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575
FXUS66 KPDT 241611
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
911 AM PDT Fri May 24 2024

.UPDATE...
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Current radar and visible satellite imagery showing very
light returns arriving along the Cascades as some mid-to high
level clouds begin to stream into the region. This is in response
to an upper level trough approaching the area as a cold front is
poised to pass through later this afternoon and evening. These
synoptic features will bring shower chances to our mountain
(50-60%) and foothill (15-25%) zones, as well as the potential for
developing thunderstorms across the Blue Mountains and the John
Day-Ochoco Highlands. Surface CAPE values of 150-250 J/kg are
expected with 0-6km shear of 15-25 kts. These low shear values
coupled with elevated CAPE will produce discrete storm cells that
will be slow to move, leading to the potential for moderate to
heavy rain to occur over a prolonged period in locations where
cells develop. Thus, localized flooding and ponding on roadways
are the primary concern, creating the potential for hazardous
travel. Make sure to have ways to receive weather alerts, and stay
updated to developing conditions - especially if you are planning
on traveling in the aforementioned areas. 75
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&&

.PREV DISCUSSION... 
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/issued 234 AM PDT Fri May 24 2024/

SHORT TERM...
Today through Sunday night...A shortwave trough embedded in NW
flow aloft will move across the area today and tonight. With
daytime heating there will be modest airmass destabilization.
The SPC HREF is indicating most unstable CAPE values reaching up
to 500 J/kg along the Blue Mountains this afternoon. The CAMS are
forecasting convective initiation around 19Z for areas from the
Blue MTNS eastward with activity becoming more widespread through
the late afternoon and early evening hours. The SPC HREF 1-hour
calibrated thunder probabilities peak at 40% at 00Z this evening
for Grant, Union, and Wallowa Counties. With storm motions around
20 kt and the potential for training, heavy rain will be the main
threat from the storms. Scattered showers will continue into the
overnight hours mainly in the far east with the TSTM threat ending
by 03-06Z.

Models and ensembles still disagree somewhat on the track of a
small upper low for Saturday, but they are converging on a track
that is mainly north of the forecast area. As a result shower
chances will be more limited than earlier anticipated, and by
Saturday afternoon it will be mainly dry. There will still be a
cold frontal passage Saturday, and during the afternoon the
pressure gradient between Portland and Spokane is forecast to
reach 10 mb. This will result in windy conditions in the Cascade
gaps and portions of the Columbia Basin (mainly Oregon portion).
Probability of winds exceeding 45 mph in these areas is 20-40%
around 2 PM Saturday afternoon based on the NBM. A westerly low
level jet of 35-40 kt and steep low level lapse rates will help
support gusty west winds.

Winds will decrease Saturday evening and it will be rather chilly
Saturday night. Lows will be in the upper 20s to lower 30s in the
mountains and high plateaus, and in the mid 30s to mid 40s lower
elevations. Ensembles agree on a ridging pattern on Sunday and
Sunday night which will result in dry weather and a nice afternoon
Sunday after a chilly start. 78

LONG TERM...
Monday through Thursday... A ridge will be over the Pacific
Northwest on Monday. This will bring dry and warm weather over the
region for Memorial Day. The ridge will move eastward during the
day, which will allow for increasing southwesterly flow ahead of a
trough offshore.

The trough will move eastward and onshore by later Tuesday into
Tuesday night and bring with it some shower and thunderstorm chances
for later Tuesday into Tuesday night.  The ECMWF EFI keys in on
CAPE/thunderstorm chances across the Blue Mountains on Tuesday, with
a broad area of 0.6 to 0.7 and a small area of 0.7 to 0.8.

The trough will then move east of the area on Wednesday, allowing
for a dry northwesterly flow which will prevail into Thursday.

Overall, the guidance is in generally good agreement through
midweek, then differences arise as the GFS is faster with the trough
moving onshore.  The ensemble clusters are fairly evenly split
through the extended, so it is difficult to select a preference at
this time.  All of the guidance does have a trough, it is just more
of a question of timing and strength as we go through the mid and
late week period.

As the trough moves through the region later Tuesday into Wednesday,
and lingering into Thursday, there will be some breezy winds,
especially in the Columbia Basin, Kittitas Valley and Simcoe
Highlands.

High temperatures will be above normal on Monday, mainly in the
upper 70s to lower 80s.  Tuesday will be a bit cooler, with highs in
the mid 70s to lower 80s.  With the trough moving onshore Wednesday,
temperatures will be close to or even slightly below normal, mainly
in the upper 60s to upper 70s.

AVIATION...
12Z TAFS...High confidence (90-100%) in VFR conditions at all TAF
sites. Winds will increase and gust to around 25 kts later this
morning or during the afternoon at all sites except YKM. DLS and
PDT will then have gusty winds through at least 09-12Z Saturday.
RDM and BDN will see decreases to around 10 kts around 25/03Z. PSC
will see a decrease around 25/06Z. ALW will see a decrease to
around 10 kts around 03Z.
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&&

.PRELIMINARY POINT TEMPS/POPS...
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PDT  68  44  61  39 /  20  20  10   0
ALW  72  48  65  44 /  20  20  20   0
PSC  75  51  70  45 /  10  10  10   0
YKM  71  42  68  38 /  20  20   0   0
HRI  73  48  67  43 /  10  10  10   0
ELN  66  43  62  41 /  30  20   0   0
RDM  66  37  58  33 /  10  20  10   0
LGD  66  42  58  36 /  60  60  30   0
GCD  67  41  59  34 /  50  50  50   0
DLS  70  48  64  45 /  10  10  10   0
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&&

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

$$

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