Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Portland or
215 am PDT Monday Apr 21 2014
Synopsis...a series of upper troughs will bring much cooler and
wetter conditions for much of the upcoming week. The next Pacific
cold front will approach the region Monday and move through Monday night.
Showers continue on Tuesday with snow levels lowering to cascae
pass levels. A warm front approaching the coast will keep a threat
for showers going Wednesday...then another cool showery trough is expected
for the latter half of the week.
Short term...satellite showed high level clouds edging into the
region early this morning as a south-southwest flow brought increasing moisture
above 700 mb. A few showers beginning to show up on radar
offshore...and as moisture increase today and baroclinic zone
approaches expect showers to increase during the day. Air mass will
remain seasonably mild through midday...but an onshore flow
developing late in the day will begin to usher in cooler marine air
again along with increasing low level moisture. Will continue with
the same trend as in previous forecasts...an increasing chance for
showers today then raise probability of precipitation to categorical for tonight as the
baroclinic zone pushes through and the main upper trough approaches
the coast. Probability of precipitation remain high Tuesday as the upper trough with coolest air
aloft moves across. GFS suggests in instability deep enough to carry
a chance for thunderstorms during the morning as the upper trough
axis moves across...but NAM and ecwmf depict shallower instability
that would likely not support thunder. Given the uncertainty with
stabilty profiles will not alter the threat for thunder across
the S in for the morning.
Tuesday night into Wednesday deep onshore flow remains keeping middle and low
levels moist. With upper heights rebuilding behind the trough...warm
air adevection kicks in for the lower levels. GFS and European model (ecmwf) suggest
some modest moist isentropic lift near 290k insentrope Tuesday
night...then increase the lift Wednesday in particular for the northwest half of
the forecast area. Will keep probability of precipitation high for Tuesday night...and increase
probability of precipitation some for Wednesday in light of the depicted isentropic lift.
Long term...Wednesday night through Sunday...the extended forecast
period continues to look cloudy...cool...and wet. Model agreement is
good that a warm front will bring increasing rain Wednesday night into
early Thursday...with the trailing cold front moving through during the
day on Thursday. It appears that there will be plenty of showers around
later Thursday and Friday as the parent upper level trough moves through the
Pacific northwest. We may also need to watch for isolated thunderstorm
potential during this time as the cold pool associated with the
upper trough passes over the region. Expect showery conditions to
continue through next weekend. A shortwave trough is modeled to move
onshore sometime later Sat or sun...which will likely bring a round
of more organized precipitation. Pyle
Aviation...VFR conditions under high clouds early this morning
as an offshore front slides closer to the coast and continues
feeding moisture into the region. Lower levels remain somewhat
dry but will moisten through the day. Increasing middle clouds but
expect VFR conditions to continue through middle day. MVFR
conditions possible as rain reaches the coast around 21z...and
pushes inland across the interior after 01z Tuesday.
Kpdx and approaches...VFR conditions expected to continue through
at least this afternoon....but increasing middle clouds this
as lower level moisture increases. Lowering ceilings and rain
expected after 01z Tuesday as front reaches terminal with brief
reduction to MVFR possible this evening and tonight. Cullen
Marine...seas remain 13 to 15 feet over the waters early this
morning...and will continue to slowly subside today but remain
near 10 feet through at least late Monday. While seas may drop
slightly below 10 feet on Tuesday...seas rebuild to near 12 feet by Wednesday
as the next westerly swell arrives.
A relatively quiet start to the week over the waters in terms of
wind. The next significant system arrives Wednesday or Thursday...likely
bringing small craft winds to all waters. Cullen
Pz...Small Craft Advisory for hazardous seas until 11 am PDT
Wednesday for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar until
3 am PDT Tuesday.
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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..