462
FXUS66 KPDT 160321
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
821 PM PDT Sun Sep 15 2024

.UPDATE...
latest radar imagery showed some isolated showers mainly over
eastern Oregon. Guidance has these showers lingering through the
night. However, latest guidance also has cut back on the western
extent of the showers so have lowered or eliminated pops along the
western extent of where precip was. Otherwise, no significant
changes to the forecast, just made some adjustments to low
temperatures, where needed.

&&

.AVIATION...
06Z TAFS...VFR conditions with winds 10 kts or less are expected
through the TAF period.

&&

.PREV DISCUSSION... /issued 432 PM PDT Sun Sep 15 2024/

Updated for Aviation...

SHORT TERM...This afternoon through Tuesday night...A band of
precipitation remains set up across the Blue Mountains, aided by
broad forcing for ascent beneath the right entrance region of a
250-mb jet. This jet is located on the east side of a
southeastward-propagating upper-level closed low centered near
the OR/CA border. Multiple distinct shortwaves rounding the base
of the trough are evident on water vapor imagery.

Ensemble guidance is in excellent agreement that the main low will
continue to track southward into central CA overnight through
Monday morning, which removes the jet support across our forecast
area. However, some mid-level WAA and a weak shortwave impulse
rounding the low over eastern OR may facilitate some showers
overnight, though confidence in details remains low.

Monday afternoon through Tuesday, the upper low will eject
northeast from CA into the Great Basin and ultimately through the
Intermountain West. Uncertainty remains medium (50% confidence in
forecast PoPs/QPF) due to subtle differences in the low`s track.
One scenario (~50% of ensemble members) favors a track that would
result in a wetting rain for the northeast mountains while the
other scenario (~50% of ensemble members) places drier conditions
across the forecast area with the precipitation axis displaced to
the south and east.

Monday evening through Tuesday, breezy to windy westerly winds are
forecast to push through the Cascade gaps as surface pressure
gradients tighten in response to a second incoming low. This
second trough will increase precipitation chances across the
Cascades and into central Oregon Tuesday afternoon through Tuesday
night as well. Plunkett/86

MORNING UPDATE...issued 1008 AM PDT Sun Sep 15 2024...A band of
light, stratiform precipitation has set up across the Blue
Mountains this morning, aided by broad forcing for ascent beneath
the right entrance region of a 250-mb jet located on the east side
of a southeastward-propagating upper-level closed low currently
centered over the OR coast. Surface observations indicate the dry
sub-cloud layer that was causing precipitation to evaporate prior
to reaching the ground has finally saturated; recorded
precipitation across regional gauges is generally less than 0.10",
locally higher along preferential upslope terrain in the Blue
Mountains.

Through the day, precipitation is expected to remain focused
across the Blue Mountains with the best chances of measurable
precipitation in a narrow band stretching from eastern Deschutes
County to western Wallowa County. Of note, 12Z CAMs continue to
suggest weak instability (generally 250 J/kg or less) collocated
with synoptic-scale forcing in southeast Deschutes/Wheeler, and
southern Grant counties this afternoon and evening. Have opted to
re-introduce a slight (15%) chance of thunder to the forecast,
though confidence remains low given that the best forcing will be
south of our CWA. Plunkett/86

LONG TERM...Wednesday through Sunday...The long term remains
progressive in nature through the period. Ensembles indicate a
deep low, following on the heels of this weekend`s system, will be
moving southwards along the coastline of the PacNW and into
Northern California through the day Wednesday. Unfortunately this
track does prevent the system from producing noteworthy
precipitation for our region, with any light showers that were
expected from it moving out through the morning hours. This will
keep conditions quiet Wednesday evening through Friday, with
temperatures near normal (widespread NBM probabilities of highs
70+ degrees are 50-100%) and skies mostly clear until Friday
evening as the next system begins moving into the region.

Forecast uncertainty is high once we reach the weekend, as
ensembles show multiple possible outcomes at that time. This
includes a new trough approaching our area, high pressure ridging
strengthening over the area and staving off this system, or even
general zonal flow between ridging to the south and troughing to
the north. It`s tough to believe the deep ridging outcome due to
how progressive our pattern has been coupled with the strength of
the system expected to come out of the Gulf of Alaska, and the NBM
is advertising temperatures below normal with a shot at some
rainfall over the weekend from a potential trough. Zonal flow may
be the most likely outcome and we remain between systems, but
until ensemble guidance comes into better agreement on next
weekend, there is low confidence (20-30%) in the forecast beyond
Friday. Goatley/87

AVIATION...
00Z TAFS...VFR conditions are expected through the TAF period. A
passing shower can not be ruled out, especially for ALW, BDN or
RDM this evening. Otherwise, winds should mainly be 10 kts or
less. DLS, BDN and RDM will see some winds above 10 kts through
early this evening then they will drop to 10 kts or less for the
remainder of the period.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  51  71  50  70 /  20  20  20  10
ALW  52  74  55  73 /  20  20  20  20
PSC  56  80  56  77 /  10  10  10  10
YKM  52  82  47  78 /  10   0   0   0
HRI  54  78  55  75 /  20  10  10  10
ELN  50  82  50  74 /  20   0   0   0
RDM  44  69  42  65 /  20  10   0  10
LGD  50  70  49  68 /  50  40  30  30
GCD  47  70  44  68 /  70  30  30  20
DLS  54  82  55  70 /   0  10   0  20

&&

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

$$

SHORT TERM...77
LONG TERM....87
AVIATION...77