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452
FXUS66 KPDT 231558
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
858 AM PDT Thu May 23 2024

.UPDATE...
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Current radar and visible satellite imagery showing
light returns moving north to south through the Basin under partly
cloudy skies. These light showers are now approaching central and
southern Morrow and Umatilla counties and are continuing to
dissipate. This is expected to continue over the next two hours as
showers dry and temperatures warm. These showers are associated
with a pocketed area of cooler temperatures and elevated dewpoints
as a slight increase in precipitable water is also present.
However, as temperatures warm these showers will quickly end and
radar returns will clear. Only minor edits were conducted with
this morning`s forecast update related to rain chances associated
with this pocket of moisture in the Basin and Blue Mountain
foothills. 75
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&&

.PREV DISCUSSION... 
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/issued 234 AM PDT Thu May 23 2024/

SHORT TERM...
Today through Saturday night...Latest water vapor imagery shows
an upper level low over southern Idaho. This feature will move
eastward today. Lingering showers over the far eastern mountains
will end this morning and the rest of the day will be dry but cool
with high temperatures about 5 degrees below normal for this time
of year. Tonight will be mainly clear and cool with lows in the
40s except 30s in the mountains.

The next weather system will impact the region on Friday through
Saturday. The deterministic models and ensembles are in good
agreement on Friday but not on Saturday. On Friday afternoon and
evening scattered mostly mountain showers are expected to develop
with 30-55% POPS. Instability will also increase over the eastern
mountains, especially Grant, Union, and Wallowa Counties with at
least a 20% chance for thunderstorms. Forecast soundings from the
NAM show most unstable CAPE of 200-500 J/kg and very weak
tropospheric flow/shear with winds mostly 15 kt or less through
the dept of the troposphere. This will result in slow moving pulse
type storms that will be capable of producing brief heavy
rainfall.

On Saturday the GFS closes off an upper low and moves it across
Oregon while the ECMWF has a weak shortwave exiting in the morning
followed by flat NW flow aloft. The GEFS and ECMWF ensemble means
support each deterministic solution so confidence is not high in
the forecast for Saturday. The NBM was used for the forecast and
it placed 30-40% chances for showers over the mountains and 5-15%
POPS over the lowlands through the day Saturday ending early
Saturday evening. The probability of thunderstorms on Saturday is
less than 10%.

The pressure gradient between Portland and Spokane will increase
to 9-10 mb Saturday afternoon which will favor gusty west winds in
the typically favored lower elevation locations. The latest NBM
has a 20-40% chance of wind gusts exceeding 45 mph in the Cascade
gaps Saturday afternoon. The deterministic NAM and GFS both show
850 mb jets of around 35 kt and steep low level lapse rates which
will promote the downward mixing of horizontal momentum. The winds
are forecast to diminish in the 03-06Z timeframe Saturday
evening. 78

LONG TERM...
Sunday through Wednesday...Dry westerly flow will be over the region
for Sunday, with a ridge building in for Monday.  The ridge will
move eastward on Tuesday and southwesterly flow will prevail ahead
of an approaching trough.  The trough will start to make its
presence known later Tuesday or Wednesday bringing the next chance
of rain for the region, and thunderstorm chances for the eastern
mountains.

Sunday and Monday look to be dry, with Sunday being a bit breezy in
spots and Monday a bit warmer. High temperatures on Sunday will be
close to normal, in the low to mid 70s and above normal on Monday in
the low to mid 80s as the ridge builds over the area, so overall
nice conditions these two days of the Memorial Day weekend.

Tuesday and Wednesday will see some breezy conditions and by later
Tuesday into Tuesday night there could be rain chances developing,
first over the Cascades, then by Wednesday over the Blue Mountains
and eventually low chances over some of the lower terrain as well.
There are some timing differences with the speed of the trough
moving in, with the ECMWF being a bit faster and weaker than the
GFS, but overall model agreement is fairly good through the
extended.  The ensemble clusters support the weaker ECMWF solution
at this time, though it is still about a week away.

High temperatures on Tuesday will be similar to Monday, and a few
degrees cooler on Wednesday.


AVIATION...
12Z TAFS...High confidence (90-100%) in VFR conditions at all TAF
sites.  Winds will generally be 10 kts or less except for DLS which
will see gusts to 20 kts, which will decrease during the evening
hours.
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&&

.PRELIMINARY POINT TEMPS/POPS...
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PDT  66  42  69  47 /  20   0  20  30
ALW  69  47  72  50 /  10   0  20  30
PSC  74  49  75  52 /  20   0  10  10
YKM  73  43  72  44 /   0   0  10  10
HRI  73  46  74  49 /  20   0  10  10
ELN  69  43  69  46 /   0   0  10  10
RDM  66  39  64  39 /   0   0  10  10
LGD  62  39  67  44 /  10   0  40  50
GCD  63  39  67  42 /  10   0  40  50
DLS  71  48  70  49 /   0   0  20  10
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&&

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

$$

UPDATE...75
SHORT TERM...78
LONG TERM....77
AVIATION...77