481
FXUS66 KPDT 242225
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
325 PM PDT Fri May 24 2024

.SHORT TERM...Today through Sunday...

Key Messages:

1. Thunderstorms develop through this evening.

2. Breezy to windy conditions Saturday.

3. Cooler temperatures Saturday before warming trend begins Sunday.

Current radar and visible satellite imagery showing isolated storm
cells developing across the John Day-Ochoco Basin, Blue Mountains,
and southern Wallowa county as light returns move onto the east
slopes of the Cascades under partly to mostly cloudy skies. These
thunderstorms are expected to continue through the evening before
lingering over Wallowa and eastern Grant county into the early
morning hours on Saturday. The HREF advertises mean CAPE of 350-450
J/kg with 0-6 km shear of 15-25 kts, which should produce discrete
storm cells that will travel relatively slow due to the low shear
values and storm relative motion. This will lead to the primary
concern being that of localized flooding and ponding on roadways
as moderate to heavy rainfall will be slow to move out of the
area, and may train as indicated by the HREF PB products
(highlighting elevated reflectivity and CAPE) which showcase a
consistent southwest to northeast motion centered over northeast
Grant and southwest Union Counties. These low shear values also
provide high confidence (80-90%) in storms staying sub-severe as
the cell`s life will be brief due to the updraft and downdraft
working against each other. This could still lead to breezy winds
and the potential for small hail in stronger cells, but confidence
is rather low (10-20%). Storm activity is expected to peak
between 3-6PM this evening over the aforementioned areas. These
conditions are in response to an upper level trough and associated
cold front slowly transiting the area through Saturday morning,
allowing for showers to accumulate along the Cascades, Blues, and
foothills. Minimal rain amounts (<0.05 of an inch) are anticipated
along the Blue Mountain foothills, with higher amounts over the
northern Blue Mountains (0.10-0.20 of an inch) and through Eastern
Grant, Union, and Wallowa counties (0.25-0.35 of an inch).

An upper low pressure arrives on the backside of the slowly passing
troughing feature on Saturday, but looks to stay just to our north
and along the Canadian border. This will lead to breezy winds across
the Simcoe Highlands, Kittitas Valley, and portions of the Basin and
Blue Mountain foothills as isobars tighten associated with an upper
level ridge approaching the coast. A developing pressure gradient
will also occur along the Cascades as a result. The GFS, NAM, and
SREF all advertise pressure gradients of 10-11 mb from Portland to
Spokane, peaking between 1-8 PM as gusts of 35 to 45 mph will be
possible. These values are just shy of advisory criteria, which
correspond to forecast pressure gradients as the normal advisory
threshold is 12 mb. Confidence in these winds staying below advisory
criteria is moderate (50-60%) as the NBM shows primarily a 50-60%
chance of gusts of 47 mph or greater over central Morrow and
Klickitat Counties, with isolated pockets of 60-80% chances. This
may warrant the issuance of Wind Advisories, but at this time
confidence is lacking in these gusts reaching advisory criteria.

The passing cold front today will lead to a slightly cooler airmass
to advect into the area in its wake, leading to high temperatures
dropping about 5 degrees from Friday to Saturday before warming back
up 4 to 8 degrees on Sunday. Highs will peak in the low to mid-70s
Sunday across the Basin and foothills, which is a slightly above
normal for this time of year. Lingering cloud cover tonight,
associated with the passing system, will keep overnight temperatures
in the upper 40s to low 50s across the Basin and foothills. However,
clearing skies Saturday night into Sunday morning will lead to low
temperatures dipping near normal values (low to mid-40s for lower
elevations of the Basin). 75


.LONG TERM...Monday through Friday...

Key Messages:

1. Mountain rain showers on Tuesday through Thursday with possible
thunderstorms.

2. Breezy winds (25-35 mph) late Tuesday through Thursday.

3. Above normal high temperatures Monday through Tuesday.


Tuesday through Thursday, an upper trough over the BC coastline
occurs with a front extending over the PacNW, thus increasing
chances of rain showers and thunderstorms for the mountain zones.
Afternoon thunderstorms are likely to develop for Tuesday through
Thursday with CAPE values of 500 J/Kg or above Tuesday as the front
brings additional lift and moisture (30-40% confidence). Chance of
precip remains 15-30% at the Blues and the Cascades and <15% in the
Columbia Basin and central OR, but 40-50% for eastern mountains
before decreasing Tuesday night. The models and ensembles are in
great agreement with the upper trough moving in late Tuesday morning
as the passing ridge exits the PacNW. Next, there is low confidence
with the depth of the upper trough developing Thursday with 29%
total cluster members favoring a deep trough over PacNW whereas 11%
members has it near OR coast.

Observed from NBM, chances remain high for breezy winds at 25-35 mph
for Yakima Valley, Lower Columbia Basin and Simcoe Highlands late
Tuesday afternoon into Thursday (>70%). Wind gusts might exceed up
to 40 mph but chances are on the low end (<20%). This is due to the
strong surface pressure gradient as the upper trough passes across
the PacNW. Breezy winds decrease to around 17-25 mph Thursday as the
upper trough exits the region (40% confidence).

High temperatures remain above normal for Monday through Tuesday in
the mid to upper 80s in the Columbia Basin, as the upper ridge
passes. However, high temperatures drop a few degrees to slightly
below-normal in the upper 60s and lower 70s for Wednesday into
Thursday once the upper trough moves over the PacNW. Temperatures
warm up again Friday when another upper ridge approaches the OR
coast (50% confidence). Feaster/97


&&

.AVIATION...18Z TAFS...VFR conditions prevail for this period. Winds
will be gusting at 15-27 kts with sustained winds at 10-20 kts
around late morning/afternoon for most sites, calming slightly later
this evening then returns at 00Z-16Z Saturday for KRDM/KALW/KPSC.
Winds at KYKM will be moderate this evening but light again tonight.
Feaster/97


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  45  61  40  71 /  20  10  10   0
ALW  49  65  45  73 /  30  20  10   0
PSC  52  70  47  76 /  20   0  10   0
YKM  43  66  41  72 /  10   0  10   0
HRI  48  67  44  76 /  20  10  10   0
ELN  43  61  42  67 /  10  10  10   0
RDM  38  57  34  72 /  20  10  10   0
LGD  42  58  37  68 /  60  30  10   0
GCD  41  59  35  70 /  50  50  10   0
DLS  48  63  47  73 /  20  10  10   0

&&

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

$$

SHORT TERM...75
LONG TERM....97
AVIATION...97