Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 241141
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
441 AM PDT Sun Mar 24 2024

.AVIATION...12z TAFs...Periods of MVFR conditions this morning,
mainly from showers ans lowered anticipated to impact KPDT, KALW,
and KPSC. Mention of VCSH continues across that similar area through
around 06z Monday. Otherwise, VFR conditions across area terminals,
with gusty afternoon wind expected at KDLS, KRDM, and KBDN.
Branham/76

&&

.PREV DISCUSSION... /issued 304 AM PDT Sun Mar 24 2024/

SHORT TERM...Today through Tuesday...Current radar and infrared
satellite imagery showing light to moderate returns along the Blue
Mountains and foothills traveling west-southwest to east-
northeast under partly cloudy to mostly cloudy skies. This is in
response to the upper level low pressure offshore pushing inland
along the southern Oregon coast overnight, and continuing to
travel south into northern California as it opens to a trough
later this morning. The trough will continue to drop south into
southern California this afternoon as a transient upper level
ridge moves onshore over the Pacific Northwest in its wake. The
weak ridge weakens as a shortwave rides overtop to keep
precipitation primarily over mountain zones on Monday before a
much stronger upper level ridge approaches and moves into the area
on Tuesday. These synoptic features will allow for conditions to
slowly dry through the afternoon and evening today before chances
bump back up Monday afternoon before waning Monday evening through
the day on Tuesday.

The primary weather concern resides with breezy winds across the
Eastern Gorge and Kittitas Valley each afternoon through Tuesday,
as a pressure gradient sets up along the Cascades as a result of
the incoming ridge features. The GFS, RAP, NAM, and SREF all
showcase a 3-4 mb pressure gradient between Portland and Spokane
this afternoon, with the GFS, NAM, and SREF showing a 4-5 mb
difference Monday afternoon and a 4.5-6 mb difference on Tuesday
afternoon. Wind gusts of between 25 and 35 mph out of the west
will be possible each day. Confidence in these wind values is
moderate to high (60-80%) as a 75-95% chance of wind gusts of 25
mph or greater is advertised by the NBM, and a 40-60% chance of
wind gusts of 30 mph or greater is suggested by the HREF this
afternoon. Both models hint at a better and more widespread
chance of these elevated gusts taking place on Tuesday, extending
across the Blue Mountains and foothills. The NBM highlights a
75-100% chance of wind gusts of 25 mph or greater with a 50-80%
chance of those gusts reaching 39 mph or greater over the
aforementioned areas.

The only other weather concern through Tuesday will be persistent
snowfall at higher elevations of the northern Blue Mountains over
the next 48 hours, as 3-7 inches is expected above 4500 feet.
However, confidence is quite low (20-40%) as guidance is not in
agreement. The NBM only shows a 5% chance of achieving 6 inches or
more snowfall over the next 48 hours over the northern Blues, but
the HREF suggests a 30-50% chance of 6 inches or more snowfall
over the next 24 hours. These chances bump up when the default
10:1 snow ratio that the HREF uses is considered, as forecast snow
ratios are between 11:1 and 13:1 (inches of snow:inches of
water/rain). Thus, a higher snow ratio would yield more snow per
amount of rain. In the end, wouldn`t be surprised if higher
elevations above 5000 or 5500 feet see 6 inches or more, but
expecting a steep gradient or decline as elevation is decreased to
4000 feet.

Measurable rainfall will stay at elevation over the Cascades,
Blues, and John-Day/Ochoco Highlands/Basin after today, with lower
elevations picking up less than 0.10 of an inch across the
Columbia Basin and 0.10-0.20 of an inch along the foothills of the
Blue Mountains today. These showers will primarily occur during
the morning and early afternoon period before waning and
congregating along our mountain zones this evening and into Monday
morning. Snow levels will be slowly increasing today, hovering
between 4000 and 4800 feet northwest to southeast, dropping to
3200-4200 feet Monday and to between 3200 and 4000 feet on
Tuesday. This slow decline in snow levels is a result of northwest
flow aloft initiating later today and extending through Tuesday
as multiple ridging features arrive from our west. This will keep
high temperatures 1-4 degrees below normal each day, with lows
actually 1-4 degrees above normal due to the lingering cloud cover
overnight. High temperatures will stay in the mid-50s to low 60s
as lows hover between the mid-30 to low 40s for lower elevations
through the period. 75

LONG TERM...Wednesday through Sunday...The forecast trend for
active conditions continues with most recent analysis, at least
through the work week. Wednesday will see the ridge axis sliding
further east out of the forecast area, with the next area of low
pressure sending moisture across the forecast area, and lingering
into Thursday. The initial push of moisture will see the most impact
over the Cascades where a high chance (80-100%) of snow/rain will
occur through Wednesday morning. The low lands will see a moderate
chance (40-60%) of mainly rain with snow above 4000 ft into
Wednesday morning. Probabilities for precipitation only increase
through the evening, with probabilities lowering by Thursday
morning.

This break will mainly occur as the area of low pressure advances
closer to the PacNW, with a low to moderate chance (20-50%) of
precipitation continuing through Thursday morning. Another push of
moisture later Thursday will bring a return of the moderate to high
chances (60-80%) for precipitation, especially over the higher
terrain. Conditions begin to improve on Friday as the area of low
pressure begins to dive south, though remnant moisture will remain
through Friday afternoon. Cluster analysis from this Wednesday-
Friday system has improved in recent analysis. Though variations
still exist on placement and intensity, the general idea and
time-line have seen increased agreement, bringing a bit more
confidence in this solution.

For the weekend, the aforementioned area of low pressure is expected
to dive south for California, taking the majority of the moisture
will it. This will allow for ridging to develop over the PacNW.
Drier conditions generally expected across the northern half of the
forecast area with a very low chance (5-20%) for any additional
precipitation. Areas south will see a slight chance (20-40%) for
lingering moisture. Sunday will see drier conditions with a very low
chance (5-15%) of precipitation expected across the area. The higher
end of that range will be across the higher terrain.

Near normal temperatures anticipated through Friday under the
influence of the low pressure. As ridging develops, warming
temperatures are favored, with locations seeing values 5-10 degrees
warmer than seasonal normals. Branham/76

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  51  36  53  38 /  60  40  20  30
ALW  53  38  55  40 /  80  20  30  40
PSC  59  38  60  41 /  30  20  10  10
YKM  60  33  58  33 /  30   0  10  10
HRI  58  38  58  40 /  40  30  10  20
ELN  56  33  54  34 /  20   0  10  10
RDM  51  31  50  30 /  10  10  10  20
LGD  47  33  48  35 /  60  30  40  50
GCD  49  33  48  32 /  40  40  30  50
DLS  59  41  55  42 /  20  10  20  20

&&

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

$$

SHORT TERM...75
LONG TERM....76
AVIATION...76


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