Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
215 am PDT Wednesday Mar 12 2014
Synopsis...after a chilly start once again...Wednesday has the
potential to be the warmest day of 2014 thus far for much of the
Willamette Valley. There may be some fog...but whatever does develop
should mainly be near rivers and should be very shallow...burning
off early. Expect plenty of sunshine...with a mild springlike
afternoon. Increasing onshore flow may keep temperatures a little cooler
Thursday... then the next chance for rain comes Thursday night into
Friday. There could be another chance for rain late this weekend as
a disturbance moves across Washington.
Short term...clear skies across much of region early this morning.
The ttd-dls gradient is around -4 mb...and thus moderate offshore
flow in place. A weak northerly gradient exists in the valley an
thus northerly flow is showing up in observation. This flow is in response
to the thermal trough pushing north up the Oregon coast. Fog
satellite product suggest fog is confined to river valleys with
patches about elsewhere. Soundings show pretty dry air above the
surface thus expecting much less in the way of fog this
morning...but with the lower dewpoints in place expect areas of
frost and a chilly start again.
Today still looks to be a sunny and seasonally warm day. Climatology
from sle for 850 temperatures suggest normal values...or 50th
percentile values...around 1c. Forecast values will be about 5-6c per
the NAM or 6-8c per the GFS/ec and slightly higher per the rap...and
these readings fall into the 85th-90th percentile. Normal highs in
the valley are 55-56. MOS guidance is going for low to middle 60s and
sref plumes show minimal spread and are in a similar ballpark. Feel
the higher end is warranted given slightly better mixing today and
the thermal low moving overhead a simply a nice Spring day. Meanwhile
at the coast...the offshore gradients will be strongest this
morning...but the offshore flow will not likely hold off the
seabreeze later today. So expect highs this morning before the
onshore flow kicks in and possibly some marine clouds moving in
later it the day. Will attempt to reflect this in the hourly temperature
grids. There may be some coastal low clouds developing as well.
High pressure will flatten late Wednesday as an upper disturbance
cruises by to the north. A decaying front will push into our northern
coastal waters on the southern end of this system. Moisture still
appears very limited with this system...with likely just some low
clouds and maybe some drizzle for the northwest coast. The ridge axis will
shift southward into western Washington later Wednesday night into
Thursday morning. This should result in some marine clouds pushing up
the Columbia River to the possibly to the Portland metropolitan area...but
the onshore gradient does not look too strong at this point. Onshore
flow and some marine low clouds Thursday morning will keep temperatures a couple
degrees cooler for Thursday...but readings remain above normal with
sun and high clouds. Looks like upper 50s.
Clouds thicken up considerably Thursday evening as the GFS and ec
remain in good agreement that a shortwave trough will slide eastward
across Washington on Friday. A warm frontal feature moves through
Thursday night for some lighter rain. Trailing cold front sinks in
Friday morning. This should send a an accompanying band of rain into
the area..with the main batch due in during the morning.. due the
best upper level dynamics and instability remaining over
Washington...there may some tendency for the Willamette Valley to be
rain shadowed Friday...especially southward towards Salem and
Eugene...with the elevated terrain favored in the afternoon as the
front moves through. The front stalls and lifts north Friday evening
as a surface high builds in...which will dry things out. /Kmd
Long term...no changes. Previous discussion follows...Friday night
through Tuesday...both the European model (ecmwf) and GFS show upper level heights
building Friday night. Models disagree whether a system will clip
the forecast area Saturday into Sunday. At this time...only the ec
is allowing this system to reach the region...with a stationary
front/boundary potentially setting up along the Washington/Oregon
border as the upper wave moves through British Columbia.
Meanwhile...the GFS maintains dry conditions beneath a stronger
ridge. The ec allows the ridge to rebound a bit on Sunday...and
models agree that another weak front will approach the region late
Sunday or Monday. Bb
Aviation...dry air mass over the region...with light offshore
flow. Will have patchy fog/LIFR stratus between 12z and 17z.
Otherwise VFR today. Flow will turn back onshore later today as
high pressure offshore builds a bit. VFR tonight...but expect areas of
IFR or low MVFR clouds spreading inland late tonight/Thursday am with
the light onshore flow.
Kpdx and approaches...VFR today and most of tonight. May have
patchy shallow fog between 12z and 15z...but should not be enough
to affect Ops. Rockey.
Marine...no much in way of winds over next several days.
Gradients remain weak...so winds stay 15 knots or less. Main issue
will be the swell. Seas will increase to 10 or 11 feet at 15 to
17 seconds around daybreak today. Wavewatch3 model showing 19 feet
for the 06z time frame and is close to buoy 46036 observation.
Swell will eventually decay to below 10 feet Thursday am.
Pz...Small Craft Advisory for hazardous seas today through
Thursday am on all coastal waters.
Small Craft Advisory for rough Columbia bar conditions
from this afternoon through Thursday am.
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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.