Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
227 PM PDT Wednesday Apr 1 2015
Synopsis...a broad cool upper trough will remain over the Pacific
northwest today with one last main short wave on the back or west
side of the trough moving through southwest Washington and northwest
Oregon this afternoon and evening for more showers...possible
thunderstorms...and small hail. This last short wave moves east of
the Cascades after midnight for more substantial drying through
Thursday as a brief trailing upper ridge moves across the Pacific
northwest. The models show the next decent late winter/early Spring
front moving into our forecast area Friday...with the associated
parent cool upper trough digging southward along the coast and
spreading onshore on the weekend...possibly lingering into the early
part of next week. Snow levels will be below the Cascade passes much
of the time through early next week.
Short term...tonight through Saturday...a broad cool late
winter/early Spring upper trough will remain over the Pacific
northwest today for more unsettled cool weather. One last main short
wave...as evidenced by the bright cloud tops on the infrared satellite
imagery moving in from the northwest...will move through southwest
Washington and northwest Oregon this afternoon and evening. This
feature will produce more showers that could be heavy at times...
isolated thunderstorms...and small hail across the area through the
remainder of the afternoon and a good part of the evening. The models
continue to show decent cape over our area...close to 400 j/kg. There
is a jet maximum from the northwest with this short wave...but the shear
seems to be somewhat modest. So mainly expect more pulsing storms
The short wave does not move east of the Cascades until late
tonight...after which the showers will dramatically decrease through
Thursday as a brief short wave ridge moves into the forecast area.
Thursday may not be totally dry though...as the models continue to
paint a bit of quantitative precipitation forecast due to a few lighter showers in the residual
onshore flow...and possibly due to a bit of afternoon heating such as
it is...that could produce a few showers over the higher terrain
along the coast and in the north that could drift out over the
adjacent valley areas.
The next front currently out beyond 140w is still on track for
Friday. 700 mb moisture and vertical motion fields suggest the front
will move through the forecast area during the day on Friday...and
this is supported by the core of a 90 knots 300 mb jet moving onshore
and the 700 mb relative humidity fields. Precipitation amounts appear modest as the
front moves through.
The associated upper low then digs south off the coast Friday night
and Saturday...a track which would keep the bulk of the showers off
the coast for a while though a few could spread onshore. This upper
low eventually expands onshore into the Pacific northwest Sunday for
more showers and greater instability.
Snow levels will remain below pass elevations through Saturday...
close to 3000 feet today...up a touch ahead of the front Friday but
still rather low...then falling this weekend again. Tolleson
Long term...Saturday night through Wednesday...cool and unsettled
weather continues late this weekend as an upper level trough digs
south into the Pacific northwest. Both the 12z GFS and European model (ecmwf) both
agree low pressure settles just off the Washington/Oregon coastline
by Sunday...bringing a threat for afternoon/evening thunderstorms.
After that model solutions diverge. Either way...showers...Cascade
snow showers and cooler temperatures continue into Monday and Tuesday
as the overall pattern is for the upper trough to dig further south
into northern California then move inland by Tuesday night. We should
see a brief break as weak upper level ridging develops by next
Wednesday...with temperatures slowly rebounding by midweek. /27
Aviation...prevailing VFR ceilings of 040-050 across the area and should
continue through at least 03z. Similar to yesterday...expect to see
some small hail and isolated thunder in some of the stronger showers.
Ceilings and showers will break up somewhat overnight for radiational
cooling producing ground fog and/or a MVFR ceiling around 015 developing
at or after 12z across the interior.
Kpdx and approaches...terrain obscuration with broken ceilings 040-050 will
limit visual approaches. Showers to continue at least through the
evening. Much like yesterday...expect heavier showers to contain
small hail and MVFR ceilings. There will be a chance for vicinity
thundershowers through 03z. Showers to taper off later this evening
along with ceilings breaking up. Will then see a 015 ceiling develop at or
after 12z answer remain through at least 02/17z. /Jbonk
Marine...expect westerly winds less than 20 knots through Thursday evening as
high pressure remains over the NE Pacific. Seas are also expected to remain
steady at 8 to 9 feet during this time with a westerly swell at 12 or
13 seconds. Showers will continue through this evening...with the
continued possibility of isolated thunderstorms and convective wind
gusts up to around 25 knots.
A front is modeled to move into the coastal waters Friday morning and
push inland Friday afternoon. It appears that the front will be
weakening significantly as it nears shore...but it may still produce
a period of Small Craft Advisory winds. A large upper level low pressure
system will move in over the weekend and then a progressive weather
pattern is expected into next week. Forecast details are somewhat
uncertain going into the weekend...but there do not appear to be any
major wind impacts. However...increasing swell is expected to arrive
Friday and into the weekend...likely pushing seas into 10 to 12 feet
Pz...Small Craft Advisory for rough Columbia River bar until 7 PM
PDT this evening.
Small Craft Advisory for rough Columbia River bar from 3 am
to 7 am PDT Thursday.
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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.