Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
220 PM PDT sun Mar 29 2015
Synopsis...mild and mostly dry weather will continue through Monday as
a flat upper level ridge remains over the Pacific northwest. A cold front will
push across the region Monday evening ushering in a much cooler and
more seasonable air mass along with showers and lower snow levels
that will last through the end of the week.
Short term...light rain spread down to the S Washington coast this
afternoon...driven by weak isentropic lift down near 290k isentrope.
Ecwmf indicates this area to lift north overnight...while GFS hangs on
to some weak lifting into Monday morning so will need to hang on to
some low probability of precipitation there into Monday morning. Elsewhere expect dry weather
to continue through Monday as the flat upper ridge holds. Good
radiational conditions tonight over much of the southeast part of the are
will allow temperatures to dip down close to normal...with above normal
afternoon temperatures returning Monday.
Models remain similar with the next system arriving Monday night...
pushing a cold front inland during the evening. Time height cross
sections show strong lift and moisture below 500 mb to justify
categorical probability of precipitation for Monday night. This Ushers in a cooler unstable air
mass for Tuesday and continuing into Wednesday as a cold upper trough moves
across. With 800 mb temperatures expected to drop a few degrees below zero c
surface high Tuesday and Wednesday are likely to come back down closer to
normal. Model soundings indicating good instability Tuesday...at least
by pacnw standards...with GFS and NAM cape values maxing out in
triple digits. With the top of the unstable layer approaching 20k feet
in addition to showers likely will need to include a chance for
thunder in the afternoon and evening as the cold trough aloft moves
across. Another shortwave with a cold pool aloft...500 mb temperatures again
down near -32...will add in a slight chance for thunder again Wednesday.
Long term...the cold upper trough of midweek is depicted by models
exiting the region Thursday...with a weak ridge moving across late Thursday
and Friday. GFS and European model (ecmwf) both indicate some light residual
precipitation Thursday...but with upper dynamics shifted east and mainly
just some shallow low level moisture left will cut back on probability of precipitation a
bit for Thursday. 12z model runs also a bit slower in digging in the next
trough and cold front late Friday so will back off on probability of precipitation across the S
part of the forecast area Friday. Over the weekend models now tending
to dig the next trough down off the coast...close enough to push a
front through Sat and keep the region in a cool unstable air mass
Aviation...VFR conditions expected to continue through today and
tonight with variable high clouds. VFR ceilings above 6000 feet continue
along the north coast near kast...but may gradually lower into
lower VFR or perhaps higher MVFR after 03z. A possibility exists
that these will make their way further south towards konp after
07z...but confidence is lower for this. Elsewhere...VFR likely
persists with variable high clouds. May again see shallow fog or
stratus develop late tonight/early Monday particularly in the South
Valley and in more fog-prone locations. Increasing and lowering
middle/high clouds into the coast after 09z and interior after 12-18z
Kpdx and approaches...VFR conditions continue today and tonight
with a broken high cloud deck. Increasing and lowering middle/high
clouds late after 15z Monday but remaining VFR through next 24 hours.
Marine...southerly winds continue this afternoon and have
remained a little gusty...around 20 knots...particularly in the
northern zones as anticipated in the morning update. Winds will
maintain or ease slightly early tonight... but then again increase
on Monday ahead of the next frontal system that will cross the
waters tomorrow. Small Craft Advisory for winds will begin at 5 am
in the north where winds will increase to 20 knots. Expect winds to
peak around 25 knots with gusts to 30 knots during the late afternoon
and evening hours. Models are in good agreement that stronger
winds will not arrive in the southern waters until the afternoon
so the advisory begins at 2 PM for the central Oregon waters.
Winds should ease by 11 PM across all waters.
Meanwhile seas remain around 10 to 11 feet in the northern zones.
Sparse observations make it a bit tougher to discern if seas have
dropped off a bit in the south...so elected to maintain the
advisory for hazardous seas as is...through 5 am. This will likely
need to be extended in the north...but suspect enough of a respite
in seas will occur in the south to let it expire. However...seas
again will build to near 12 feet across all waters behind the front
and into Tuesday. Cullen
Pz...Small Craft Advisory for winds from 5 am to 11 PM PDT Monday
for coastal waters from Cape Shoalwater Washington to Cascade Head or
out 60 nm.
Small Craft Advisory for hazardous seas until 5 am PDT Monday
for coastal waters from Cape Shoalwater Washington to Florence or out
Small Craft Advisory for winds from 2 PM to 11 PM PDT Monday
for coastal waters from Cascade Head or to Florence or out
Small Craft Advisory for rough Columbia River bar from 11 PM
this evening to 4 am PDT Monday.
Small Craft Advisory for rough Columbia River bar from noon to 4 PM
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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.