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000
FXUS66 KPDT 210357
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
857 PM PDT Sat Apr 20 2024

.SHORT TERM UPDATE...A cold front sweeping across WA/OR has a
well-defined back edge just west of the Cascade Range on satellite
loops. There has been limited moisture with weak returns over
south central WA and a brief period of light rain in Yakima. The
biggest impact with the front will be the increasing winds just
ahead and behind the front that will decrease early Sunday
morning. WSW winds will gust to 30-45 mph in many areas but small
probabilities of gusts greater than 45 mph. The 00Z HREF has only
small areas of the Rattlesnake Hills and the Simcoe Highlands
with 70% or higher for gusts greater than 45 mph. 00Z UIL sounding
shows 35 kt winds between 850-700mb which increases confidence
that the approaching front will not warrant wind advisories. Minor
updates were made to the current forecast, mainly to introduce
isolated showers for portions of south central WA this evening.
Wister/85



&&

.AVIATION...00Z TAFs (previous discussion)...A cold front will be
traversing across the forecast area this evening, and its main
impact for the terminal airports will be increasing wind. Most
sites will see a sudden increase in winds from the WSW at
10-15G20-25kt...local gust to 30kt. Any precipitation will be
light and over the Cascades and Blue Mountains. Gradients will
relax and skies will clear behind the front early Sunday morning.
Wister/85


&&

.PREV DISCUSSION... /issued 450 PM PDT Sat Apr 20 2024/

SHORT TERM...
Tonight through Monday night...High pressure will continue moving
east of the area tonight. As it does, an area of low pressure will
move into British Columbia. The pressure gradient between these
two features will cause an increase in winds later this evening
through Sunday morning, followed by a slow decrease through
Sunday. A cold front will move across the area this evening
through tonight and will cause a further enhancement to the winds.
A few showers (rain or snow) are possible mainly over the
mountains as the front passes.

After Sunday, the remainder of the short term period is dry, first
under a westerly flow and then a ridge build in for later Monday
into Monday night.

As far as winds, they will increase this evening and become gusty,
generally in the 25 to 35 mph range with some gusts as high as 40
mph, especially over the Simcoe Highlands and possibly portions of
the Columbia Basin and Blue Mountain Foothills. The NBM
probabilities for winds >= 39 mph probabilities range from about
30 to 80% across the Simcoe Highlands and about 20 to 40% across
the Columbia Basin and Blue Mountain Foothills. The NBM
probabilities for winds >= 47 mph are significantly less. They are
about 20 to 50% in the Simcoe Highlands and generally less than
20% in other areas.

Winds should peak late tonight into Sunday morning. Winds will
decrease during Sunday but will remain breezy, generally in the 15
to 25 mph range. Winds will further decrease Sunday night. At this
time, no wind advisories are being issued, but will keep watching
to see as the front approaches if winds are higher than currently
expected.

Temperatures will be a bit lower on Sunday, behind the front and
will be slightly below normal. With highs returning to near normal
on Monday. Sunday night looks to be another colder night and while
temperatures look marginal at this point, some freeze warnings may
end up being needed.

LONG TERM...
Tuesday through Saturday...Models start out in good agreement
Tuesday then differences increase over time, so forecast
confidence is low. On Tuesday, a transitory ridge will move out of
the area while an upper low and trough sinks south out of the
Gulf of Alaska and down the British Columbia coast. 85 percent of
the GFS ensemble members favor rain showers over the mountains
Tuesday evening with little support from the ECMWF or Canadian.
Allowing for the GFS, have a slight chance of mountain rain
showers (15-25 percent) with amounts of less than a tenth of an
inch while snow levels will be at 6000-8000 feet. The ridge and a
southwest flow over the area will warm temperatures around 5
degrees from Monday with highs in the upper 60s to lower 70s in
the lower elevations and in the 60s in the mountains.

A few differences arise in the models on Wednesday as the trough
moves overhead. The ECMWF and Canadian ensemble members mostly favor
the upper low being over central British Columbia giving us a
westerly flow. 83 percent of the GFS ensemble members keep the low
offshore with a southwest flow over our area which is more favorable
for rain. Have kept a slight chance of very light rain over the
mountains with barely measurable amounts. Snow levels will remain at
5000-6000 feet. With the trough tightening pressure gradients,
breezy winds are expected in the afternoon. Temperatures will be a
few degrees cooler than Tuesday.

Model differences increase on Thursday as 90 percent of the GFS
ensemble members and 30 percent of the ECMWF members keep the upper
low along the British Columbia coast and even strengthen it a bit
while the rest of the ECMWF and nearly all of the Canadian ensemble
members have a weaker upper low crossing the Canadian Rockies.
Models all show a front or open wave arriving in the late afternoon
and overnight with the GFS having a strong well defined front while
the other deterministic runs looking weak and disorganized. Have a
chance of rain showers over the mountains and in central Oregon.
Rain amounts will be one to two tenths of an inch in the Cascades
and less than a tenth of an inch elsewhere. Snow levels will remain
around 5000 feet. Temperatures will cool a few more degrees with
highs in the 60s in the Columbia basin and in the mid 50s to lower
60s elsewhere.

By Friday, forecast confidence decreases further as model ensemble
members have conflicting solutions. 85 percent of the GFS members
and 40-50 percent of the ECMWF and Canadian members have closed low
or an open wave along the Washington Oregon coast. Other model
ensemble members have a shallower trough centered over the Rockies
with some members having a closed low forming over central or
southern California. Have gone with a chance of rain over the area
(25-55 percent) mainly in the afternoon with up to a quarter inch in
the mountains, around a tenth of an inch in the Blue Mountain
Foothills and just a few hundredths of an inch in the rest of the
lower elevations. Temperatures will drop another degree or two from
Thursday.

On Saturday, the GFS members (75 percent) continues to favor a
trough over the area with only mild support from the ECMWF and
Canadian. The ECMWF and Canadian tend to favor the trough over the
Rockies with a ridge developing out in the eastern Pacific. Have a
20-40 percent chance of rain over the mountains with the lower
elevations dry. Temperatures will warm 2-3 degrees to the 60s and
lower 70s with 50s in the mountains. Perry/83

AVIATION...
00Z TAFs...A cold front will be traversing across the forecast area
this evening, and its main impact for the terminal airports will be
increasing wind.  Most sites will see a sudden increase in winds
from the WSW at 10-15G20-25kt...local gust to 30kt.  Any
precipitation will be light and over the Cascades and Blue
Mountains.  Gradients will relax and skies will clear behind the
front early Sunday morning.  Wister/85

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  38  58  33  63 /  10   0   0   0
ALW  41  61  36  65 /  20   0   0   0
PSC  43  65  39  67 /  10   0   0   0
YKM  35  63  32  66 /  20   0   0   0
HRI  41  63  35  67 /  10   0   0   0
ELN  36  57  32  64 /  20   0   0   0
RDM  31  57  29  66 /   0   0   0   0
LGD  37  55  30  63 /  10   0   0   0
GCD  37  58  30  66 /  10   0   0   0
DLS  41  63  37  68 /  10   0   0   0

&&

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

$$

SHORT TERM...77
LONG TERM....83
AVIATION...85