557
FXUS66 KPDT 212212
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
312 PM PDT Tue May 21 2024

.SHORT TERM...Tonight through Thursday....Active mid-week weather
pattern is occurring as a Canadian low is currently sweeping a
warm front across the region, with stratiform rain overspreading
today followed by breezy winds and a chance at showers and
occasional thunderstorm tomorrow before clearing out in Friday.

The aforementioned precipitation overspreading the region is
expected to continue through the evening and overnight hours,
heaviest across the Blue Mountains. Probability of 24 hour
rainfall amounts shows high confidence (70-100%) that locations
across the foothills and into the mountains themselves will
receive 0.5 inch; increasing this threshold to 1.0 inch, there
remains moderate confidence (40-60%). Into Wednesday, the focus
shifts to then showers, thunderstorms, and breezy winds. Showers
will be a bit more scattered in nature than compared to the
widespread stratiform rain ongoing today, but with moderate to
high confidence still (60-90%). Instability has seen a noteworthy
jump, with a bit more widespread coverage of low amounts of CAPE
from the Ochoco-John Day Highlands through the Blues and Wallowas.
Still, with how weak parameters are, there`s low confidence
(10-30%) in thunderstorms, and any that do occur should just have
occasional lightning and perhaps some locally breezy winds, though
with cooler temperatures aloft the chance of graupel with an
isolated storm cannot be ruled out. On the subject of winds,
confidence is moderate to high (60-80%) in widespread wind gusts
of 30-50 mph, strongest across the lower Columbia Basin of Oregon.
A wind advisory for this region has been issued in response with
the expectation of gusts 45+ mph.

Finally, as this system cools our temperatures, snow levels
descend to between 4.5-5k feet. This should promote late season
mountain snow, heaviest across the Strawberry Mountains into the
Wallowas, and even a noteworthy amount falling in the Blue
Mountains. Heavy accumulations should be limited to the highest
peaks and crests of this region, but there is moderate confidence
(60%) in accumulations of 1-3 inches for some of the passes
including locations such as Santiam Pass and Tollgate.

Late Thursday into early Friday the low progresses eastwards out
of our region, with a chance of some light wrap around moisture
possible in the Wallowas but by Friday morning we should be dry
and cleared out with another system expected into the weekend.
Goatley/87


Key Messages:

1. Breezy winds Friday and Saturday.

2. Rain showers and isolated thunderstorms return late in the week.

3. Above normal high temperatures Sunday onward.

The extended period is characterized by an upper level low pressure
system dropping along the British Columbia coast before driving
through the Pacific Northwest early in the holiday weekend. This
synoptic feature, coupled with a passing cold front, will allow for
a pressure gradient to develop along the Cascades that will cause
breezy conditions to occur over the east slopes of the Cascades,
Simcoe Highlands, Lower Columbia Basin, foothills of the northern
Blue Mountains, and the Kittitas Valley Friday and Saturday -
peaking during the afternoon hours. Confidence in elevated winds is
high (90%), especially on Saturday as the ECMWF EFI highlights
unclimatologically high sustained winds and gusts across the
aforementioned areas. Thus, wind gusts of 25 to 35 mph on Friday and
30 to 40 mph on Saturday will be possible out of the west northwest.
Confidence in these wind gusts is moderate to high (60-80%) as the
NBM a 70-90% chance of 39 mph gusts across the Simcoe Highlands on
Friday before bumping up to a 75-95% chance of wind gusts of 47 mph
on Saturday. Saturday does look to be the windier day, which is also
advertised by the GFS and NAM as incurring a pressure gradient of 9-
10 mb between Portland and Spokane - just shy of the normal advisory
threshold of 12 mb. However, these forecast gradients may continue
to trend upward as the NBM showcases a 70-80% chance of 47 mph winds
on Saturday across the northern Blue Mountain foothills and Kittitas
Valley. Thus, there is a moderate (70%) chance of a Wind Advisory
being issued for the earlier mentioned areas on Saturday.

The upper level low and associated cold front arrival will also
bring showers along the Cascades Friday afternoon before extending
across the Blue Mountains and foothills Friday evening through
Saturday afternoon. Minimal rain amounts are expected along the
foothills (less than 0.05 of an inch), with only 0.10-0.20 of an
inch expected at elevation over the Cascades and Blues. The only
exception will be over Wallowa County, which should get 0.15-0.25 of
an inch. This additional moisture will also allow for the potential
for afternoon thunderstorms both Friday and Saturday, with a much
better chance on Saturday. Friday`s storm chances (10-15%) will
primarily stay confined over Wallowa county, extending south and
west over the Blue Mountains and John Day-Ochoco Basin on Saturday.
Storm probabilities will also bump up to a 15-25% chance as SBCAPE
increases from 50 J/kg to 150 J/kg and shear increasing from 20-30
knots Friday to 30-45 knots on Saturday. One thing to note, is that
these higher SBCAPE and shear values are primarily associated with
the GFS solution, which does drive the upper low further south
across the area - correlating to a better potential for
thunderstorms. At this time, 52% of ensembles on Friday and 60% on
Saturday align more with a shallower path of the upper level low as
advertised by the ECMWF. As such, it is expected that isolated,
discrete storm cells will have the potential for development as the
low tracks over mountainous terrain over the eastern mountains, but
will not exhibit severe characteristics in the form of large hail,
damaging winds, or tornadic activity.

The upper low pressure continues to depart to our east over the
latter half of the holiday weekend (Saturday evening onward), drying
conditions and clearing skies into the beginning of the workweek.
This is a result of an upper level ridge that builds across the
Pacific Northwest as an upper level low pressure lingers over the
Gulf of Alaska and slowly drops south. Guidance is in good agreement
of this feature as 86% of ensemble members show an upper level ridge
over the Pacific Northwest and southwest flow aloft on Monday. This
flow will advect warmer and drier air into the region to allow above
normal high temperatures to arrive Monday onward as highs will break
into the low to mid-80s across lower elevations of the Basin.
Confidence in reaching high temperatures of 80 degrees or more on
Monday is as followed: 72% for Pasco, 6t4% for Hermiston, 69% for
The Dalles, 55% for Yakima, 45% for Pendleton and Redmond, 44% for
Bend, and 42% for Walla Walla via the NBM. These probabilities ramp
up to between 80-90% on Tuesday as southwest flow aloft continues to
enhance. 75


&&

.AVIATION...18Z TAFS...All sites are currently VFR with
high clouds until CIGS decrease to MVFR conditions tonight for
KPDT/KRDM/KBDN/KALW before improving Wednesday. CIGs are expected to
drop to OVC020-030 this evening through the early morning hours.
Wind gusts will be also be elevated through this TAF period at 20-30
kts with sustained winds at 10-25kts along with light rain as the
front passes through the area. Feaster/97


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  44  56  42  67 / 100  70  50   0
ALW  47  59  45  68 / 100  70  40  10
PSC  50  64  48  74 /  90  50  20   0
YKM  40  64  41  74 /  40  30  10   0
HRI  47  62  45  72 / 100  50  30   0
ELN  40  59  43  70 /  40  30  10   0
RDM  42  54  31  66 /  70  30  10   0
LGD  43  52  39  61 / 100  80  60  10
GCD  43  53  37  61 / 100  80  80   0
DLS  47  61  46  70 /  70  40  10   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 5 AM to 5 PM PDT Wednesday for ORZ044-508.

WA...None.
&&

$$

SHORT TERM...87
LONG TERM....75
AVIATION...97