevations, there will be considerably less.  Along the
Foothills of the Blue Mountains, generally around one-quarter inch,
and most other places just a few hundredths.  The ECMWF EFI shows an
area of 0.7 to 0.8 in the northern Blue Mountains for QPF on
Wednesday.

The second low looks to bring less precipitation, Generally up to
one-quarter inch in the mountains, with some locally higher amounts
and hundreds to around one tenth elsewhere.  However, it will
ultimately depend on the track and strength of the low.  The ECMWF
solution is supported by about 40 percent of the ensemble clusters as
we approach Saturday, and the ECMWF ensemble mean is in better
agreement with its deterministic run, giving more support to its
solution.  However, there is always natural variability in solutions
at this time scale.

Wednesday into Wednesday evening also looks to be breezy with the
trough moving through, especially across the lower Columbia Basin,
with wind gusts 25 to 35 mph, possibly as high as 40 mph.  The ECMWF
EFI keys in on a large portion of eastern Oregon with values of 0.7
to 0.8 during this time.

High temperatures will be 5 to 10 degrees below normal on Wednesday,
with the trough moving through, but will return closer to normal
Thursday, except central Oregon.  By Friday, most areas should be
close to normal.

&&

.AVIATION...12Z TAFS...High confidence (90-100%) in VFR conditions
through the period. Gusty winds to around 25 kts are expected to
redevelop at DLS and PDT and around 20 kts at BDN and RDM. All
winds should become less than 10 kts by this evening.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  63  39  69  42 /   0  10   0   0
ALW  66  43  72  47 /  10  10  10   0
PSC  70  47  77  50 /   0  10   0   0
YKM  67  41  75  45 /   0   0   0   0
HRI  69  43  75  48 /   0  10   0   0
ELN  63  40  71  44 /  10   0   0   0
RDM  56  31  65  36 /   0   0   0   0
LGD  57  36  64  38 /  10  10  20   0
GCD  57  34  64  37 /   0  10  10   0
DLS  65  45  72  48 /   0   0   0   0

&&

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

$$

SHORT TERM...74
LONG TERM....77
AVIATION...77