120
FXUS66 KPDT 161129 AAA
AFDPDT

Area Forecast Discussion...Updated
National Weather Service Pendleton OR
429 AM PDT Fri Aug 16 2024

Updated aviation discussion

.SHORT TERM...Today through Sunday...Skies are beginning to clear
out across the forecast area after a day of scattered showers and
thunderstorms over the eastern mountains. For today, we`ll be left
in the wake of an upper-level low with dry SW flow aloft, before a
more robust low pressure system centers itself off the PacNW coast
over the weekend and advects moisture across the western half of the
forecast area, while the eastern half sees more southerly flow, and
thus an influx of hot and dry air. This will make for an active
Saturday, with showers and storms across the Cascades and east
slopes, as well as the potential for critical fire weather
conditions across the southeastern portion of the area as winds pick
up under a very dry air mass.

For Saturday, as the aforementioned low drops down from the Gulf of
Alaska, a band of amplified SW to S flow aloft accompanying it will
usher in formidable moisture transport across the spine of the
Cascades, while picking up on hot and dry desert air from the Great
Basin and spreading it into eastern Oregon. We`ll have quite the
stark contrast in the forecast as a result, with showers,
potentially heavy at times, along with thunderstorms across the
Cascades, spreading into the east slopes at times in the afternoon
and early evening. Not particularly concerned about critical fire
weather conditions across our Cascade zones with how wet storms will
be (QPF guidance suggesting widespread >0.20 rain accumulations,
thus the Flash Flood Watch for our WA Cascade burn scars), but more
for our east slope zones like WA690 and parts of 691 should
convective activity spread that far eastward. Confidence is too low
to pull the trigger at this time, especially with the 06z HRRR
suggesting limited activity outside of high terrain, and CAPE being
limited enough as is. Instability won`t be a limiting factor across
the mountains with how favorable the flow aloft is, but may be
lacking once you trek further into the east slopes and beyond.

Guidance has trended upward for temps across the eastern half of the
forecast area, depicting more robust warm air advection as temps in
the lower Basin flirt with the upper 90s. This can be seen in the
drop in forecasted RHs as well, with the high terrain across south-
central and southeastern Oregon bottoming out in the single digits.
This, combined with potent flow aloft, will trigger breezy
conditions and thus critical fire weather conditions in spots.
Issued a Fire Weather Watch for OR 642 as a result. Winds don`t look
to be quite strong enough elsewhere, but with the weather pattern
shaping up to be as it is, unstable conditions for 642 and nearby
zones cannot be ruled out as well.

Once this low settles in off the coast, a blocking pattern keeps it
in place for the next several days, however the upper-level flow
backs off, leaving Saturday as the main day of impacts. Temps cool
down on Sunday back towards seasonal norms, what with more mild SW
flow aloft overhead. Evans/74

.LONG TERM...Monday through Friday...Models continue to show very
good agreement in having a persistent weather pattern over the
western states next week at least through Thursday. A trough and
strong closed low will be off the Pacific Northwest coast and a
strong ridge will be centered over the Rockies and the Midwestern
states. The trough will keep temperatures several degrees below
normal and send a southwest flow over our area. Instability looks
limited with NBM thunder probabilities no more than 10 percent
through the period. The NBM does give 15-30 percent pops along the
Washington Cascade crest and have a slight chance to chance of rain
showers there each day Tuesday through Friday while the rest of the
area remains dry.

Monday will start out with the trough and its upper low a few
hundred miles off the Olympic Peninsula and by Tuesday the low will
be slightly weaker and will have moved south to the Oregon coast.
Weather will be fairly similar each day. Temperatures will be a
little warmer Monday with highs in the 80s to around 90 in the Tri-
Cities while the mountains will be in the mid 70s to mid 80s. NBM
probabilities give a 65-85 percent chance of 90 degree temperatures
in the Columbia Basin and the Grande Ronde Valley, a 40-50 percent
chance in the Blue Mountain Foothills but less than a 25 percent
chance elsewhere. Tuesday will be a degree or two cooler. This drops
the chances of 90 degree temperatures to 45-60 percent in the
Columbia Basin and less than 30 percent elsewhere. The NBM gives a
less than 10 percent of showers across the area Monday and most of
Tuesday, but by late afternoon the low moves closer to the coast and
this introduces a 20 percent chance of showers along the Washington
Cascade crest with barely measurable amounts. Pressure gradients
each day support a mild marine push through the Cascade gaps
yielding 15-20 mph winds in the Columbia Gorge and Kittitas Valley.
The chances for 25 mph winds in those areas are less than 20 percent
both days.

On Wednesday, models show the upper low moving ashore and getting
absorbed into the main flow but at the same time, a shortwave will
dive south out of British Columbia. This will restrengthen the
trough overnight and Thursday after it briefly weakens Wednesday
afternoon. So this will mean only minor changes in the weather.
Temperatures both days drop to the upper 70s to mid 80s with 70s in
the mountains. The Tri-Cities will have a 30-35 percent chance of 90
degree temperatures but the rest of the area will have less than 15
percent chance. Precipitation will continue to be limited to a
slight chance of showers with very light amounts along the
Washington Cascade crest. The Cascade gaps will have 15-20 mph winds
Wednesday afternoon though chances of 25 mph winds are less than 20
percent. Winds will be lighter on Thursday.

On Friday, models develop significant differences about the offshore
trough. Model ensemble members are fairly evenly split about having a
strong low, a weak low or one somewhere in between. They also vary
the low location from about 200 to 700 miles offshore. Additionally,
about 15 percent open the low into a wave and have a shallow trough
centered over the Willamette Valley. The forecast follows the NBM
and keeps temperatures similar to Thursday, maybe a degree or so in
places. It keeps a slight chance of showers along the Washington
Cascade crest once again while the rest of the area is dry.
Perry/83

&&

.AVIATION...12Z TAFs...VFR conditions are expected for the next 24
hours. Mainly clear skies will become FEW-SCT at 150-250 this
afternoon and tonight. An exception will be KBDN which has been
having FEW-SCT at 040-050 and expect that will continue through 18Z
before clearing. Also expect skies to become BKN150 at KRDM and KBDN
after 06Z. Winds will be mainly below 10 kts though KDLS will become
northwest at 10-15 kts with gusts to 25 kts from 18Z-04Z. Perry/83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  82  54  92  55 /   0   0  10  20
ALW  86  58  97  60 /   0   0  10  20
PSC  87  59  97  64 /   0   0   0  30
YKM  84  54  91  56 /   0   0  10  70
HRI  86  57  96  60 /   0   0  10  20
ELN  84  54  91  58 /  10   0   0  60
RDM  81  49  86  47 /  10  10  20  50
LGD  83  51  93  52 /  10  10  10  10
GCD  84  53  93  51 /  10  10   0  10
DLS  83  60  93  60 /   0   0  20  70

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Saturday afternoon through Saturday
     evening for ORZ642.

WA...Flash Flood Watch from Saturday afternoon through late Saturday
     night for WAZ522-523.

&&

$$

SHORT TERM...74
LONG TERM....83
AVIATION...83