145
FXUS66 KPDT 151708
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1008 AM PDT Sun Sep 15 2024

.MORNING UPDATE...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

&&

.AVIATION...18Z TAFs...VFR conditions expected over the next 24
hours, with the exception of sites receiving precipitation.
PDT is the most likely site to see rain, but BDN/RDM could also
see local showers that may move into the sites briefly. Rain not
expected to be heavy enough to reduce visibilities below 6 miles,
but lowered ceilings flirting with MVFR conditions possible.
Breezy DLS with gusts around 20 knots through the afternoon,
otherwise all other sites should see winds 10 knots or less
outside of localized breeziness from showers. Goatley/87

&&

.PREV DISCUSSION... /issued 348 AM PDT Sun Sep 15 2024/

SHORT TERM...Today through Tuesday...

Bottom Line Up Front

1. Rain across central OR and central OR mountains

2. Breezy winds through the Gaps Tuesday

3. At or near normal temperatures

Models are in relatively firm agreement through the short term
with an upper level low making its way down the coast of Canada
and into the forecast area. Models show the low to elongate along
the Cascades and bringing moisture up the leading edge of the low
across central OR through the eastern mountains. Southwesterly
flow aloft will dominate the region bringing with it near normal
temperatures. Models at the surface show the pressure gradients
tightening along the Cascades by as much as 12 mb which will bring
breezy conditions to the Cascade Gaps on Tuesday. As the low
tracks to the south undercutting the region, some wrap around
moisture will make it to the Wallowa Mountains.

The upper level low headed this way will bring increased chances
of rain across portions of the CWA. Rainfall is expected to
stretch across central OR, along the eastern mountains and into
Wallowa. Looking at the raw ensemble rainfall totals, 50-70% of
the raw ensembles show the 48 hour total for central OR to be
roughly 0.05 inches of rain. 80-100% of the raw ensembles have
0.15-0.3 inches of rain along the eastern mountains and 50-70%
have 0.10 inches along the Wallowas. As the low moves off towards
the east, models show some wrap around moisture pushing rainfall
into the Wallowa again, however, rainfall amounts will vary
depending the extent of the track. As of now, NBM show a 50-70%
chance of an additional 0.05- 0.01 inches of rain through Wallowa
County.

Models at the surface show a tightening of the surface pressure
gradients as the upper level low elongates and moves southward.
GFS and the SREF models both show the cross Cascade pressure
gradients tightening between 9 to 12 mb Tuesday. This will bring
breezy conditions to the Cascade Gaps with 60-80% of the raw
ensembles in agreement that the Gorge, lower Columbia Basin,
Kittitas Valley and the foothills of the Blues will see sustained
winds of 35 mph or greater. 30-60% of the raw ensembles also how
the same areas seeing gusts to 40 mph. After Tuesday, models begin
to show the surface pressure gradients relaxing bringing winds
back to normal across much of the area.

Lastly, with the incoming flux of southerly flow aloft, EFI shows
temperatures through the short term to remain at or near normal.
The NBM shows much of the lower elevations, central OR and
Kittitas/Yakima Valleys to remain in the upper 60s to low 70s with
a few isolated locations in the Basin seeing mid to upper 70s
today with over 60% of raw ensembles agreement. Temperatures will
increase slightly tomorrow with the NBM showing the Basin,
adjacent valleys and the Gorge in the 80s, foothills of the Blues
and central OR in the low to mid 70s and the higher terrains in
the upper 50s to low 60s with roughly 60% of the raw ensembles in
agreement. Temperatures will decrease a few degrees Tuesday.
Bennese/90

LONG TERM...Wednesday through Sunday...Models are in pretty good
agreement through the period with only minor differences in timing
and strength of the broad upper level trough and amplifying ridge
late in the period. Cluster analysis reflects this nicely.
However, two clusters (40%) show a more troughy solution over the
weekend. We`ll have to keep a close watch to see if other clusters
begin to converge on this solution. Ensembles show the main
energy over CA and NV by the start of the period with drier,
northerly flow developing over the region Thursday. The GFS
ensemble shows a stronger ridge developing over the eastern
Pacific continuing northerly flow over the area while the ECMWF
ensemble is a little less amplified resulting in the flow becoming
northwest. They come back into agreement Friday with both showing
northwest flow aloft as the upper trough moves east and a long
stretch of zonal flow develops over the eastern Pacific. The
Northwest flow continues into the weekend as a ridge develops over
the eastern Pacific. This pattern is generally a dry one, but
favored northwest-north facing slopes could see an isolated
shower.

There`s a low-moderate chance (20-35%) of receiving 0.10 inch of
rain Wednesday over our southern CWA, mainly Oregon, but
probabilities drop to less than 10 percent for the reminder of the
period as drier, northwest-northerly flow filters into the area.

There`s a low chance (10-15%) of breezy conditions developing
through the Columbia River Gorge and Cascade gaps Thursday and
Friday.

Daytime highs are expected to remain near to slightly below normal
through the period. Earle/81

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  66  49  71  50 /  20  20  20  10
ALW  69  52  74  55 /  30  20  20  20
PSC  73  57  79  56 /   0  10  10  10
YKM  74  52  81  47 /   0   0   0   0
HRI  71  54  79  55 /  10  20  10  10
ELN  73  49  80  50 /  10   0   0   0
RDM  65  46  69  42 /  20  10  20   0
LGD  64  50  69  49 /  50  50  30  30
GCD  63  47  69  44 /  80  70  30  20
DLS  75  54  82  55 /   0   0   0   0

&&

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

$$

MORNING UPDATE...86
SHORT TERM...90
LONG TERM....81
AVIATION...87