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000
FXUS66 KPDT 122157
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
257 PM PDT Fri Apr 12 2024

.SYNOPSIS...A broad and deep upper-level low offshore of northern
California will pump moisture into the region and facilitate
multiple rounds of showers and thunderstorms this afternoon
through the weekend. A low-end severe weather threat, highlighted
by a Marginal risk (5% chance of severe hail or wind within 25
miles of point in the risk area) from the Storm Prediction Center,
exists across much of eastern Oregon this afternoon before
shifting to central and north-central Oregon on Saturday.
Thereafter, the upper low will weaken and track inland across
California and the Great Basin while another upper-level trough
dives southeast into the PacNW and Northern Rockies during the
first half of next week. Tangible weather impacts will transition
to likely (80-95% chance) advisory-worth west to northwest winds
for the Cascade gaps and Columbia Plateau on Sunday and Monday,
and a low-end freeze potential (<30% chance) for eligible lower-
elevation zones through mid-week.

&&

.SHORT TERM...This afternoon through Sunday night...A low-end
risk of severe thunderstorms will continue this afternoon across
eastern Umatilla, and most of Grant, Union, Wallowa counties where
clearing skies this morning set up a differential heating
boundary. Across the aforementioned areas, the latest RAP-based
mesoanalysis paints effective shear of 40-60 kts, MUCAPE of
500-1000 J/kg, and effective inflow-layer SRH of 25-100 m2/s2 so
the environment will be supportive of some organized convection.
Isolated strong cells have already developed in Harney, Lake, and
Malheur counties and are tracking north into the CWA. As the
afternoon progresses, 12Z HREF guidance suggests the best chances
for organized convection will be over the northern Blue Mountains
into Wallowa County where observations further indicate boundary-
layer moisture and temperatures are most supportive of CAPE in
excess of 1000 J/kg. While there is some spread in the precise
location of organized cells, neighborhood probabilities of 4-hr
max updraft helicity (a proxy for rotating updrafts) peak in this
region through this evening. The main hazards with cells will be
marginally severe hail (1" diameter or greater), marginally severe
wind gusts (58 mph or greater). Cannot rule out a very low-end
threat of a weak tornado, but 50th percentile 12Z HREF LCL heights
are high enough (1-1.5 km) to preclude what would otherwise be a
decent setup for our CWA.

The threat of severe weather continues Saturday for central and
north-central Oregon where upper-level forcing is anticipated to
be most supportive. The threat is once again highlighted by a
Marginal risk from the SPC with severe hail and severe wind being
the advertised hazards. 12Z HREF members are also supportive of a
low-end flash flood threat - neighborhood probabilities of 30%
for 3-hr max QPF of 1" or greater for Deschutes, Crook, and
Jefferson counties Saturday afternoon through the evening. PWATs
of 0.6-0.8" and storm motion of 20-30 kts (except slower for
right-moving supercells per Bunker`s storm motion) should reduce
the threat outside of training cells. Of note, the ECMWF EFI
(values of 0.8-0.9) is highlighting central OR for good ensemble
agreement in unusual precipitation relative to model climatology.

Sunday, the threat of convection appears to shift to the Blue
Mountains again. Severe threat appears low at this time. However,
advisory-level west to northwest winds appear likely (50-90%
chance per NBM probabilities) for the Cascade gaps and Columbia
Plateau as another trough dives southeast towards the PacNW and
northern Rockies. Plunkett/86

&&

.LONG TERM...Monday through Friday...Sensible weather concerns
through the period will revolve around an incoming upper trough
and cold front Monday that will produce strong winds across the
forecast area. Otherwise, while uncertainty in the synoptic
pattern is present, the overall trend is for light precipitation
in the mountains through the midweek, then dry and quiet
conditions prevailing into next weekend.

