-- Discussion containing changed information from previous version are highlighted. --
480
FXUS66 KPDT 281102
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
245 AM PDT Tue May 28 2024

.SHORT TERM...
Today through Thursday night...

Bottom Line Up Front: (BLUF)

1. Isolated to scattered thunderstorms will affect eastern
portions of the region this afternoon and evening.

2. Some thunderstorms, especially across Wallowa County could be
severe, with heavy rain, gusty winds and small hail.

3. An upper level trough and surface cold front will move across
the region from west to east this afternoon and evening, with the
trough moving east of the area by Wednesday morning.

4. Breezy conditions are expected this afternoon and evening and
again on Wednesday for portions of the area.

An upper level trough and surface cold front will move onshore and
then cross the area from west to east during the afternoon and
early evening hours today. The trough will then exit the region
early Wednesday morning.

Behind the trough, dry northwesterly flow will prevail for later
Wednesday through Thursday, with ridging building in for Thursday
night.

As the trough and cold front move eastward, expect some showers to
develop across the Cascades, but the more important activity will
be showers and thunderstorms that are expected to develop mainly
across the eastern mountains during the afternoon and evening
hours.

By mid to late afternoon showers and isolated thunderstorms will
begin to develop across the higher terrain of eastern Oregon and
Washington. These showers and thunderstorms should become more
numerous as the afternoon progresses, especially over Wallowa
County. The Storm Prediction Center has placed much of Wallowa
County and a small portion of Union County under a marginal risk
of severe thunderstorms today. Isolated to scattered thunderstorms
with heavy rain, gusty winds and small hail are possible in this
area (30% chance of thunderstorms, but only about a 5 to 10%
chance of severe). Further west, along and east of a line from
Walla Walla to Pendleton to Monument, there is a chance (20-30%)
of isolated thunderstorms, mainly with gusty winds.

The best CAPE values, between 500 and 1000 J/kg are from the Blue
Mountains eastward and progress east with time, and should be
generally east of our area by 29/03Z. Any activity looks to be
winding down during the mid to late evening as the atmosphere
stabilizes behind the passage of the cold front. The ECMWF EFI
supports the thunderstorm potential today in the eastern Oregon
mountains, with CAPE value anomalies 05 0.6 to 0.7 and even a
swath of 0.7 to 0.8.

The other concern will be breezy conditions. Winds will begin to
increase mid morning and increase further during the afternoon and
evening hours, before decreasing overnight. Wind gusts in the 25
to 35 mph range, with some gusts as high as 40 mph are possible in
the Oregon Columbia Basin, Columbia Gorge, Simcoe Highlands, and
Foothills of the Blue Mountains. NBM probabilities of wind gusts
>=39 mph are 80-90% NBM probabilities of wind gusts >= 47 mph are
generally 40-50% percent. Other areas will see winds in the 20 to
30 mph range.

Winds will increase again on Wednesday afternoon in the northwest
flow behind the frontal passage, but they should be lower than
today. Winds should generally be 20 to 30 mph, with gusts as high
as 35 mph. NBM probabilities of winds >= 39 mph are 70-90%. NBM
probabilities of winds >=47 mph are generally less than 30% except
for the Kittitas Valley, which is around 70%. Other data does not
support winds that high in the Kittitas Valley, but will need to
keep an eye on that.

High temperatures today will be cooler than Monday and closer to
normal for this time of year, with highs ranging from near 70
degrees in central Oregon to near 80 degrees in the Columbia
Basin. On Wednesday, high temperatures will be even cooler and
will end up 5 to 10 degrees below normal on average and range from
the mid 60s to around 70 degrees. Thursday will see a warming
trend, with highs back into the 70s everywhere and back close to
normal.


.LONG TERM...
Friday through Tuesday...

Bottom Line Up Front: (BLUF)

1. Dry and warm conditions to start ahead of a pattern shift.

2. Rain returns to the area Saturday increasing through the period.

3. Afternoon thunderstorms along the eastern mountains Sat, Mon &
Tues.


Models are in great agreement with the upper level ridge in place in
the PacNW with the axis just off to the west of the Cascades Friday.
Clusters show the main variance being with the amplitude of the
ridge as it sits overhead. Regardless, dry and warmer temperatures
will prevail through Friday. Raw ensembles show 60-80% probabilities
the low and mid level elevations will see temperatures in the mid to
upper 70s with the EFI ensembles showing these temperatures to be at
or near normal for this time of year. However, there is a isolated
area showing the low temperatures in potions of the Basin and along
the Simcoe Highlands will be slightly below normal with raw
ensembles showing near 70% probabilities temperatures in these
mentioned areas will be below 45 degrees.

Models then shift patterns bringing in an upper level trough late
Friday night and again are in firm agreement with clusters showing
the variance again being with the amplitude of the trough. All
ensembles show the upper level trough to bring with it precipitation
beginning with showers over the mountains late Friday before
becoming widespread by Sunday afternoon as another shortwave ripples
across the region. Friday night there is 20% POPs along the Cascades
with less than 10% probabilities of 0.01 inches of rain. As the next
shortwave approaches Sunday, chances of rain along the mountains
increase to over 30% POPs with 50-70% probabilities of 0.01 inches
of rain along the Cascades and 25% for the remainder of the forecast
area. Monday shows chances along the eastern mountains increasing to
30-40%, Cascades increasing to 60-70% and the remainder of the
region remaining below 35% for 0.01 inches of rain. Temperatures
will remain in the 70s across much of the region with greater than
70% probabilities.

With the trough and shortwaves over the region the atmosphere will
be remain slightly unstable. While not all of the ingredients for
thunderstorms is present, ensembles show there to be enough daytime
heating (temperatures in the 70s), CAPE (over 50 J/kg), higher lapse
rates (above 7.5 C/km) with some mid level moisture that
thunderstorms cannot be ruled out. Especially along the eastern
mountains in Wallowa County. Ensembles however, only show 15%
chances for Saturday afternoon, 20% chances Monday and 15% Tuesday.
Bennese/90

&&

.AVIATION...
-- Changed Discussion --
06Z TAFs...VFR conditions will prevail through the
period with the only concern being the winds. Westerly winds will
dominate with sustained winds across the TAF sites of 10-16 kts with
gusts to 26 kts to start before increasing at DLS, and PDT to 20-25
kts with gusts to near 30 kts around 22Z. Bennese/90
-- End Changed Discussion --

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  72  43  64  40 /  20  10  10   0
ALW  76  47  67  45 /  20  20  10   0
PSC  81  48  71  45 /  10  10   0   0
YKM  75  38  68  37 /  10   0   0   0
HRI  76  46  69  43 /  10  10   0   0
ELN  68  41  62  41 /  10   0  10   0
RDM  68  37  62  33 /  10  10   0   0
LGD  73  42  60  37 /  50  40  10   0
GCD  76  42  63  36 /  60  20  10   0
DLS  68  47  66  45 /  10  10   0   0

&&

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

$$

SHORT TERM...77
LONG TERM....90
AVIATION...90