Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
218 PM PDT Tuesday Apr 21 2015
Synopsis...a series of weak fronts will bring a few light showers
and drizzle to mainly the coast and Cascades through Thursday...but
most locations will remain predominantly dry. After a few showers
Friday...a wetter storm system will move into the area this
weekend...with possibly more wet weather early next week.
Short term...tonight through Friday...visible satellite imagery
reveals marine clouds pushing into western Oregon and Washington.
Areas of drizzle continue along the coast...most concentrated
between seaside and Astoria...but drizzle has been reported as far
south as Newport. Expect the soupy mess along much of the coast to
gradually improve this evening as more unstable air moves into the
Meanwhile...a weakening south to north oriented front continues to
push eastward towards the coast. However...Doppler radar shows shower
activity associated with the front has weakened considerably over the
past several hours. Nonetheless...there is a slight chance a few
showers and/or sprinkles may push into the Willamette Valley this
evening as this system attempts to move inland. Otherwise...shallow
instability...low level moisture and continued onshore flow will
favor our typical orographically favorable locations such as the
coast...Coast Range and Cascades for shower activity overnight and
early Wednesday. Even though model quantitative precipitation forecast is spotty at best near the
coast during this period...suspect the north Oregon and South
Washington coast will be the wettest area given the flow will be a
bit more westerly in this area and the large amount of cold air
cumulus upstream over the northeast Pacific apparent via satellite.
Onshore pressure gradients continue to increase this afternoon with
kast-kpdt topping 10mb. This has resulted in west to northwest winds
increasing across the area. Expect winds to increase further...into
the 15 to 25 miles per hour range...near the Coast Range gaps and likely a bit
higher in the central Columbia River gorge.
A southwest to northeast oriented ridge axis will shift northward
into western Washington on Wednesday...which should allow most of the
Oregon portion of the County Warning Area...including the coast...to clear more
quickly than southwest Washington and extreme northwest Oregon. 500mb
heights do begin to lower a bit late Wednesday night so marine clouds
may creep up the Columbia River Thursday morning. However...an
incoming front and surface trough may relax pressure gradients enough
that inland cloud penetration may not occur until later in the day.
While the main upper level support for this frontal system generally
stays in Washington and points northward...the close proximity of it
should bring enough support to result in showers spreading into the
area Thursday or Thursday night. Probability of precipitation were nudged upward...but due to
uncertainty in timing between these two time periods...probability of precipitation were
generally left in the chance range.
Models seem to be trending towards an additional shortwave trough or
two pushing southeastward into the area on Friday...reinforcing
onshore flow...and bringing another round of showers to the area.
While snow levels will lower to near pass level...quantitative precipitation forecast should still
be light enough to preclude any snow highlights for the
Cascades...but it looks like it may end up the wettest day in the
short term. /Neuman
Long term...Friday night through Tuesday...models rotate a closed
upper low through southern British Columbia Friday night into Sat...but differ on
timing and strength of the main shortwave moving across the Pacific
northwest. Timing and strength issues remain a problem throughout the
extended period as a long wave trough digs down over the east Pacific
and sends a series of shortwaves across the Pacific northwest. While the
longwave trends appear reasonably consistent...a moderate amount of
spread in ensembles with smaller scale features suggests a need to
lean towards climatology probability of precipitation early next week. Through the weekend
though models consistent enough to carry generally likely probability of precipitation or
better for the warm and then cold fronts Sat night into Sun night.
Overall with an active flow off the Pacific temperatures will generally run
near normal through early next week.
Aviation...inland webcams show some breaks in the clouds this
afternoon so expect lingering MVFR ceilings to lift to low end VFR by
22z. Coastal sites will see MVFR ceilings...and locally IFR near
konp...through 00z Wednesday before briefly lifting to low end VFR
through 04-06z Wednesday. Should see MVFR ceilings return along the coast
and for inland valley sites tonight...by 08z coast and by 10z
Kpdx and approaches...MVFR stratus starting to break up this
afternoon and should see VFR ceilings by 23z Tuesday with northwest winds
picking up middle afternoon into early evening. After VFR ceilings
through this evening...MVFR ceilings around 2500-3500 feet return
between 10-12z Wednesday. /27
Marine...after quite some time...buoy 50 appears to have returned
this afternoon! Northwesterly Small Craft Advisory winds with gusts 25-30
kts will continue across the coastal waters through late tonight/
early Wednesday morning. Seas slowly rising this afternoon as
well...with a northwesterly swell around 10 feet expected to arrive by this
evening and peaking around 11-12 feet tonight. Seas gradually
subside below 10 feet midday Wednesday. A much stronger front looks
to move across the waters over the weekend and will likely bring
gusty southerly winds and another round of building seas.
Expect rough Columbia River bar conditions during the next few
ebbs. Will have to watch the tonight period if seas rise a little
higher than anticipated rough Columbia River bar conditions may
occur during non Ebb time. /27
Pz...Small Craft Advisory for winds until noon PDT Wednesday for
coastal waters from Cape Shoalwater Washington to Florence or out 60 nm.
Small Craft Advisory for hazardous seas from 4 PM this
afternoon to 6 PM PDT Wednesday for coastal waters from Cape
Shoalwater Washington to Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar from 7 PM
to 10 PM PDT this evening...then again from 5 am to 10 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.