Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
226 am PDT Monday Oct 20 2014
Synopsis...a slow moving cold front has moved onshore early this
morning...spreading primarily light rain into the inland valleys. The
front will continue eastward today...moving across the Cascades by
afternoon. Behind the front...expect showers with perhaps an embedded
thunderstorm or two along the coast. Showers will decrease tonight
and Tuesday morning as the associated upper trough moves east of the
forecast area. The next front will be quite wet as it slowly moves
north to south through our forecast area Tuesday night and
Wednesday...with rain likely lingering into Thursday. This front may
also produce strong coastal winds. Another front is expected by the
Short term...today through Wednesday...a cold front is inching its
way eastward across western Washington and Oregon early this morning.
Mainly light rain developed along the coast Sunday evening and has
spread as far inland as the Interstate 5 corridor early this morning.
Expect the light rain to continue to spread eastward into the
Cascades this morning...perhaps becoming a little heavier as another
vortmax travels northward along the front. Behind the front...rain
will taper to showers along the coast this morning and inland by
afternoon. Latest model runs have been slowing down the cold pool
aloft...with it forecast to move across the forecast area overnight
rather than this afternoon. This slower timing limits instability a
bit...keeping most convection rather shallow. Still cannot rule out a
stray lightning strike or two this afternoon...but the best chance
will be along the coast.
00z NAM shows some decent q-vector convergence and residual
deformation as the upper trough axis moves through this evening...
which could keep decent shower coverage into the the evening hours.
The upper trough axis then shifts east of the Cascades overnight with
showers likely dwindling after midnight into Tuesday morning. The cold
pool with this system is not all that impressive...850 mb temperatures get
down to around +3 degree c with 1000-500 mb thicknesses around 544
decameters Tuesday morning. Thus snow levels will have a hard time
getting much below 6000 feet...and by then shower coverage will be
decreasing anyway. Perhaps some of the higher ski resorts will get a
dusting of snow but that is about it.
Tuesday may actually turn out to be a decent day especially for our
southern zones. Shortwave ridging develops aloft...which should dry
things out for most of our forecast area aside from the north coast.
Eugene and much of Lane County may break out into some sunshine for
the afternoon...allowing temperatures to push well into the 60s...perhaps
even making a run for 70 degrees in some spots.
Meanwhile the next frontal system will be strengthening over the NE
Pacific and approaching the Washington and northern Oregon coast Tuesday
afternoon and evening. This front will be a slow mover...with
conditions favorable for cyclogenesis along the front which would in
turn slow the front further. Depending on where and when waves of low
pressure develop along the front...strong winds may occur along the
beaches and headlands...especially for our S Washington/north or coast zones.
Models differ on where and when lows will develop along the front...
but as an example the 06z NAM develops a 994 mb low with an
associated coastal jet producing 950 mb winds as high as 60-65 knots
as far south as Newport by midnight Tuesday night. Then the 06z NAM has
an additional 998 mb low developing along the front by Wednesday afternoon
with another round of 50 kt+ winds at 950 mb along the Oregon coast.
For now suffice it to say there is a decent potential for strong
winds at times along the coast...particularly the beaches and
headlands...Tuesday night through Wednesday night.
Models also remain consistent in showing this front will have access
to abundant moisture with a deepening atmospheric river developing as
it approaches the coast. Moisture transport is impressive in all
model solutions...and it shows in the model quantitative precipitation forecast. With waves of low
pressure developing along the front...orographic enhancement will be
strong at times for south-favored slopes. For now...our quantitative precipitation forecast forecast
calls for 2 to 5 inches throughout the forecast area by the time rain
tapers off Thursday. Heaviest rain will be in the Coast Range and the
S Washington Cascades. This may be enough to cause some flood threat for our
more responsive rivers such as the Grays River draining out of the
Willapa Hills. Of course we will monitor this closely and will have a
better idea on the rainfall distribution as we get closer to the
Long term...Thursday through Sunday...Thursday will likely be the
tail end of our midweek rain event as the frontal system sags further
south and weakens over northern California. Medium range guidance
continue to struggle with how to handle the late week and weekend
period...with the 00z GFS/ECMWF/Gem seeming to trend toward shifting
the longwave pattern to favor troughing over western North America.
This would be a significant shift from the overall longwave ridging
pattern that has dominated the west for the past several months. As
such...the 00z GFS/European model (ecmwf) now bring another upper trough across the
Pacific northwest for more showers and cool temperatures through the weekend. Am a bit
skeptical of this large scale pattern shift... especially considering
the large pool of above normal sea surface temperatures over the NE Pacific.
Therefore we remain a bit conservative... keeping probability of precipitation/temperatures closer
to climatology over the weekend than these 00z models would suggest.
Aviation...moist southerly flow aloft as front pushes inland.
Mostly VFR...but MVFR now increasing along the coast. Should have
areas of MVFR further inland until 17z. Expect mountains/passes to be
frequently obscured. Air mass becoming unstable after 17z...with
showers increasing. At that time...generally VFR with areas of
MVFR ceilings/visible under showers and isolated thunderstorms.
Kpdx and approaches...VFR today...with rain until 17z...then
brief break before showers increase. Still looks reasonable to
expect areas of MVFR ceilings 12z to 17z as front moves slowly across
Ops area. Then only occasional MVFR ceilings under showers. Rockey.
Marine...broad low pressure stays anchored over the Gulf of Alaska.
Several fronts will push across the coastal waters this week.
First is approaching the coast early this am...and will move
onshore this morning. Winds have eased a bit...but should maintain
enough pressure gradient for gusts 20 to 25 knots today and tonight.
Winds may relax at times...but with showers and a few thunderstorms over
region...will still have gusty winds at times. So...will extend
Small Craft Advisory for winds out through Tuesday.
Next front will be much stronger...and will approach region later
Tuesday and slowly move across the coastal waters Tuesday night into
early Wednesday. Looks like solid southerly gales...with gusts 40 to 45
knots Tuesday night.
Seas now running 12 to 13 feet...but will hold rather steady through
Tuesday. But with increasing winds later Tuesday and Tuesday night...expect
combined seas to build back to 15 to 20 feet for Tuesday night...but
highest still still likely to be over the outer waters...or from
10 to 60 miles offshore. Rockey.
Pz...Small Craft Advisory for winds and hazardous seas today
through Tuesday on all coastal waters.
Small Craft Advisory for rough Columbia bar conditions
from now through Tuesday afternoon.
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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.