Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Portland or
259 PM PDT Sat Apr 19 2014

Synopsis.... showers will decrease tonight as an upper trough moves
east . Weak higher pressure will move in Sunday for generally dry
weather on Sunday...except some moisture may brush the far north
coast zones. The next front will become north south oriented Sunday
night and stall off the coast as energy digs toward California for
more mostly dry weather...but then move onshore Monday and Tuesday
for more rain or showers. The remainder of next week looks unsettled
as more precipitation moves in at times.

&&

Short term...tonight through Tuesday...main cold front has cleared
and now onshore flow has resulted in a steady stream of Post frontal
showers. Satellite depicts still quite a bit of cumulus offshore
working its way this direction and increased convergence as the moist
flow hits land. Some of these showers will have brief moderate to
heavy downpours as low level lapse rates are modestly steep. There
may even be a rumble of thunder. Surface high pressure is already
nosing into the SW corner of the area near Florence and Newport.

As surface high pressure builds in over the remainder of the area
late tonight and sun...expect a considerable decrease in the shower
activity. Soundings are moist and somewhat unstable...but the lowest
levels stabilize enough to mention patchy fog...favored most for
points farther south. Depending on the amount of
clearing...temperatures fall into at least the lower 40s...with middle
to upper 30s possible in outlying areas and the farther south you
head into the valley.

Flat ridging tonight amplifies some on Sunday. Therefore expect a
return to dry conditions for Sunday along withwarmer temperatures after
the cool start. Middle to upper 60s not out of the question with
filtered sunshine as higher clouds push in ahead of a compact
circulation out near 46n/145w at the base of the Gulf of Alaska upper
low. This circulation will head northwest of the area as high
pressure builds north into the northern rockies. Surface observation this is happening
the Gulf of Alaska low continues to deepen a push a cold front toward
the area...but the moisture remains offshore Sunday night for
continued dry weather.

The upper trough and associated cold front moves to near the coast
Monday afternoon. Models have been insistent on breaking out showers
in the Cascades to some degree on Monday afternoon. Model soundings
suggest air mass at best marginally unstable...and with dynamics
digging the trough S prefer to limit probability of precipitation out ahead of the front Monday
to chance category inland...but higher along the coast closer to the
front.

SW onshore flow coupled with falling heights aloft point towards
cooler marine air pushing back in Monday evening into Tuesday for the
most organized period of showers. Snow levels drop to around the
passesmid level lapse rates steepen as the cold pool moves
overhead. Normally -28c to possibly cooler at 500 temperatures brings
concerns for isolated thunder. Main forcing passes by south of the
area which will preclude a thunder mention yet...but can see it being
added a future time. /Kmd

Long term...Tuesday night through Saturday... the extended portion of
the forecast is looking increasingly wet with below normal temperatures.
We start out fairly quiet Tuesday night and early Wednesday...with a few
showers around on the backside of an upper level trough that is
departing to the east. The next frontal system is expected to bring
steady rain beginning Wednesday or early Thursday. Confidence in the timing of
this system is not high at this point. The 12z European model (ecmwf) run came in
much more progressive than past runs...bringing rain onshore Wednesday
afternoon. The 12z GFS and the majority of its ensembles hold the
rain off until overnight Wednesday into Thursday. At any rate...precipitable
water values with this system are looking fairly high so we will
likely get a pretty good dose of rain through Thursday night. A broad and
cold upper level trough will build into the NE Pacific for Friday into next
weekend...bring more rain chances. Pyle
&&

Aviation...VFR prevailing with a mix of MVFR ceilings in the
2000 to 3000 feet range across the area. Expect MVFR ceilings to
decrease along with the showers in the early evening. High pressure
building over the area brings stable conditions which may lead to
patchy fog or low stratus Sun morning. Higher terrain frequently
obscured through

Kpdx and approaches...VFR prevailing with occasional MVFR ceilings
around 2500-3000 feet through 00z or so. After 12z sun model relative humidity
fields and sounding suggest a cloud layer around 2500 feet could
develop in the area. If it does form the layer would dissipate by
18z sun.

&&

Marine...Post frontal southwesterly winds 15 to 25 knots continue
early this afternoon. The trends is for the winds to decrease
below advisory criteria from south to north and that appears to be
happening. Will keep current advisories unchanged. A relatively
quiet period starts the work week with weak system nearing the
waters. The next significant front arrives late Wednesday or
Thursday...likely bringing small craft winds.

A large westerly swell train arrives tonight into sun. Enp wave
model is doing well based on comparison with offshore buoys.
Buoy 46036 which about 500 nm off the Washington coast was 31 feet with a
16 second period at 18z. This was within 1 feet of the enp output.
The enp also compared well with buoy 46002 - about 300 nm off the
south Oregon coast - measuring 15 feet at 18z.

Expect seas will rapidly increase to 17 to 20 feet at 16 seconds
when the swell moves into the waters this evening...and peaking
around midnight through 5 am sun. Seas will then gradually
subside through Sun afternoon...diminishing to around 15 feet Sun
night...and to 10 feet early Tuesday.

A high surf advisory continues for later tonight through Sun
morning for the South Washington...north and central Oregon
coasts. The westerly swell will rapidly increase between 6 PM and
10 PM and likely peak near 20 feet around midnight through early Sun
morning then begin to subside. Not a good time to be in or near
the surf zone.

&&

Pqr watches/warnings/advisories...
or...high surf advisory from 10 PM this evening to 10 am PDT Sunday
for north Oregon coast.

High surf advisory from midnight to 10 am PDT Sunday for
central Oregon coast.

Washington...high surf advisory from 10 PM this evening to 10 am PDT Sunday
for South Washington coast.

Pz...Small Craft Advisory for winds until 9 PM PDT this evening
for coastal waters from Cape Shoalwater Washington to
Cascade Head or out 60 nm.

Small Craft Advisory for hazardous seas until 6 am PDT
Monday for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.

Small Craft Advisory for winds until 4 PM PDT this
afternoon for coastal waters from Cascade Head or
to Florence or out 60 nm.

Small Craft Advisory for rough Columbia River bar
until late Sunday afternoon.

&&



$$

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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..