Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
845 am PDT Sat Jul 30 2016
Synopsis...an upper level trough will support cooler temperatures
this weekend. The marine layer will deepen tonight and Sunday. Weak
high pressure will quickly move over the area Monday before a deeper
upper trough transits over the Pacific northwest Monday night through
Tuesday night. This feature may produce rain showers across extreme
southwest Washington and northwest Oregon. Seasonal conditions return
the middle part of next week.
Short term...(today through monday)...minimal changes to the current
forecast. Morning visible imagery shows marine stratus along much of
north and central Oregon coast, the SW Washington interior lowlands
and up the Columbia River to near kpdx. Water vapor imagery shows an
upper trough over southern British Columbia with another one out near
53n 155w. The 12z ksle sounding came in with an 850 mb temperature of
17.4c, slightly higher than the 6-hr GFS forecast. However, the GFS
and other models show 850mb tempeeratures continuing to cool through
the day, lowering to as low as 12c in the north Willamette Valley and
SW Washington. Daytime highs today will be tricky. MOS guidance has
had a trend of being too cool, so have went slightly above most
Also of note is the kttd-kdls gradient, which was 6.5 mb as of 15z.
The NAM is too weak with the gradient. Should easily see gusts up to
35 mph east of Stevenson. In fact, a few coop sites around Hood River
were gusting 25-30 mph as of 15z.
The marine layer deepens tonight as the upper trough moves over
Washington and extreme north Oregon. There will be more extensive
inland clouds Sunday morning, and the deeper clouds are more likely
to produce patchy drizzle along the coast and inland areas near the
Columbia River in the morning. The clouds will be slower to clear
Sunday afternoon, and the afternoon temperatures will peak below
normal in most areas.
A weak transitory 500 mb ridge moves over the forecast area late
Sunday night and Monday morning then moves east Monday afternoon
ahead of the next approaching upper trough. This approaching upper
low will be deeper than the current one and have a sharper trough
axis. 06z GFS looks to be a little slower with this feature, similar
to the 00z European model (ecmwf). The best dynamics with this trough will be north of
the Portland forecast area. Bulk of the energy moves across
Washington and extreme northern Oregon Monday night. A lot of
uncertainty with this system. Weishaar
Long term...(tuesday night through friday)...the threat for showers
will decrease Tuesday as the upper level low moves east of the
Cascades. A weak transitory upper level ridge will move over the area
Wednesday and Thursday. This will help warm temperatures a little,
but morning clouds and persistent onshore winds will keep daytime
temperatures near or slightly below seasonal normals. Another upper
trough is expected Friday. Tj
Aviation...MVFR stratus continues along the coast and western
slopes of the Coast Range and inland over southwest Washington. A
few fingers of of stratus in the kpdx area will probably bring
scattered-broken layer between 2000 and 2500 feet through 18z. Expect
interior cigs burn back to the coast 19-20z. Stratus lingers
longer along the coast, but still expect some signs of clearing
Kpdx and approaches...sct-bkn layer between 2000 and 2500 feet in
the area and may affect kpdx through 18z. Otherwise VFR through
late tonight with MVFR ceilings returning after 09z sun. Mh
Marine...surface high over the NE Pacific will weaken somewhat,
but remain in place through the weekend. Small craft advisories
for winds and seas continue through tonight. The strongest winds
to remain over the outer waters while near shore winds will be
sub-advisory criteria through midday then increase for the
afternoon and evening.
Seas remain quite steep over the waters due to a combination of
locally generated wind wave and a fresh swell. While the
individual contribution of each of these may be indistinguishable
at times, the overall steepness and wave height will be apparent.
As such, the Small Craft Advisory for seas will continue through
tonight for all waters.
Advisory strength winds may continue over the central Oregon
waters on Sunday, but should ease below criteria in the north.
Then expect weaker northerly winds over all waters through around
midweek, when strengthening high pressure may bring a return to
gustier northerly winds. Seas will remain elevated early Sunday
morning, but gradually subside through the first part of the week.
Pz...Small Craft Advisory for winds until 2 am PDT Sunday for
coastal waters from Cape Shoalwater Washington to Florence or out
Small Craft Advisory for hazardous seas until 5 am PDT Sunday
for coastal waters from Cape Shoalwater Washington to Florence or
out 60 nm.
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This discussion is for northwest Oregon and southwest Washington from
the Cascade crest to 60 nautical miles offshore. The area is commonly
referred to as the forecast area.