Area Forecast Discussion

from: NWS Pendleton, OR

FXUS66 KPDT 280545 AAA

Area Forecast Discussion
National Weather Service Pendleton OR
1039 PM PDT Mon Sep 27 2021

updated the aviation discussion

.SHORT TERM...The latest ir satl was showing a large area of light
to moderate pcpn over north central Oregon and south central WA
with locally moderate to heavy pcpn along the foothills of the
blue mountains. Further west along the wa/or cascades the pcpn
area lessens. Meanwhile the entire area can expect showers to
persist overnight especially in the eastern sections of the region
and the forecast was updated to increase pops along the foothills
of the blue mountains and eastward tonight. Toward morning somewhat
drier air will work its way into the area from the west.

Otherwise minor changes were also performed on pops and sky and
pcpn amounts and the present short term forecast appears on track
for tonight.

.AVIATION...An area of light to moderate rain will continue to affect
taf sites kpdt and kalw until 08z and lcl mvfr conditions are
possible. Meanwhile mostly vfr conditions should prevail at all taf
sites despite showers into Tuesday. Sfc wind gusts between 20 to
25 kts are possible for taf sites kpdt...kalw after 08z.


.PREV DISCUSSION... /issued 455 PM PDT Mon Sep 27 2021/

SHORT TERM...Tonight through Wednesday...Satellite and radar
imagery this afternoon is showing a cold front slowly moving
across the PacNW with showers developing across the eastern half
of the forecast area. Meanwhile surface observations are
indicating that the frontal boundary at the surface has moved
across the central OR and the OR/WA Columbia Basin. An upper level
jet that is parallel to the frontal boundary will continue to slow
the system this afternoon. Behind the surface frontal boundary, a
wide rain band will slowly move across the forecast area into
this evening, providing a chance of rain across most of the
forecast area. Breezy westerly winds will also persist into this
evening across those areas that have seen the frontal boundary
come through. Behind the rain band, precipitation chances will
begin to trail off across the Columbia Basin and adjacent valleys,
while the Cascades and eastern mountains will continue to see
rain and possibly snow impacts well into tomorrow. Snow levels
will begin to drop to about 5,500 feet across the forecast area
tonight, as a cold airmass infiltrates into the region behind the
frontal passage.

Following closely behind the cold front, an upper level trough
will move across the forecast area starting early tomorrow. This
will help sustain precipitation chances across the mountains and
foothill locations, however the Basin and adjacent valleys, as
well as portions of central OR will likely remain dry tomorrow.
Breezy westerly winds will redevelop across the Basin and north
central OR tomorrow afternoon as the upper trough axis passes
overhead. Behind the trough axis passage late tomorrow afternoon,
precipitation chances in the mountains and the breezy westerly
winds will begin to diminish, becoming dry and calm by midnight.

Wednesday, a ridge of high pressure will build into the region
from the northern CA, providing mostly dry and calm conditions
across the forecast area. However, by Wednesday afternoon, a
shortwave trough swinging into central BC will provide increasing
cloud cover from the west and a chance of rain showers across the
WA Cascades that will spill over into portions of the WA east

Temperatures behind the frontal passage starting tonight will
become below normal as a colder airmass advects into the
intermountain northwest. Afternoon highs tomorrow will mostly
be in the upper 50s to 60s, though in the mid 40s to mid 50s in
the mountains. Overnight lows Wednesday morning are expected to
be the coldest for the week, with 30s to low 40s expected across
much of the forecast area. Wednesday, afternoon highs will
continue to be below normal, with temperatures mainly in the 60s,
and 50s in the mountains. Lawhorn/82

LONG TERM...Thursday through Monday...Another upper-level low is
forecasted to pass over the CWA on Thursday. Guidance in good
agreement on depicting this system, however QPF ensembles not
producing much east of the Cascades. EPS a bit wetter than GEFS,
however wouldn`t be surprised, given the system`s northeastward
trajectory, if the lower Basin and central OR stay mostly dry
through the progression of this low. In the WA Cascade crests,
ensembles range between a half inch to an inch of precip, with very
little if any rain over the OR Cascades. Even most of the Blues
outside the northern portion along SE WA look to see only isolated
light showers at best. Overall, sensible weather impacts look to be
pretty limited with this system.

Beyond Thursday, the majority of the long term looks to be quiet as
ridging builds overhead behind the exiting low. Ensembles differ on
the position of the ridge`s axis and the timing of its
amplification, but overall pattern looks to keep us dry with warmer
than normal high temperatures. Weekend highs will depend heavily on
how the ridge develops, but looking at low to mid 70s across much of
the forecast area to start off October. A few ensemble members
depict a weak cutoff low offshore over the weekend, which may inject
some Pacific moisture aloft and impact temps, but looks to be weak
enough to avoid any significant impacts on the forecast. 74


PDT  45  64  39  67 /  90  20  10   0
ALW  47  65  42  69 / 100  30  10   0
PSC  51  68  43  69 /  90  20   0   0
YKM  40  64  37  65 /  20  20   0  10
HRI  47  67  40  70 /  90  10   0   0
ELN  41  60  38  61 /  40  30  10  10
RDM  37  59  30  69 /  80  20   0   0
LGD  42  58  37  64 /  90  50  10   0
GCD  40  58  35  67 /  90  30  10   0
DLS  49  65  44  70 /  70  40  10  10





NWS PDT Office Area Forecast Discussion