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000
FXUS66 KPDT 260344
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
844 PM PDT Thu Apr 25 2024

.UPDATE...

Key Messages:

1. Thunderstorm potential Friday afternoon.

2. Breezy afternoon winds over the weekend.

The upper level trough that is currently pushing onshore will
quickly drop through California as upper level ridging occurs to
our east. This will lead to the possibility of thunderstorm
development along the Blue Mountains and east during the afternoon
and early evening, as advertised by being included in the General
Thunderstorm (0 of 5) category via the Climate Prediction
Center`s Convective Outlook. The best chances will be over Grant,
southern Union, and Wallowa counties. The HREF highlights surface
CAPE values between 150-200 J/kg and 0-6km shear of 35-45kts,
which is enough forcing to expect some discrete cells to form.
However, lingering cloud cover and delayed mid-level moisture
arrival will lead to storm development peaking between 3 PM and 8
PM, with high confidence (80%) in sub-warning storm cells.

A building upper level ridge offshore will flatten as a shortwave
travels onshore on Saturday, tightening a pressure gradient along
the Cascades to allow for breezy winds to occur across the Simcoe
Highlands and Lower Columbia Basin as gusts of 25 to 35 mph out of
the west will be possible. Confidence in these values is high
(80%), as the NBM suggests a 70-90% chance of wind gusts of 30
mph or greater over the aforementioned areas. An approaching upper
level closed low off the British Columbia coast on Sunday will
further tighten isobars to provide even windier and more
widespread conditions across the Simcoe Highlands, east slopes of
the Cascades, Eastern Gorge, Lower Columbia Basin, northern Blue
Mountain foothills, and the Yakima/Kittitas Valleys. Highest gusts
are anticipated through the Simcoe Highlands, with gusts up to 40
mph. Elsewhere, gusts of 25-35 mph are expected out of the west.
The NBM showcases a 75-95% chance of 35 mph gusts or greater over
the Simcoe Highlands, and a 50-75% chance across the Lower
Columbia Basin and northern Blue Mountain foothills. 75


&&


AVIATION...00Z TAFS...VFR conditions currently at all sites, with
the exception of MVFR at KPDT due to reduced ceilings of BKN013
and visibilities of 5SM. These conditions will persist over the
next couple hours in response to the light rain occurring at the
sites. There is also light rain over KALW and KYKM, but will taper
off through this evening. Winds will also be slackening over
KPSC, KALW, KBDN, and KYKM as showers exit to our east. Ceilings
will stay BKN040-060 for all sites through the period, with
ceilings at KPDT improving through the evening. 75


PREV DISCUSSION... /issued 243 PM PDT Thu Apr 25 2024/

SHORT TERM...Today through Sunday...Current radar is showing the
leading edge of the upper level trough has begun to push along the
Cascades and spill over to the east slopes. Current observations
show 0.01 inches along the OR east slopes and 0.01-0.03 inches along
the WA east slopes. Current satellite shows mid and high level
clouds over the majority of the CWA. This will assist with keeping
temperatures more on the seasonal side. However, if there is a break
in the clouds this afternoon, daytime heating coupled with some
orographic lift and mid level moisture, the eastern mountains could
hear a rumble or two of thunder this afternoon.

Today through Friday night models are in relatively firm agreement
with the upper level trough moving slowly across the region. With it
there are elevated probabilities of some high mountains snow and low
elevation rain. Looking at the raw ensembles 6 hour potential for
0.05 inches of rain, there is and 80-95% chance along the Cascades
crests and along the east slopes of the Cascades through tonight
while  only a 5-15% chance along the eastern mountains, while the
lower elevations will see a 30-40% chance of 0.01-0.06 inches of
rain. Snow levels will decrease from over 5000 ft to near 4500 ft
allowing the crests of the Cascades and portions of the Wallows to
see an 80-90% probability of 0.5 inches along the Cascades and
higher at the crests while the Blues and Wallowas have 60-80%
probabilities for 0.5 inches as well. Along the OR Cascades through
Santiam Pass there is a 30% probability of up to 3 inches. Not
enough to warrant an advisory but still enough to say be cautious
while driving across. As for the temperatures, due to the increased
cloud cover, temperatures will remain near seasonable with the EFI
showing seasonable temperatures and the ensembles showing low to mid
60s along the lower elevations and high 50s elsewhere. With the
majority of the cloud cover currently over the eastern portion of
the CWA, there is a slight chance with low confidence (5-15%), that
the daytime heating coupled with a lifted index of above 6.5°C/km
and MUCAPEs of 150J/kg, the eastern mountains could here a rumble or
two this afternoon. CAMs models show these ingredients to uptick
Friday afternoon to MUCAPEs of over 200 J/kg, lifted index cresting
7.4°C/km, mid level moisture and southwest flow, isolated
thunderstorms could pop up over the eastern mountains Friday
afternoon with confidence slightly higher (15-25%).

