Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
931 am PDT Monday Oct 20 2014
Synopsis...a slow moving cold 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 weekend.
Update...low pressure trough axis has pushed just inside 130w along
the pacnw coast. The back edge of a north/S oriented front is located
just west of I-5 with rain bands to the east transitioning to
showers off to the west. Energy diving south along the base of the
trough will maintain the slow eastward progression of the front.
Still appears that best chance for thunderstorms will be over the
waters this afternoon and evening with an outside threat of seeing
one develop inland this afternoon and evening. Only significant
change was to push the text wording over to Post frontal shower and
thunderstorm coverage wording.
An early look at morning model run data maintains confidence in
another slow moving and moisture laden front crossing the area over
the next few days. Current quantitative precipitation forecast forecast looks fairly good against
the recent observation. Esrl reforecast analogs of similar past events
provide similar quantitative precipitation forecast amounts and pattern giving further confidence of
total rain for the next 72 hours. As mentioned below and in our
social media posts...its time to ensure drains and culverts are
ready to receive a prolonged several hour period of moderate and
briefly heavy rain. Updated grids are available on the web with text
products to follow shortly. /Jbonk
Previous disc... /issued 226 am PDT Monday Oct 20 2014/
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. Weagle
Aviation...with front pushing slowly inland today precipitation
will become more convective in nature. Expect VFR conditions to be
more dominant today and tonight...but with occasional MVFR ceilings and
visibilities generally associated with areas of rainfall and
showers. Expect mountains/passes to be frequently obscured. Isolated
thunderstorms will be possible near the coast through 02z.
Kpdx and approaches...VFR today and tonight...but with occasional
MVFR conditions mostly associated with showers. Showers will be less
frequent tonight after 02z.
Marine...S winds behind a cold front that moved ashore this early
this morning will remain near small craft criteria into Tuesday...with
the main threat of wind gusts in the 20 to 25 knots range coming in
areas of showers. West swell of 10 to 14 feet around 15 second is expected
to only slowly subside through Tuesday.
Winds expected to pick up during the day Tuesday into Tuesday night as a
frontal system approaches. Winds are likely to reach gale force
throughout the waters from Tuesday afternoon through much of Tuesday
night...until a cold front moves southeast into the waters. The westerly
swell will continue to subside Tuesday and Tuesday night...but seas are
likely to build some and become steeper Tuesday night due to the
southerly gales. Seas may approach 20 feet Tuesday night...most likely
over the outer waters.
Pz...Small Craft Advisory for winds until 6 PM PDT Tuesday for
coastal waters from Cape Shoalwater Washington to Florence or out
Small Craft Advisory for hazardous seas until 6 PM PDT
Tuesday for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar until
6 PM PDT Tuesday.
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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.