Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Portland or
915 PM PDT Wednesday Apr 23 2014

Synopsis...a frontal system will move across the region tonight and
move east of the area Thursday morning for continued persistent rain
and higher elevation snow. The parent upper low or trough will dig
south toward California Thursday afternoon and Thursday night before
moving onshore and through the area Friday and Friday evening for
more showers. Another significant frontal system is expected late
Saturday and Saturday night.

Short term...rain and mountain snow continues this evening as a warm
front lifts north across northwest Oregon and SW Washington and a cold front
approaches. It is difficult to determine where the warm front is on
satellite imagery as it mainly shows a blob of unorganized
high-level clouds over the forecast area. Surface observations
suggest that the warm front is draping east to west somewhere between
Salem Oregon and Kelso Washington.

Water vapor imagery shows a complex low offshore. Subtle features in
water vapor and infrared satellites show that there actually may be
two close-knit cold fronts offshore. The main one associated with a
low near 48n 128w. There appears to be signs of another weaker
surface low around 43n 129w. Expect breezy winds especially for the
coast tonight as these fronts approach.

The north Oregon Coast Range has seen quite a bit of rain this
evening with many rain gages reporting an inch of rain in the past 6
hours.The Willamette Valley has measured around 0.3 inch the past 6
hours..and the South Washington Cascades around a half of an inch.

Snow levels have been rising and are currently around 5500 feet.
Still think the higher elevations will get a good dose of snow
overnight and will let he Winter Weather Advisory for the Cascades
ride out.

We should see plenty of showers Thursday behind the cold front.
Model lifted index values look a little weak for thunder but we
should at least see some small hail...with model soundings showing a
bit of a cap at 700 mb as the main colder air aloft is still
offshore. On Friday the main upper trough will be moving through
southwest Washington and northwest Oregon and thunder chances look a
bit better as temperatures at 500 mb fall to minus 28 degree c.

While the upper trough moves east of the area Friday night...some
showers will continue Friday night and Saturday as onshore flow
continues and there is not much upper ridging in between systems.
The next stronger front is expected Saturday night. Hartley/Tolleson

Long changes. Previous discussion follows...Saturday night
through Wednesday...the latest medium range forecast models continue to
advertise a frontal system moving onshore Sat night and early sun.
Kept categorical probability of precipitation for this time period...with a transition to
showers later in the day Sunday behind the trailing cold front. A
weak warm front may brush the forecast area with some light precipitation
on Monday...but the trend will be toward drier weather as upper level
ridging builds over the Pacific northwest. Right now...confidence is pretty high
that Tuesday will be warm and dry with the ridge directly overhead...and
high temperatures were raised significantly with the latest forecast package.
Model uncertainty begins
to increase the 12z GFS and some of its ensembles indicate
that a trough over the NE Pacific will split...with the northern energy
pushing into the Pacific northwest Wednesday as an open trough. The 12z European model (ecmwf) maintains
a NE Pacific closed low that hardly moves through the end of the
week...allowing ridging to remain in place over the Pacific northwest for
several days. Have kept some low chance probability of precipitation on Wednesday due to this
uncertainty. Pyle

Aviation...disorganized yet potent frontal system continues to
spread rain and general MVFR and local IFR conditions across the
forecast area this evening. This system will also continue to
produce gusty south winds...especially along the coast where winds
may gust to 40 knots. Cold front will push onshore 08z-10z...then
push inland to the Cascades by 13z-15z.

Behind this front the air mass will become more convective in
nature...with better mixing and ceilings pushing back up into the
2000-3000 feet range. Steady rain will switch to showers behind the
front with improved visibilities outside of showers. Showers will persist
through the day Thursday...along with a few breaks in the clouds.
Heavier showers may produce small hail in the afternoon.

Kpdx and approaches...ceilings likely to hold around 1500 feet in
occasional rain through 12z. S winds will become gusty at times...
up to 25 knots with the passage of a cold front 12z-14z. After that
winds turn west-SW with improving ceilings/visibilities but continued showers.
Expect general MVFR with showers Thursday...though periods of VFR
are likely between showers. Weagle

Marine...complex low pressure off the Pacific northwest coast already
brought one surge of S winds to the coastal waters this evening...
and is expected to bring another very early Thursday morning. Gusts to
gale force will be common...especially in the coastal jet...prior
to 12z. Decided to end the Gale Warning for our southern waters
just after the cold front moves through early Thursday morning...but
will keep it through midday for the northern waters as a few gale
gusts could linger as the parent low pushes into Vancouver Island.

Otherwise after sunrise...blustery and squally west-SW flow will
dominate Thursday...though winds will be gradually easing
throughout the day. Very choppy combined seas 12-16 feet likely
through Thursday morning...but this should slowly subside later Thursday/Thursday
night. Lighter conditions expected Friday/early Sat as the low
pressure system fills over British Columbia and pressure
gradients slacken over the area. Weagle

Pqr watches/warnings/advisories...
Pz...Gale Warning until 11 am PDT Thursday for coastal waters from
Cape Shoalwater Washington to Cascade Head or out 60 nm.

Gale Warning until 8 am PDT Thursday for coastal waters from
Cascade Head or to Florence or out 60 nm.

Small Craft Advisory for rough Columbia River bar until 6 PM
PDT Thursday.


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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..