Saturday the models show the upper level trough to have its axis
over the region with the backside of the trough over the Cascades
and the leading side over the eastern mountains. Guidance shows the
majority of the precipitation will linger along the Cascades as well
as the northern Blue Mountains and the Wallowas. With temperatures
above freezing overnight and snow levels above 5000ft, much of the
precipitation that will fall will be rain at pass levels and rain
snow mix along the crests with 20-40% chances of up to 0.10 inches
of rain. EFI continues to show the temperatures at seasonable with
temperatures increasing ever so slightly to the upper 60s to low 70s
through the lower elevations and mid to upper 50s elsewhere.

Lastly, winds through the period will be west to southwest with a
diurnally driven increase. Ensembles show high confidence (70-90%)
that sustained winds will be 10-20 mph through the Cascade Gaps and
along the lower Columbia Basin daily through Saturday and decreasing
overnight. Bennese/90

LONG TERM...Sunday through Thursday...The long term period is
expected to see unsettled weather as a trough will be over the area
through most of next week and a series of weak and moisture deprived
fronts bring light amounts of rain mainly to the mountains.
Temperatures will be generally near to slightly below normal for
late April. There are growing differences between model ensemble
members over time so while the overall pattern is clear, the finer
details are less certain. The Extreme Forecast Index indicates
little in the way of unusual weather aside from elevated winds
Sunday and Monday afternoons in the Columbia Basin (EFI = 0.75-0.80).

Models start out in good agreement on Sunday in having a strong
upper low off the northern British Columbia coast and a shallow
trough centered off the Pacific Northwest coast. A weak front and a
generally west to southwest flow into the Cascades will bring a 60-
80 percent of rain along the Cascade crest with amounts of about a
tenth of an inch or less. The northern portions of the eastern
mountains will have a 20-35 percent chance of very light rain. Snow
levels will be around 4500-5500 feet. The main concern will be winds
with the frontal passage and 15-30 mph westerly winds are expected
over most of the area in the afternoon. Temperatures will be
slightly warmer than Saturday with highs in the upper 50s to mid 60s
with 50s in the mountains.

For Monday, models are still in reasonable agreement though model
ensemble clusters show some differences. 70 percent of model
ensemble members have the upper low over Vancouver Island or southern
British Columbia though they have significant differences about the
strength of the closed low. The other 30 percent have the low
offshore near central British Columbia. They also indicate some form
of weak front crossing the area in the afternoon with a 40-60
percent chance of light rain in the Cascades and a 15-25 percent
chance in the eastern mountains. Amounts will be mainly just a few
hundredths of an inch. The front will again bring some breezy
westerly winds of 15-25 mph in the afternoon. Temperatures will be 3-
5 degrees cooler in the afternoon.

At this point, model ensemble members diverge significantly. On
Tuesday and Wednesday, 55 percent of the members have the upper low
and trough centered off of Vancouver Island and the other 45 percent
favor a shallower trough and low centered over Montana. The National
Blend of Models favors the solution with the trough off the coast,
though with pops of 20-40 percent in the Cascades and 15-25 percent
in the eastern mountains with the rest of the area dry. Rain amounts
will be minimal. The version with the trough offshore gives us a
southwest flow which pumps up a ridge off to our east, which will
warm temperatures a few degrees each day reaching the mid 60s to
lower 70s with upper 50s to mid 60s in the mountains on Wednesday.
It should be noted that the 25th percentile and 75th percentile of
temperatures have a broad range of 10-15 degrees, so forecasted
temperatures have low confidence.

For Thursday, 62 percent of the models continue to keep a strong
trough along the Pacific Northwest coast and a strong ridge over the
Midwest with a southwest flow over our area while the others show a
weak trough developing along the coast or just ashore and a weaker
ridge over the midwest. Both solutions favor rain, so have a 40-60
percent chance of rain over the mountains and a 15-30 percent chance
over the lower elevations. The mountains will get up to a tenth of
an inch of rain while amounts will be very light in the lower
elevations. The trough and greater precipitation and cloudiness will
cool temperatures to the 60s with 50s in the mountains.
Temperature spread remains wide and confidence remains low. Perry/83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  42  61  42  62 /  80  20   0   0
ALW  47  65  45  65 /  80  20  10   0
PSC  50  68  46  69 /  50  10   0   0
YKM  42  64  40  66 /  30  20   0   0
HRI  45  67  45  67 /  60  10   0   0
ELN  40  62  40  62 /  50  30  10   0
RDM  38  54  34  58 /  20  20   0   0
LGD  41  57  39  58 /  90  40  10   0
GCD  40  56  37  59 /  90  50  20   0
DLS  47  62  46  61 /  60  30   0   0

&&

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

$$

UPDATE...75
SHORT TERM...90
LONG TERM....83
AVIATION...75