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000
FXUS66 KPDT 211744 AAA
AFDPDT

Area Forecast Discussion...Updated
National Weather Service Pendleton OR
1044 AM PDT Sun Apr 21 2024

Updated aviation discussion

.UPDATE...
Only minor updates made to current forecast. Wind will decrease
through the day behind the passage of a cold front and become
light tonight. With the exception of the Cascades, generally dry
weather is expected today.

&&

.AVIATION...18Z TAFs...VFR conditions are expected for the next 24
hours. SCT-BKN cirrus at around 25k feet AGL will become FEW250-SKC
this afternoon. KALW and KPDT will have FEW-SCT cumulus at 4K-5K
feet for the next few hours before it dissipates. Columbia Basin
sites will have west to northwest winds at 10-15 kts with gusts to
25 kts this afternoon through around 03Z. Otherwise, winds will
remain below 12 for the next 24 hours. Perry/83

&&

.PREV DISCUSSION... /issued 429 AM PDT Sun Apr 21 2024/

SHORT TERM...Today through Tuesday night...Latest water vapor
imagery shows a sharp upper level trough and vorticity maximum
moving across British Columbia and Washington State. PDX-GEG surface
pressure gradient of 11 mb is resulting in gusty sw-w winds tonight.
The pressure gradient and resultant winds are expected to weaken
over the next several hours. However it will still be breezy to
windy in the Kittitas Valley today with northwest winds of 20-30 mph
and gusts 35-38 mph. The SPC HREF probability of wind gusts
exceeding 40 mph across the Kittitas Valley is 20% or less and the
latest LAMP guidance is forecasting peak gusts of 32-33kt at KELN
this afternoon. Thus will not issue a wind advisory with this
forecast package. Otherwise, expect just a few rain and snow showers
along the Washington Cascade crest today into this evening with POPS
30-50% and snow levels 2500-3500 feet MSL. The NBM probability of 1
inch of snow at Snoqualmie Pass through this evening is 25%.

The next potential impactful weather will be the possibility of a
freeze Monday morning. Winds will be decreasing overnight and
skies clearing. This will provide for good radiational cooling
late tonight. Low temperatures will be close to freezing across
much of the Blue Mountain foothills, Columbia Basin, and Yakima
and Kittitas Valleys. At this time it appears that there is
moderate potential (40-50% chance) that portions of these areas
will need freeze highlights. Otherwise Monday will be dry with
highs in the 60s except 50s in the mountains.

The GEFS and ECMWF ensemble are in excellent agreement on moving
an upper trough to along the coast late Tuesday and Tuesday
night. Southwest flow aloft ahead of the trough will cause an
increase in moisture over the eastern mountains leading to a
slight chance of showers (15-20% chance). Analyzed the potential
for thunderstorms but forecast CAPE is less than 100 J/KG.
Additionally NBM and SREF probability of TSTMS are less than 10%.
Thus have opted not to include thunderstorms in the forecast at
this time. 78

LONG TERM...Wednesday through Sunday...Overall confidence in
forecast details is medium on Wednesday followed by low-medium
confidence during the remainder of the period, especially Saturday
and Sunday, due to significant spread among ensemble solutions.

Wednesday, there is high confidence (>90% chance) that an upper-
level closed low or open wave from the Pacific will push into the
PacNW (26% of members) or BC (74% chance). The former solution would
provide more synoptic forcing for ascent across the forecast area
while the latter would place the best forcing north of our area. The
result is a slight chance (15-24%) of showers for the mountains, and
a very low chance (<15%) of thunderstorms for the Blue Mountains.

An active pattern is then favored Thursday through Friday as a
second wave and developing surface low move into the PacNW from the
Pacific. Ensemble clusters indicate uncertainty in where the system
will track through the period -- either tracking over the Great
Basin (41% of members), W OR (22% of members), NW CA (19% of
members), or Ern OR (18% of members). The last scenario would
provide the wettest outcome for the forecast area while the other
scenarios would result in lighter precipitation.

Significant uncertainty exists in the pattern Saturday and Sunday
with solutions on Sunday ranging from another Pacific shortwave (27%
of members) to broad troughing (22% of members) to dry zonal flow
(23% of members) to a ridge building in from the Pacific (28% of
members).

Additional notes:

1. Low freeze potential for eligible forecast zones (<15% chance)
through the period.

2. Medium-high confidence (60-90% chance) in active weather and at
least light area-wide precipitation Thursday through Friday. Chance
of wetting rain (>= 0.10") for the lower elevations is low (<35%
chance), low-medium (30-50% chance) for the Blues, and medium (40-
70% chance) for the Cascades.

3. Low probabilities (<30% chance) of 24-hr accumulating snow >1"
for mountain passes through the period.

Plunkett/86

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  58  33  62  37 /   0   0   0   0
ALW  61  37  65  40 /   0   0   0   0
PSC  65  37  67  41 /   0   0   0   0
YKM  64  32  66  38 /   0   0   0   0
HRI  64  35  67  38 /   0   0   0   0
ELN  58  33  64  37 /   0   0   0   0
RDM  59  26  66  35 /   0   0   0   0
LGD  56  29  62  35 /   0   0   0   0
GCD  58  31  67  38 /   0   0   0   0
DLS  62  38  68  44 /  10   0   0   0

&&

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

$$

SHORT TERM...77
LONG TERM....86
AVIATION...83