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000
FXUS66 KPDT 241003
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
303 AM PDT Wed Apr 24 2024

.SHORT TERM...Today through Friday Night...

Key Messages:

1. Breezy winds this afternoon and evening.

2. Widespread rain returning Thursday afternoon through Friday.

3. Thunderstorm potential Friday afternoon and evening.

Current radar and infrared satellite imagery showing spotty, light
returns passing through eastern Grant and southern Union counties as
mid-level clouds begin to push in from the northwest. This is in
response to a weak upper level shortwave and associated cold front
that will be passing through the area later today. These features
will attribute to a pressure gradient developing across the
Cascades this afternoon and evening to allow for breezy winds to
develop along the Eastern Gorge, Simcoe Highlands, northern Blue
Mountain foothills, and the Kittitas Valley. Wind directions will
be out of the west through the Basin and out of the northwest
over the Kittitas and Yakima Valleys. Sustained winds of 15-25 mph
with gusts of 25-35 mph will be possible, with gusts over exposed
areas at higher terrain in the Simcoe Highlands and the Kittitas
Valley nearing 40 mph. Confidence in these wind values is high
(80%) as the NBM suggests a 60-80% chance of wind gusts of 25 mph
or above over the aforementioned areas, dropping to a 30-50%
chance when increasing gusts to 30 mph or greater. When bumping up
to 35 mph gusts, the only spots that are highlighted include
areas around Slacker Butte and eastern Kittitas County with a
20-40% chance. The GFS, NAM, and SREF all showcase winds peaking
between 11 AM and 5 PM, with a pressure gradient range of 6.9 mb
and 8 mb between Portland and Spokane. These values provide
further confidence in the previously mentioned wind values, and
that these winds will stay below advisory level.

A stronger upper level shortwave and associated cold front
approaches and passes through the area late Thursday afternoon and
evening, bringing a return to showers across the area as it taps
into subtropical moisture. Rain chances (50-80%) will begin to
materialize along the Cascade crest early Thursday morning before
pushing into the east slopes by late morning, and across the
entire region through the afternoon and early evening. Snow levels
will stay consistent Thursday and Friday between 5000-6000 feet
northwest to southeast. This will keep snowfall at higher
elevations of the Cascades and Elkhorns, and accumulations below 2
inches both days. Rain amounts also stay similar each day, with
Friday having a slightly better potential for rain to occur at
lower elevations of the Basin. Rain amounts will be highest over
our mountain zones (0.20-0.40 of an inch), with 0.10-0.20 of an
inch through the John-Day Basin, and less than 0.10 of an inch
across the northern Blue Mountain foothills and Central Oregon.
Similar rain amounts are anticipated on Friday, but a better
chance of experiencing a Trace to 0.02 of an inch exists over the
Tri-Cities area and the Yakima Valley.

A reinforcing upper level shortwave arrives along the coast late
Thursday into Friday morning, digging south into central and
southern California through the day. This feature will tap into
additional moisture to improve lower elevation rainfall on Friday,
and will also erode the backside of a developing upper level ridge
located over western Montana. This will lead to the potential for
thunderstorms to develop through the afternoon and evening on Friday
across the Blue Mountains and east. The best chances will be over
Grant, eastern Crook, southern Union, and southern Wallowa counties
as these areas are included in the General Thunderstorm category (0
of 5) of the Climate Prediction Center`s Severe Weather Outlook.
Currently, confidence is low (20-30%) for developing thunderstorms
due to surface CAPE of 50-100 J/Kg and 0-6km shear of 20-30kts.
These parameters indicate a slight potential for discrete storm
cells to form over the aforementioned areas, with the main concern
of small hail and breezy winds as indicated by forecast soundings.

Temperatures will drop with the arrival of stronger systems Thursday
and Friday as near to below normal high temperatures are
anticipated. Afternoon temperatures will peak in the upper 60s to
low 70s across the Basin today, dropping into the low to mid-60s
Thursday and Friday. Overnight lows will stay above normal as
additional cloud cover and increased moisture arrive with both
systems. Morning low temperatures will drop into the mid-to upper
40s across the Basin on both Thursday and Friday. 75


.LONG TERM...Saturday through Wednesday...Guidance continues to
depict a progressive pattern through the weekend before bringing
in a broad low pressure system from the Gulf of Alaska that could
impact our weather for much of the next week. The progressive
nature of the synoptic pattern makes for timing differences across
ensemble members, but agreement does exist in depicting this
broad low next week across the majority of ensemble members.

The start of the weekend will see a low cutting off and sinking down
towards the Four Corners region, leaving us under a belt of zonal
flow. Such a pattern may yield light shower activity across the
Cascade Crests (confidence 40-50%), but snow levels will remain well
above pass level, and QPF guidance is on the lower end for Saturday.
This starts to shift heading into Sunday, as the Gulf of Alaska low
starts to encroach upon the forecast area, amplifying the flow aloft
and making for stronger chances for precip, at least across the
Cascades. Colder air will start to seep into the region, albeit SW
flow will start to dominate ahead of the low, which will preclude
lower snow levels until the low really starts to push inland, likely
more toward the middle of next work week.

As mentioned earlier, the progressive pattern between Friday`s low
and the next oncoming system makes for timing differences across
models. The relatively torpid movement of the Gulf of Alaska low
also introduces complications: some ensembles suggest a more
prolonged period of amplified zonal flow, which would yield a wetter
forecast for the Cascades while the lee side into the Basin remains
dry, while others move the low in a bit quicker and make for more SW
flow aloft, which could yield better chances for area-wide precip.
Confidence is on the lower end for the forecast late Sunday onward
as a result (30%), but the general picture ensembles paint is that
of a cool and wet forecast heading into the last full week of April.
Evans/74


&&

.AVIATION...12z TAFs...VFR conditions expected. Winds will pick up
during the afternoon across many sites, primarily at DLS and PDT,
where gusts out of the W/NW could approach 25 kts at times.
Elsewhere, expect W/NW winds around 10-15 kts, with clouds building
throughout the day, starting with sct-bkn high clouds early,
becoming bkn at most sites around 10-15 kft by the late afternoon
and early evening. Evans/74

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  66  42  61  45 /   0   0  20  60
ALW  69  46  65  48 /  10   0  20  80
PSC  72  47  67  50 /   0   0  20  30
YKM  69  42  61  42 /  10  10  50  20
HRI  71  45  65  47 /   0   0  20  40
ELN  62  41  59  42 /  10   0  50  40
RDM  63  36  55  39 /   0   0  50  50
LGD  65  39  60  43 /   0  10  20  90
GCD  68  38  61  41 /   0   0  30  90
DLS  68  47  59  48 /  10  10  70  60

&&

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

$$

SHORT TERM...75
LONG TERM....74
AVIATION...74