Monday will start with the arrival of an upper trough and surface
cold front to the PacNW, resulting in surface pressure gradients
continuing to tighten across the Cascades and into the Columbia
Basin. Current NBM guidance shows 80-95% probabilities for wind
gusts to exceed 45 mph in the eastern gorge, OR Columbia Basin, OR
Blue mountain foothills, the Kittitas valley, and along the
higher terrain of the WA Cascade east slopes including Simcoe
Highlands; the NBM also shows 55-75% chance of reaching warning
level wind gusts across the OR Columbia Basin, Kittitas valley,
and areas of the Simcoe Highlands. If this trend continues, wind
highlights will be warranted across these areas. Otherwise, the
trough and cold front passage will be relatively dry with mostly
light precipitation across the region. That said, the ingredients
and signal are there for a Puget Sound convergence zone to develop
behind the frontal passage, which if it materializes could produce
moderate snow accumulations from Snoqualmie Pass north. In fact,
NBM shows a 45-55% probability of 6 inches of snow and a 60-75%
chance of 4 inches of snow at pass level Monday night.

Late Monday through Wednesday, ensemble guidance is in great
agreement that the trough passage will remain mostly dry as it
moves across the region. Ensemble cluster solutions generally show
a less than 30% chance of 0.1 inches of QPF in any 24 hour period
through Wednesday. On the topic of the trough passage, while
ensemble guidance was in good agreement with it`s approach early
Monday, uncertainty grows with track and timing of the upper
trough late Monday through the remainder of next week. First off,
ensemble cluster members, and more specifically between the ECMWF
and GFS, are in disagreement in how far south the trough will drop
into the PacNW Monday into Tuesday. Current GFS ensemble guidance
is in favor of the further south trough passage, which would
result in cooler temperatures (lower to mid 50s in the afternoons)
across the lower elevations. The further south approach of the
trough would also introduce chances of freezing morning lows along
the Blue Mountain foothills, Kittitas valley, central OR, and
north central OR (confidence 30%). Tuesday and Wednesday mornings.
Meanwhile, the ECMWF members favor the trough to stay further
north, keeping warmer temperatures (upper 50s to mid 60s in the
afternoons). Wednesday, the trough will expand as its axis is
progged to push east into the Rockies and the northern Great
Plains, while ridging builds offshore the PacNW. Overall, ensemble
guidance is in good agreement with nearly 80% of the members
showing the PacNW under a northwest flow aloft Wednesday. Thursday
into Friday, ensemble guidance is in disagreement on the
progression of the upper ridge offshore. While about 50% of the
members, mostly from the GFS, favor the trough to stick around
across the northern Rockies and intermountain PacNW, the other
half of the members show the ridge moving inland over the PacNW.
While a general warming trend is expected in the later half of
next week, there is low confidence (30%) in how strong the warming
trend will be and if the lower elevations will see a return to
the 70s in the afternoons. Lawhorn/82

&&

.AVIATION...Previous Discussion...18Z TAFs...VFR conditions are
expected to prevail through the period, though showers and
thunderstorms impacting some sites may produce MVFR to IFR
conditions this afternoon. Best chance for showers will be at
PDT/RDM/BDN/ALW with showers within vicinity of PSC. As for
thunderstorm chances, moderate confidence (45-55%) site PDT will
see the best chance of any thunderstorms tracking overhead this
afternoon, while confidence is low (25%) that any thunderstorms
will impact sites RDM/BDN/ALW directly, though have introduced
VCTS at these sites for this afternoon. Winds will mainly be 12 kts
or less, though outflow winds could result from vicinity
thunderstorms at sites PDT/ALW/RDM/BDN. Lawhorn/82

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  43  71  48  72 /  60   0  10  20
ALW  46  77  51  76 /  70  10  10  20
PSC  46  77  51  80 /  30   0   0   0
YKM  41  75  46  79 /  10   0   0   0
HRI  44  76  50  78 /  50   0  10  10
ELN  42  74  47  73 /   0   0   0   0
RDM  39  64  43  64 /  40  50  50  20
LGD  43  73  47  69 /  60  10  10  50
GCD  43  74  46  67 /  70  30  40  60
DLS  46  75  52  70 /  20   0  10   0

&&

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

$$

SHORT TERM...86
LONG TERM....82
AVIATION...82