Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
939 am PDT sun Aug 30 2015
Synopsis...cloudy and cooler weather today as a weak front moves
across northwest Oregon. Light showers today will decrease in the afternoon.
A warm front will bring light rain to the coast and northern areas
early Monday morning. A weak cold front will move across the region
late Monday night and Tuesday. An upper level trough will support
continued cool and showery weather through Thursday. High pressure
and drier weather expected next weekend.
Update...radar imagery shows plenty of showers across the region
this morning. It is difficult to discern the current location of the
front...but satellite imagery suggests that it is in the vicinity of
the Willamette Valley. Surface observations show that the
coast...Coast Range...and the Cascade foothills have observed between
0.2 and 0.5 inch of rain the past 6 hours. The rainfall has been
less for the Willamette Valley with 6 hour totals between 0.10 and
0.30 inch. 6-hour rainfall totals for the Cascades are between 0.3
and 0.7 inch...but several weather stations in the SW Washington
Cascades have measured over an inch of rain the past 6-hours. Showers
will be decreasing early this afternoon...but will likely linger over
the Cascades until late this afternoon.
The next frontal system is approaching the b.C. Coast this morning.
The warm front will brush the northern portion of the Portland
forecast area with light rain mostly for the coast...the northern
Oregon Coast Range...and areas north of Portland late tonight through
Monday morning. The rainfall totals will be light...less than 0.10
inch. There will be a break in the rain Monday afternoon and evening.
The trailing cold front will stall offshore Monday night and Tuesday
morning with SW Washington the main location for rain...then it will
move inland Tuesday. Tj
Previous short term...(today through tue)...pattern more resembles a
late September or October rather late August. A broad area of low
pressure remains just offshore. Cold front has pushed across much of the
region earlier this am...and is now pushing over the Cascades.
Steady rain generally east of i5 this am and will continue to shift
eastward with the front. Like usual... a cooler more unstable air
mass will shift inland over the region. This will maintain showers
today...though best threat will be over Washington.
Coolest air...with -20c or colder temperatures aloft near 22k feet...
is just off the Washington coast. This air will push into Washington
state this am...and over far northwest Oregon. Though timing is not most
favorable for strong convection...will keep a slight chance of
thunderstorms for areas north of a Tillamook to Scappoose to Montana St helens
for this am. As air mass begins stabilizing later today... the
showers will be decreasing...especially over Oregon.
Decent amount of clouds over region tonight... but showers will be
rather sparse as will be between weather systems. The next front will
arrive on Monday...with another round of rain later Monday and Monday night
and Tuesday. Temperatures will remain rather pleasant...with highs on the
coast in the 60s and inland valleys in lower 70s.Quinton.
Long term...no changes. Previous discussion follows...Tuesday night
through Saturday...an upper trough will remain just off the Pacific northwest
coast through Friday. Models show another decent piece of energy
swinging through around Wednesday or Thursday for a better chance of
showers...but the timing varies a bit. The models are indicating it
may dry out and start to warm up some next weekend. Tolleson
Aviation...a majority of the shower activity this morning has
moved over the Cascades and foothills. However...another line of
showers is starting to move into the north and Central Valley.
Will likely see brief periods of MVFR ceilings and visibility under moderate
to heavy showers north of ksle through around 19-20z. Otherwise...
expect mostly VFR conditions across the region today and tonight
with scattered showers possible this afternoon/evening. Marine
stratus looks to return to the coast after 06z and will likely
bring MVFR ceilings and visibility. Periods of IFR ceilings and visibility also
possible tonight...mainly south of ktmk.
Kpdx and approaches...VFR the next 24 hours. Will see scattered
cumulus with a slight chance of showers this afternoon/evening. /64
Marine...winds have eased across all the waters this morning with
occasional gusts to 20 knots so will let the Small Craft Advisory
for winds over the outer waters expire. Fresh swell continues to
bring square seas to the waters so will keep the Small Craft
Advisory for hazardous seas as is. However...buoy 46089 is starting
to show a longer period swell as the dominate period so will
likely be able to cancel the hazardous seas advisory earlier than
A couple weak systems will brush the northern waters first this
afternoon and evening then again from around daybreak Monday
through the early evening with gusts around 25 knots. As such have
issued a advisory for winds starting this evening.
Seas will remain 7 to 10 feet for the upcoming week as the
unsettled pattern remains over the region. It appears longer
period distant swell will dominate after today...but may get
additional sets of square seas/fresh swell at times. /Jbonk/64
Pz...Small Craft Advisory for winds from 7 PM this evening to 6 PM
PDT Monday for coastal waters from Cape Shoalwater Washington to
Cascade Head or out 60 nm.
Small Craft Advisory for hazardous seas until 8 PM PDT this
evening for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar from 3 PM
this afternoon to 10 PM PDT this evening...and from 4 am to
9 am PDT Monday.
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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.