Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 252143
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
243 PM PDT Mon Mar 25 2024

.SHORT TERM...Tonight through Wednesday...Radar and satellite
imagery this afternoon shows light showers continuing across the
west slopes and the crest of the Cascades, while light showers
are also beginning to fire off across the eastern mountains and
hills/elevated terrain in the Columbia Basin. This shower activity
is associated with a weak upper shortwave in northwest flow
aloft, as well as a weakening cold front at the surface. Light
snow showers will continue across the eastern mountains and
Cascades tonight as the shortwave tracks overhead, with snow
accumulations mainly between 1 to 3 inches. In the northern Blues
and along the OR Cascade crest, however, short-term hires
guidance indicates that upslope enhancement will result in
chances for heavier snow showers, with 50-70% chances of snow
accumulations of 5 inches by late tomorrow morning. Tightened
pressure gradients this afternoon and overnight will also result
in breezy west winds continuing across the OR Columbia Basin and
through the Cascade gaps.

Tomorrow, northwest flow aloft will prevail across the forecast
area through the afternoon, resulting in persistent but light
snow flurries across the mountains. Breezy winds overnight will
continue and strengthen again through tomorrow morning as surface
pressure gradients remain tightened. While gusts will generally be
between 20-35mph, stronger gusts of 35-45mpg will be possible
through the Kittitas valley and out in the OR Columbia Basin
between Hermiston and Lexington. By the late afternoon, a brief
period of dry conditions will develop as a transient upper ridge
pushes east across the PacNW, which will also lead to winds
decreasing area wide through the evening.

Early Wednesday, the upper ridge will be forced east into the
Northern Rockies by a deep low approaching from the northeast
Pacific. Light rain/snow showers will begin developing across the
Cascades by the morning as a small warm front pushes ahead. By the
late morning, this warm front will be overrun by a more potent
cold front boundary, which will push rain and high mountain snow
showers across the forecast area throughout the daylight hours
Wednesday. The Cascade crest, Wallowas, and the interior northern
Blues will see a 60-80% chance of exceeding 0.5 inches of QPF by
Wednesday night; meanwhile the remainder of the eastern mountains
will see a 55-75% chance of exceeding 0.25 inches of QPF; in the
lower elevations, rain shadowing will limit the rain
accumulations, with only a 25-50% chance of exceeding 0.15 inches
across much of the area, though the Blue mountain foothills will
see a 50-75% chance of the same amount. As for snow, a northwest
to southeast snow level gradient between 4.5kft to 5.5kft will lay
across the forecast area when the heaviest precipitation is
occurring. This will result in very little snow accumulation
below 5kft, with chances of 3 inches of snow accumulations only
around 25% in the OR Cascade crest, and less than 25% at pass
level in the WA Cascades; while the NBM shows near zero chances
for 3 inches of snow in the northern Blues and eastern mountains,
1 to 2 inches is expected Wednesday night when snow levels lower
behind the cold front. Winds early Wednesday morning will remain
mostly light, but will begin turning to the south with the
approach of the cold front boundary. As the cold front reaches the
west side of the Cascades, expect southerly winds to strengthen
across central OR with gusts between 30-40mph, and through the
southern Grande Ronde valley, where current forecast guidance is
indicating gusts between 40-55mph through Ladd and Pyles canyon.
Lawhorn/82

.LONG TERM...Thursday through Monday...The last few days of March
will be met with showers each day along with occasional breezy
winds, mainly on Thursday. The first day of April will be met with
dry and warm conditions.  Confidence in the overall forecast is
high, despite some discrepancy in the models in the April 1
forecast. Discrepancies are mainly downstream with a large upper low
over the Great Basin.

A deep closed low will set up off the WA coast Thursday.  The low
will quickly travel south along the coastline Thursday night into
Friday in response to a weaker secondary low diving south along the
backside of the main upper low.  This will also cause both lows to
pinwheel around each other and travel south along the CA coast
on Friday. Numerous showers (70-90% chance) are forecast over the
Cascades and the eastern mountains, and there is a 15-30% chance
of showers elsewhere.  The instability is marginal and confidence is
not enough to include in the forecast for now, but this may be
needed if future runs advertise steeper lapse rates and mid-level
ascent.  The chance of showers decreases Friday, especially in our
WA zones.  Precipitation wrapping around the tight circulation will
primarily be confined to the southern half of OR.  There are minor
differences in the position of the low during this time.

As the closed low spins off the CA coast on Saturday, a deformation
band will set up along the northern periphery of the low and bring
scattered showers (40-60% chance) over central and northeast OR.
Meanwhile, a shortwave trough and attendant cold front will travel
across BC and Alberta, and the tail end of the front will bring
slight chance (20%) of showers to the WA Cascades and northern Blue
Mountains.  The potential for showers on Sunday is only 10-
30% with highest probability over the Blue Mtns and Wallowa Mtns.
Models are in general agreement pushing the low south of the San
Francisco Bay area, but some differences on position.  The northern
shortwave trough will dive south across ID, and the northerly flow
aloft will maintain the widely scattered orographic showers over the
eastern mountains.

The shortwave trough and the closed low will merge together over the
Great Basin early next week. Meanwhile, a high pressure ridge is
progged to build over the PacNW.  This will create a Rex block that
will likely remain through the middle of the week for continued dry
and warm weather. Once again, there are model differences next
Monday but the large low will be too far south for any impacts for
our CWA. Wister/85


&&

.AVIATION...18Z TAFs (previous discussion)...Scattered showers can
be seen on WSR-88D west of the Cascades, associated with a weak
shortwave that will track across the forecast area late this
afternoon and evening. RDM and BDN have the best potential for -SHRA
as well as MVFR conditions. Included a PROB30 for ALW and PDT
tonight but kept CIGS around 3500 feet. There will be showers in the
vicinity of DLS, but YKM and PSC will remain dry with mid and high
clouds. Winds will increase ahead and behind the shortwave with WSW
winds gusting to 25G30kt. Showers may also result in stronger
outflow winds.  Wister/85



&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  37  54  34  58 /  30  30   0  60
ALW  40  57  37  58 /  50  50   0  60
PSC  42  62  39  58 /  10  10   0  60
YKM  34  59  35  51 /  10   0  10  80
HRI  40  60  37  59 /  20  10  10  70
ELN  34  54  31  48 /  10   0  10  80
RDM  28  50  33  54 /  30   0  10  70
LGD  34  49  29  51 /  50  50   0  50
GCD  32  48  30  53 /  60  30  10  70
DLS  42  58  40  54 /  30  10  20  90

&&

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

$$

SHORT TERM...82
LONG TERM....85
AVIATION...85


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