960
FXUS66 KPDT 240521
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1021 PM PDT Thu May 23 2024

Updated Aviation Discussion

.AVIATION...06Z TAFs...VFR conditions to prevail through the
period. A system arriving tomorrow will lower CIGS to between 6kft
to 12kft AGL, with rain showers confined to the Cascades and
mountains east/south of sites PDT/ALW. Light winds will prevail
tonight, then increasing to between 12-20kts with gusts up to
30kts tomorrow, except at site ALW where winds less than 12kts
will prevail through the period. Lawhorn/82

&&

.PREV DISCUSSION... /issued 448 PM PDT Thu May 23 2024/

.SHORT TERM...Today through Saturday...

Key Messages:

1. Windy conditions Saturday afternoon.

2. Thunderstorms possible Friday afternoon into early Saturday.

3. Below normal high temperatures extend through period.

The extended period is characterized by an upper level trough
that is dropping along the British Columbia coast today before
passing through the Pacific Northwest Friday into Saturday
morning. This system will bring showers to the majority of our
mountain and foothills zones, as the potential for thunderstorms
across the Blue Mountains and Wallowa County materialize late in
the morning and extend through the night on Friday. However, the
primary concern through the period resides with breezy to windy
conditions Saturday as advisory level winds may be reached across
the Simcoe Highlands, portions of the Lower Columbia Basin, and
Blue Mountain foothills. These conditions are in response to the
upper trough stalling and strengthening over the area as an upper
level ridge approaches the coast. The combination of these two
synoptic features will tighten isobars between them, and allow for
a pressure gradient to develop along the Cascades. The pressure
gradient between Portland and Spokane will increase Friday
afternoon and lead to breezy conditions, but will peak Saturday
afternoon as highlighted by GFS, NAM, and SREF guidance. Pressure
gradients on Friday are forecast to stay between 8.5-9.5 mb, with
gradients on Saturday reaching 9.0- 10.5 mb between 2 PM and 8 PM.
These values are below the normal advisory threshold of 12 mb,
but 850mb winds of 35-40 knots and steep low level lapse rates
will allow for efficient mixing of these winds to the surface to
promote wind gusts nearing advisory criteria. At this time, there
is moderate confidence (50-60%) that a wind advisory will be
necessary across the aforementioned areas as the NBM hints at a
50-80% chance of wind gusts reaching 47 mph or greater.

As previously mentioned, the arrival of the upper level trough will
bring rain showers along the Cascades Friday morning before
extending to the northern Blue Mountains, foothills, and across
Central Oregon by the afternoon and through Saturday afternoon.
Precipitation amounts will stay less than 0.05 of an inch across the
northern Blue Mountain foothills, 0.05-0.15 of an inch at elevation
over the northern Blue Mountains, and 0.20-0.30 from the Southern
Blue Mountains through south-central Wallowa County. It will also
bring the chance (20-30%) of developing thunderstorms Friday
afternoon through much of the evening across the Blue Mountains,
John Day-Ochoco Basin, and Wallowa County, which is also included in
the General Thunderstorm (0 of 5) category from the Storm Prediction
Center`s Convective Outlook. The HREF advertises surface CAPE of 100-
250 J/kg, with low level wind shear of 15-25 knots. These low values
of shear will lead to high confidence (70-80%) of sub-severe
thunderstorm development. However, due to the nature of the slow
storm motion and low shear, the primary concern related to any
developing cells will be persistent rainfall as the storms will
be slow to move. This may lead to ponding on roadways or localized
flooding if cells develop in a training pattern (new storm cells
following mature storm cells). HREF PB products, highlighting
elevated reflectivity and CAPE, showcases the majority of
development centered over Union, Eastern Grant, and north Wallowa
county as the best timing for training should occur between
1PM-4PM. Confidence in this occurring in developing storms is
still low (5- 15%), as cells will stay discrete and rather brief.

Northwest flow aloft will persist through the period, only
becoming west northwest slowly on Saturday as the upper level
ridge arrives along the coast in the wake of the departing
troughing system. High temperatures on Friday will be similar to
today, with temperatures cooling 2-6 degrees Friday to Saturday.
Mostly clear to partly cloudy skies overnight will promote cooling
and slightly below normal temperatures through Saturday. 75

.LONG TERM...Sunday through Thursday...

Bottom Line Up Front:

1. Mountain rain and thunderstorms return Tuesday.

2. Windy conditions return Tuesday and Wednesday.

3. Warm temperatures Monday and Tuesday.

Models are in decent agreement that an upper level trough will make
its way back into the region beginning Tuesday through Wednesday.
Clusters show that the main variance within the ensembles is with
the amplitude of the incoming system. This will affect both the
precipitation amounts as well as the timing of the precipitation and
thunderstorms. With that said, moisture is expected across the
Cascades as well as across the eastern mountains with ensembles
showing 20-40% probabilities of up to 0.01 inches of rain along the
Cascades and 10-20% along the eastern mountains beginning Tuesday
afternoon. Long term guidance does show a round of instability with
surface based CAPE values of over 500 J/kg, lifted index values
nearing -3 to -4 and lapse rates nearing 8-9 C/km accompanying the
initial front which will allow the likelihood of thunderstorms
across the eastern mountains beginning Tuesday afternoon. However,
as this is day 6, confidence in these storms occurring is low to
moderate (20-30%).

With the upper level trough moving across the region Tuesday through
Wednesday, models show strong southwesterly flow aloft while
guidance at the surface shows a strong pressure gradient forming
across the Cascades. The tightening of the pressure gradient by 8 to
10 mb will cause the westerly flow at the surface to increase. Winds
through the Cascade Gaps, foothills of the Blues, Columbia Basin and
adjacent valleys have 70% probabilities of seeing sustained winds
greater than 25 mph with gusts of 39 mph or greater on Tuesday and
Wednesday. Confidence in the wind forecast is moderate as all
guidance is showing similar agreement with higher probabilities.

Lastly, ahead of the next system, models are in relatively firm
agreement with and upper level ridge of high pressure settling over
the area Saturday through Tuesday. Ahead of the next system,
temperatures will be warmer with the upper level ridge. Monday and
Tuesday will be the warmest days of the period with well over 90% of
the raw ensembles showing the vast majority of the region being
above 70 with isolated areas in the Basin seeing temperatures in the
low 80s both days. Higher terrains will be in the upper 50s to lower
60s. Otherwise, the remaining days of the period, over 80% of the
ensembles have locations such as the John-Day Basin, north central
OR, the Gorge, Columbia Basin and adjacent valleys in the 70s while
the remainder of the areas are in the mid to upper 60s. Bennese/90


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  42  68  45  62 /   0  20  20  10
ALW  47  71  49  64 /   0  30  30  20
PSC  50  74  52  70 /   0  10  10  10
YKM  44  70  43  67 /   0  20  10   0
HRI  47  73  49  67 /   0  10  10  10
ELN  44  65  43  62 /   0  30  10   0
RDM  39  64  37  57 /   0  20  20   0
LGD  40  66  43  57 /   0  60  60  40
GCD  39  68  41  58 /   0  50  50  30
DLS  49  68  48  64 /   0  30  10  10

&&

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

$$

SHORT TERM...75
LONG TERM....90
AVIATION...82