165
FXUS66 KPDT 290300
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
800 PM PDT Sun Apr 28 2024

.UPDATE...
Latest radar showed some precipitation over the Cascades and some
isolated precipitation over the Blue Mountains. Based on radar
trends and latest guidance, have decreased pops over the Blue
Mountains tonight. Otherwise raised overnight lows in several
locations due to the latest guidance. However, general forecast
looks good with pops increasing over the entire area toward
morning and into the day Monday.




&&

.PREV DISCUSSION... /issued 447 PM PDT Sun Apr 28 2024/

Updated for Aviation...

AVIATION...
00Z TAFS...VFR conditions are expected at all TAF sites through
the period. Precipitation will move in toward Monday morning at
DLS, BDN, RDm. DLS will see all -RA while BDN and RDM could see a
mixture of -RASN at the onset.

The other concern is gusty winds All sites are experiencing gusty
winds, at least occasionally late this afternoon that are
expected to continue into early evening. Winds should gust 20 to
30 kts. Winds should then decrease to around 10 kts after 02-04Z.

Winds are expected to increase again on Monday morning through
early afternoon and gust to around 25 kts. High confidence
(90-100%).

PREV DISCUSSION... /issued 329 PM PDT Sun Apr 28 2024/

SHORT TERM...Today through Tuesday night...Satellite imagery
shows a broad area of low pressure with two distinct circulation
centers off the British Columbia coast. Ensemble guidance is in
excellent agreement (>99% chance) that this system will impact
the PacNW tonight through Tuesday. The first shortwave and jet
max will track into the PacNW tonight through Monday, facilitating
mountain snowfall and widespread light precipitation for the
lower elevations. Synoptic support from the jet max appears most
robust for the central OR Cascades, and forecast precipitation and
snowfall reflect this outcome. To touch on snowfall, NBM
probabilities of 5 inches of snowfall for the Cascades range from
60-90% for the central OR Cascades, including over Santiam Pass,
while probabilities are lower for the northern OR Cascades and the
WA Cascades (20-50% chance of 6 inches). Winter Weather Advisories
remain in effect 11PM this evening through 11AM Monday morning for
the Upper Slopes of the Eastern Washington Cascades (above 3500
feet), and the East Slopes of the Oregon Cascades (above 4000
feet). The westerly flow aloft is supportive of a strong rain
shadow to the lee of the crest so snowfall totals along US-97
south of Bend will be lower (<1").

Breezy to windy westerly winds will accompany the system on Monday,
though forecast surface pressure gradients (6-9 mb, locally 10 mb
PDX-GEG) do not appear supportive of widespread advisory-level winds
Monday morning through evening. Moreover, 12Z HREF members are
advertising widespread breezy to windy westerly winds, but ensemble
maximum wind gusts fail to exceed 45 mph from any member. Hence,
have held off on any wind highlights as confidence in sustained
advisory-level winds is low (<50% chance).

The second wave, a weakening closed upper low and attendant
surface low, is forecast to swing into the PacNW Monday night
through Tuesday. While confidence is very high (>95% chance) that we
will see another round of precipitation, including light snowfall
for the mountains and possibly a light snow shower (<50% chance) of
an inch or less for the lower elevations (1500-2000 feet elevation),
it is worth noting that there is still some ensemble spread
regarding the track the low takes as it pushes into the PacNW. The
most likely solution will once again favor the OR Cascade crest for
heaviest precipitation from a synoptic standpoint. As the upper low
moves overhead Tuesday afternoon, there is a low chance (10-15%)
that there will be a thunderstorm across the region as steep lapse
rates and daytime surface heating allow modest (<500 J/kg) SBCAPE.
COnfidence on location is low given the aforementioned spread in the
track of the low and the inherent randomness of convection.
Plunkett/86

LONG TERM...Wednesday through Sunday...The long term continues
to still be a bit out of sorts when it comes to the latter half of
the period however, guidance has become a bit more in agreement
regarding the first portion. With this development, confidence in
the forecast for at least Wednesday through Friday has increased.
Confidence steadily decreases through the period with
moderate/high (70-80%) Wednesday and Thursday, moderate Friday
(40-60%) and low over the weekend.

Wednesday through Thursday the guidance is in somewhat of an
agreement between the GFS, Euro and the NAM showing the upper
level trough exiting the region and putting the region under
primarily northerly flow. The next upper level low begins to slip
down the Canadian coast Thursday morning flattening the flow to a
more zonal flow before moving into the region. The main variances
within the models is the timing and positioning of the next
incoming upper level low. This will play a part in how much
precipitation will be seen. NBM ensembles have the POPs mostly
over the Cascades Wednesday before spreading to the Northern Blues
with a minimal increase across the low elevations. Raw ensembles
put 30-50% POPs over the Cascades and 30-40% over the Northern
Blues with 30-50% probabilities of up to 0.05 inches of rain. Snow
levels will be a above 4000 ft so very little snow accumulations
expected with this round. Winds will be diurnally breezy NNW winds
increasing slightly Thursday as the low crosses the Cascades. EFI
has picked up on the cooler air associated with this low and
shows a few areas in east central OR as well as the Columbia Basin
having slightly below normal seasonal temperatures with the
ensembles showing temperatures to be in the mid 60s. Ensembles
show the mid elevations to be in the low to mid 50s while the
upper elevations will be in the low to mid 40s.

Friday models are still in some agreement with the upper level low
mostly to the east and settled over the eastern mountains while the
leading edge of the upper level ridge pushes into the PacNW.  The
ridge will bring with it drying conditions Friday. Main variances
in the guidance is the amplitude of the ridge as it passes over.
EFI shows that the temperatures will begin to moderate back
towards near normal with ensembles showing the lower elevations
back in the low 70s, mid elevations in the upper 60s and the
higher elevations in the low 50s.

Moving to the weekend, models and clusters become a bit skewed
with the next incoming synoptic pattern. Some have the ridge
remaining in place over the region while others have a tight
closed low slipping into the PacNW with enhanced chances for
precipitation. Forecast confidence over the weekend continues to
be low (15-30%). Bennese/90

AVIATION...18Z TAFs...VFR conditions expected despite the
increase in cloud cover incoming with the shortwave trough.
Precipitation is expected to crest the Cascades and reach
BDN/RDM/DLS between 12-15Z with 60-80% probabilities of rain in
DLS and a 50-60% for RASN mix at BDN/RDM. CIGs will continue to to
decrease through the period to 0VC050 and winds will continue to
be breezy with sustained winds between 9-15 kts with gusts of
15-22 kts at all until this evening. Bennese/90

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  39  54  33  55 /  10  70  10  40
ALW  42  56  37  57 /  20  80  30  50
PSC  45  62  40  62 /  10  40  10  20
YKM  38  58  33  60 /   0  40   0  20
HRI  42  60  37  61 /  10  50  10  20
ELN  39  54  32  57 /  20  30   0  20
RDM  31  49  27  51 /  10  50   0  20
LGD  36  49  30  49 /  20  80  30  60
GCD  34  49  28  50 /  10  70  10  40
DLS  43  57  38  57 /  30  60  10  40

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 11 PM this evening to 11 AM PDT
     Monday for ORZ509.

WA...Winter Weather Advisory from 11 PM this evening to 11 AM PDT
     Monday for WAZ522.

&&

$$

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