Area Forecast Discussion
Issued by NWS Pendleton, OR

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000
FXUS66 KPDT 240551
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1051 PM PDT Sat Mar 23 2024

Updated Aviation Discussion

.AVIATION...06Z TAFS...Based on the radar data, rain showers are
beginning to weaken. KRDM and KBDN may remain breezy tomorrow
afternoon, bringing winds above 10kts. Currently, there are still
some isolated showers within the vinicity of KPDT, KALW, and KPSC
with more developing on Sunday thus increasing confidence a bit more
on the coverage (50%). CIGs may improve as rain showers continue
dissipating. Feaster/97

&&

.UPDATE...
Latest radar imagery showed shower and isolated thunderstorm
activity from earlier was winding down. Model guidance suggests
additional showers may develop overnight, especially over
northeastern Oregon. Current forecast generally looks on track.
For the evening update, made adjustments to POPS based on latest
radar and model trends and also made some adjustments to overnight
lows where needed.



&&

.PREV DISCUSSION... /issued 457 PM PDT Sat Mar 23 2024/

AVIATION...00Z TAFS...KPDT and KALW are currently VFR but may
decrease to MVFR at 11z as more rain showers enter their vinicity,
bringing down CIGs. KRDM and KBDN are also expected for breezy
conditions at around 22Z, which will have winds above 10kts. Other
than that, the remaining sites will be at VFR with winds below
10kts. However, CIGs could continue to decrease as more isolated
rain showers occur during this period thus lowering confidence due
to timing and coverage of the forecast area (30%). Feaster/97

SHORT TERM...Tonight through Monday...The band of precipitation
and breezy winds associated with a cold front passage earlier
today has lifted north and east of the forecast area this
afternoon. Behind it`s exit, radar and satellite imagery show a
scattered field of showers across the eastern mountains, a mid to
high level cloud shield over central OR, and mostly clear
conditions filling in across the Lower Columbia Basin at this
time. The upper low offshore and increasing instability from
daytime heating will support the development of scattered showers
and a few isolated thunderstorms across much of northeast OR and
portions of the southern WA Cascades and far southeast WA this
afternoon(confidence 55-65%). HREF members are showing surface
CAPE only increasing to between 250-350 J/kg with 0-6km shear of
30-50kts over northern Blues and areas east of there.
Additionally, deterministic guidance and the RAP indicate low and
mid level lapse rates increasing to 8 C/km, which will provide
some aid in initial uplift. While most of the developing showers
will produce rain, mountain areas generally above 4.5kft to 5kft
can expect to see snow. Shower activity will wind down into
tonight while the upper low opens into a trough as it dives into
CA, however a few shortwave impulses ejecting into eastern OR will
keep showers going over the eastern mountains with an a slight
chance of clap thunder as well.

Tomorrow, a weak shortwave is expected to dive across the PacNW in
northerly flow aloft, providing a chance of rain and mountain snow
showers through the afternoon, with the heaviest showers expected
to develop over the northern Blues. Snow amounts will generally
remain under 3 inches (confidence 60-70%), though an isolated
heavy snow shower or two could bring locally 4 to 6 inches in the
higher terrain (confidence 20%). The shortwave passage will also
result in a pressure gradient tightening in the afternoon and
evening across the Cascades, resulting in breezy west winds
through the Eastern Columbia Gorge and northwest winds through the
Kittitas valley. Strong winds are not expected, but wind gusts
between 30-40mph will be possible in the breeziest areas. Late
Sunday into Monday, a transient upper ridge will slide into the
PacNW, though flow aloft will mostly remain from the west to
northwest. While conditions are expected to dry out in the lower
elevations, the northwest flow aloft will allow for light snow
flurries to continue across the Cascade crest and the eastern
mountains.

Monday, the last vestiges of a cold front boundary and shortwave
aloft will slide down the BC coast into the PacNW. This will help
to increase chances of rain showers and mountain snow showers
throughout the day. However, any rain/snow accumulations will be
quite limited as much of the moisture associated with this system
will precipitate out west of the Cascades.

Temperatures will continue a gradual cooling trend into early
next week. While afternoon temperatures will be in the mid 50s to
lower 60s across the northern half of the forecast area, areas of
central OR and mountain valleys will struggle to warm out of the
mid to upper 40s tomorrow and Monday. Morning lows will be in the
30s, with 20s to low 30s in mountain valleys. Lawhorn/82

LONG TERM...Tuesday through Saturday...Ensembles are coming into
better agreement on the synoptic pattern through the next weekend.
Overall trend continues to be cool and wet, however some previous
discrepancies have been resolved, and the only major uncertainties
lie at the tail end of the period, stemming from how models resolve
the evolution of yet another broad low impacting the PacNW.

Tuesday`s weather looks fairly benign as transitory ridging builds
in behind light NW flow aloft. This NW flow may trigger some light
mountain showers (confidence 30-40%) with snow levels in the 3000-
4000 ft range, but given how weak the flow aloft is, not
anticipating anything particularly wet. Ensembles then seem to
suggest that the forecast area will lie on the apex of the
transitory ridge as a broad low moves in swiftly from behind.
Wednesday and Thursday look to be the wettest days of the period as
a result, as the low ushers in SW flow via its SE quadrant.
Confidence is moderate to high (50-60%) in an area-wide precip event
around this timeframe, based primarily on ensemble consensus this
far out. Snow levels start out warm around 4000-5500 feet before
dropping down to around 3000-4000 feet as the low pulls in colder
air into the forecast area. Confidence in QPF is low this far out
(<30%), but wouldn`t be surprised if the lower elevations see
another wetting rain with this system.

Ensembles suggest that this low will linger about the western CONUS
heading into next weekend, digging down towards California starting
around Friday. This would result in colder temperatures as the flow
aloft becomes more northerly, but would limit precip chances
primarily to the mountains, albeit with light amounts. Guidance then
diverges with regards to this low as it cuts off, disagreeing on how
fast it progresses downstream, as well as what lies behind it. Most
ensemble members seem to suggest ridging behind the low, however a
minority of members keep the low stagnant over the desert SW,
leaving us under the influence of cold northerly flow aloft beyond
the period. Evans/74

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  41  51  36  52 /  60  70  30  40
ALW  42  52  38  55 /  30  80  40  50
PSC  43  59  39  59 /  30  40  10  20
YKM  37  59  33  57 /  10  20  10  10
HRI  41  57  39  57 /  50  50  30  30
ELN  36  55  32  54 /  20  20  10  10
RDM  31  50  32  49 /  40  20  10  20
LGD  36  46  32  48 /  60  70  40  40
GCD  34  49  33  48 /  50  60  40  50
DLS  42  58  41  56 /  30  20  10  30

&&

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

$$

SHORT TERM...77
LONG TERM....74
AVIATION...97


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