341
FXUS66 KPDT 022315
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
415 PM PDT Thu May 2 2024

.Updated for Aviation...

.AVIATION...
00Z TAFS...VFR conditions are expected through the TAF period.
Due to the presence of some isolated -SHRA winds in some areas
are a bit gusty, but these should subside in the next hour or two.
Winds are expected to be 10 kts or less through the period.


&&

.PREV DISCUSSION... /issued 249 PM PDT Thu May 2 2024/

SHORT TERM...Tonight through Sunday...Deterministic models and
ensembles are in good agreement on the 500 mb longwave pattern
through the short term though subtle differences in the position
of an upper low on Saturday night and Sunday will determine where
the axis of heaviest QPF will set up.

For this evening scattered rain and snow showers over the central
mountains (20-45% chance) will diminish this evening before
ending after sunset. Friday will be dry as a shortwave ridge
moves across the area downstream of an approaching upper low.
High temperatures will be in the mid 60s to mid 70s with mid 50s
to mid 60s mountains.

The GEFS and ECMWF ensembles are in good agreement on moving an
upper low from off the Washington coast Friday night to southwest
Oregon into NRN CA and then to NRN NV on Sunday. The ensemble
clusters agree for the most part on this track though a couple of
the clusters are a bit farther north with the low. The ultimate
track of this low will modulate the QPF across the region. The GFS
ensemble shows a Weak AR with this system so there is a good
potential for moderate to heavy QPF. At this time it appears that
the heaviest precipitation will occur over northern and eastern
Oregon, southern Washington and along the Cascade east slopes.
Used the WPC QPF for this forecast which yields 48 hour amounts
(ending 12Z Monday) of 1-2 inches across the northeast mountains
and east slopes and .7-1.1 inches elsewhere. Areas downwind of the
Oregon east slopes are forecast to have a bit less QPF (.4-.5
inches) due to rain shadowing. These values are close to the 90th
percentile NBM values and seem reasonable to use given that the
EFI is depicting the potential for a climatologically outlier
event.

Initially there will be little concern for snow with snow levels
Friday night and Saturday above pass level. However Saturday night
snow levels across central Oregon will lower to 3500-4500 feet and
remain there through the day on Sunday. This will result in some
higher mountain snow with the probability of 4 or more inches of
snow Sunday through Sunday night of 40-65% for the eastern
mountains and the Oregon east slopes (highest probabilities over
the southern Blue Mountains).

The other sensible weather concern is strong and gusty winds. The
first period of interest is Friday night and Saturday along the
northern Blue Mountain Foothills of Oregon. NBM is showing
45-50 kt SELY wind gusts along the base of the Blue Mountains.
This could be a bit high given that the KBKE to KMEH pressure
gradient is forecast to be only 5-6 mb . Nonetheless there is
moderate potential (40-50% chance) for at least wind advisory
level winds along the base of the Blues during this period. The
second period of more widespread stronger winds across much of the
lower elevations will be on Saturday night and Sunday when winds
become westerly. Deterministic NBM is predicting 50 mph wind
gusts at KPDT at 12Z Sunday morning. NBM 24 hour probabilities
(ending 06Z Monday) of 50+ mph gusts are 30 to 70% for much of
the lower elevations. 78

LONG TERM...Sunday night to Thursday...Early next week will be
marked by somewhat unsettled weather with the greatest precipitation
chances tied to the mountains followed by a drying, warming trend
that is becoming increasingly more likely than not Thursday onward.
The main sensible weather highlights include:
- Continued mostly beneficial rain that will be tied mainly to the
mountains Sunday night through Tuesday,
- Cooler than normal highs, 5-15 degrees below normal, Monday to
Wednesday, and
- Near seasonable temperatures Thursday with low chances (15-30%) in
the high mountains.
- Modest rises on mountain rivers and streams with high confidence
(>85%) in flooding not occurring.

Ensemble guidance is in fair agreement initially with the synoptic
pattern to start the long term. Early Sunday night will be
characterized by a weakening upper-level closed low over southern ID
that will be ejecting east as an open wave over the Northern and
Central Rockies Monday. Initial clustering scenarios shows the
spread is driven by position/timing of this feature more so than
amplitude Sunday night-Monday with nearly double the spatial
variance (33%) amongst members. Much of the disagreement is centered
in this open waves position as it evolves east from near the
Central Great Basin to the Northern Rockies area. While the greatest
12-hr QPF in the long term falls on Sunday night, much of this
rainfall stems from residual moisture in place and the weak AR event
that will be winding down by early Sunday night. Of which,
confidence in wetting rain (>0.1) is moderate (50-70%) and confined
mainly to the upper eastern slopes of the WA/OR cascades and the
Blues Sunday night. Elsewhere, chances are low (15%-40%) for wetting
rain across the Lower Basin into the foothills of the Blues.

