169
FXUS66 KPDT 300946
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
246 AM PDT Tue Apr 30 2024

.SHORT TERM...Today through Thursday...Nighttime satellite imagery
tonight is showing a vertically stacked low pressure system about
200 miles due west of the WA coast, while scattered low level
clouds cover the Blues and Wallowas, and low to mid level cloud
cover developing along the Cascade crest. Meanwhile, radar imagery
is already showing showers pushing across the Western WA and
northwest OR ahead of the upper low offshore.

Today, a weakening upper low with an attendant surface low will
push across the PacNW with a renewed chance of rain and mountain
snow showers across the forecast area. The plume of moisture
associated with this system will mostly be guided across northern
OR throughout the day. NBM probabilities show a 50-60% chance of 2
inches or more across the Wallowas and peaks of the southern
Blues, while the probability of exceeding 4 inches along the
Cascade crest is 60-70%. Increasing surface CAPE and modest low
level lapse rates will once again aid in isolated thunderstorm
development across much of northeast OR, far southeast WA, and the
WA Cascade east slopes. Surface based CAPE is forecast to reach
between 200-400J/kg this afternoon, with HREF probabilities for
250J/kg around 55-65% centered along the OR/WA border, and along
the WA Cascade east slopes. The approach of the surface low will
result in breezy west winds developing by this afternoon through
the Cascade gaps and lower elevations, though winds this time
around will be weaker than yesterday, with sustained winds
15-25mph and gusts up to 35mph.

Drier conditions are in store for the forecast area Wednesday as a
dry north to northwest flow setups behind the upper low exit. That
said, the upper low now over eastern MT will guide wrap around
moisture across the northern Blues and Wallowa county during the
day, resulting in light snow showers in these areas. Otherwise, a
cross Cascade gradient will persist through Wednesday with breezy
west to northwest winds up to 20mph and gusts up to 30mph
developing through the Cascade gaps and into the Columbia Basin,
with the strongest winds(around 25 mph and gusts up to 35mph) in
the Kittitas valley.

Thursday, a weak upper low will dive through the northwest flow
aloft and into the PacNW. Current model guidance is showing the
low track into the PacNW near the mouth of the Columbia, then
track across central and eastern OR through the afternoon. This
low is expected to bring rain and high mountain snow showers, with
the OR Cascades once again favored for the heaviest precip with
this system. NBM probabilities suggest that the OR Cascades will
see the best chances of precip, with the probability of 0.1 inches
of QPF between 70-80%; across the eastern mountains and WA
Cascades probabilities are 35-45% for 0.1 inches of QPF. Unlike
the previous systems, this low will have little impact on winds
Thursday, with locally breezy conditions forecast.

The lowest temperatures into the weekend are expected this
morning/Today, with afternoon highs in the upper 40s to 50s...30s
mid 40s mountains. As for Wednesday and Thursday, a slight warming
trend will resulting in mid 50s to 60s across the lower
elevations in the afternoons. Wednesday morning lows will dip into
the upper 20s to low 30s across central OR, John Day Basin, and
the Grande Ronde/Wallowa valleys, and mid to upper 30s across the
remainder of the lower elevations. As for Thursday, expect more
warming with lows in the mid 30s to low 40s in the lower
elevations. Lawhorn/82

.LONG TERM...Friday through Tuesday...While model discrepancies
continue to complicate the long term forecast, the overall picture
becomes clearer with each subsequent set of model runs. Overall, the
forecast area is looking at a dry Friday, a wet weekend, and a
cooling trend to kick off the work week next week.

Ensembles are in strong agreement on Friday being a day of
transitory ridging as a broad low over the northern Plains shifts
out toward the east. Friday will be the warmest day of the period as
a result, with high temps rising into the upper 60s and into the mid
70s across our lowest elevations. These quiet conditions won`t last
long, however, as models then bring in a a closed low pressure
system at the start of the weekend. Biggest challenge of the
forecast period continues to be tracking the trajectory of this low,
which will ultimately determine the extent of the impacts we see
over the weekend.

The ECMWF deterministic continues to favor a more northwesterly
approach for the weekend low, while the GFS dives the low more to
our south, making for more S/SW flow aloft. The former solution
would be cooler, and result in another potential round of high
mountain snow, while the latter would be more favorable for
thunderstorm development, especially with the warm and dry
conditions expected on Friday. It should be noted, however, that a
majority of ensemble solutions, based on ensemble clustering, favor
the Euro solution of a more northwesterly approach. The NBM seems to
be consistent with this as well, as temps peak on Friday before
cooling down each subsequent day. The NBM did depict patches of
isolated thunderstorm activity across the eastern mountains over the
weekend, but with ensembles favoring the colder solution, elected to
remove any mention of storms in the forecast. Either way, both
Saturday and Sunday look to be wet as this low moves in, with NBM
probabilistic guidance suggesting a 40-60% chance of at least 0.1
inches of rain falling across our lower elevation/population zones,
and a 70-80% chance of the same amount of precip for our mountain
zones. Expect gusty conditions to coincide with the onset of precip
as well.

Models aren`t in tremendous agreement on what occurs after this
weekend system, but broadly favor a cooling pattern brought about by
an intrusion of colder air into the PacNW via troughing. How this
troughing materializes is up in the air, but some ensemble solutions
suggest that the weekend low may deepen and become more broad,
allowing for colder air to seep in, or that another system from the
northwest encroaches upon the region and advects cold air in that
way. Either way, confidence is on the lower end (20-30%) with
regards to how the synoptic pattern shapes up Monday onward, but
unseasonably cool temps do appear to be in store for us next week.
Evans/74


&&

.AVIATION...12z TAFs...VFR conditions expected for the period.
Another round of breezy winds and light showers is expected for
today, however winds will be much lighter and the chances for precip
much smaller than yesterday. Winds out of the W/NW will gust up to
25 kts across most sites this afternoon, while light shower activity
will develop through the daytime hours. Any shower that forms near
or at a site is expected to be too light to cause sub-VFR
conditions, however. Precip chances will end by nightfall, and winds
are expected to decrease to less than 10 kts by the overnight hours
heading into Wednesday. Cigs generally 5-10 kft bkn-ovc, lifting to
the mid levels and becoming more sct by the evening. Evans/74


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  54  35  60  36 /  30  40   0  10
ALW  57  38  62  39 /  30  40  10  10
PSC  60  40  67  43 /  10  20   0  10
YKM  55  33  64  38 /  40  20   0  10
HRI  59  38  65  39 /  20  30   0  10
ELN  53  35  59  36 /  30  20   0  10
RDM  49  27  55  33 /  30  20   0  40
LGD  48  33  54  32 /  60  60  20  10
GCD  50  32  55  33 /  60  70  10  20
DLS  55  41  62  43 /  70  30   0  30

&&

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

$$

SHORT TERM...82
LONG TERM....74
AVIATION...74