A second round of precipitation is anticipated Monday with chances
ramping up in the afternoon. This is attributable to a weak surge of
moisture associated with a leading shortwave trough ejecting east
across WA out of a larger longwave trough that is centered over the
Gulf of Alaska. Increased chances of 50-90% across the crest of the
Cascades and eastern mountains for more than a tenth of an inch
Monday afternoon/early Monday night. That said, there is modest
potential for higher amounts as denoted by EPS EFI exceeding 0.6
across the Blues. Raw ECMWF ensemble shows the 50th percentile at
Meacham near 0.25 over a 24-hr period ending 12Z Tuesday. Meantime,
a modest corridor of 500 mb winds will be adjacent south and west of
the area with a decent westerly 850 mb jet stream directed across
the CWA Monday. Current thinking is winds will stay elevated Sunday
night and carry/strengthen some Monday. The winds aloft in tandem
with a modest cross-Cascade pressure gradient during the day will
help promote another breezy day on Monday with the deterministic GFS
and mean GEFS showing a cross-Cascade pressure difference of at
least 10 mb. This is increasing confidence in sub-advisory wind
gusts (50-70%; up to 40 mph). Of which, max 24-hr daily peak gusts
for the NBM portray chance for exceeding 45 mph is 40-70% across the
eastern Gorge, OR Lower Basin into the foothills of the Blues and
Klickitat. However, raw EPS has scant-limited areal coverage for
chances of exceeding 34 kts/39 mph Monday.

Northwest flow is expected to remain in the wake of the passing
shortwave trough Tuesday. Of which, high chances (60-90%) will
return late confined mainly to the Cascade crest and northern Blues.
This is in part due to potential for embedded disturbances within
the mid-level flow. That said, moisture is limited and QPF is light
overall.

A drying trend is anticipated to start with only moderate confidence
in the forecast (50%) at this time. This revolves around the
strength of the building ridge in the eastern North Pacific and the
potential for lingering cyclonic flow Wednesday. This stems from the
scenario of Mondays shortwave trough elongating the upper trough
across the central CONUS into the southwest/western CONUS. A weaker
upper trough, stronger ridge scenario would be slightly warmer
compared to the cyclonic flow case; impacts will be scant regardless
of what pans out. Ultimately, this uncertainty translates then to
Thursday with gudience and 500 mb means showing a leaning for this
upper ridge to build and evolve east to extend across Vancouver
Island. However, there is appreciable disagreement with the location
of the upper ridge axis based on clustering scenarios with over 8 dm
of spread amongst ensemble member across the western CONUS. The
scenarios breakdown into near 50:50 split between a more
elongated/stronger upper trough extending southwestward across the
western CONUS and a stronger upper ridge displaced more east into
the West Coast. The latter warmer and offers low chances for precip,
while the former would be cooler and promote increased chances
(>30%) for precip in the mountains.

AVIATION...18Z TAFS...VFR conditions during this period with
currently cloudy skies (5-25 kft), except for KYKM having clear
skies. However, cloud coverage will decrease for most sites tonight
into Friday morning, excluding KYKM where it will increase.
Confidence was very low (10-15%) at first to include wind gusts for
KRDM/KBDN, but due to the surface pressure gradient weakening as the
low passes, confidence level rose higher for that possibility (70%).
Winds will be less than 10kts. Feaster/97

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  35  70  47  61 /   0   0  10  60
ALW  41  73  53  69 /   0   0  10  50
PSC  42  74  53  68 /   0   0  10  50
YKM  37  70  46  61 /   0   0  40  80
HRI  39  75  49  64 /   0   0  20  60
ELN  37  68  45  61 /   0  10  30  70
RDM  30  65  41  48 /   0  10  90  80
LGD  31  65  47  63 /  10   0  10  50
GCD  32  63  45  56 /  50   0  30  80
DLS  42  71  49  55 /  10  10  70  80

&&

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

$$

SHORT TERM...78
LONG TERM....80
AVIATION...77