Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
307 PM PDT Wednesday Oct 22 2014
Synopsis...a moist and slow-moving frontal system is bringing heavy
rain and gusty winds to southwest Washington and northwest Oregon
today. The front will slowly move southeast this evening...moving
into the Cascades overnight. Expect plenty of Post-frontal showers on
Thursday...and possibly a thunderstorm along the north coast. Another
potentially strong system may bring more rain and wind to the area on
Saturday. Yet another front is possible early next week.
Short term discussion...tonight through Saturday...a look at the
latest Doppler radar imagery shows a wide band of moderate to heavy
rain associated with a slow moving cold front. AMSU satellite derived
precipitable water values are showing around 1.25 inches feeding into
the frontal boundary. The rain band has moved very little through the
course of the day...as a disturbance riding along the boundary led to
a well defined baroclinic Leaf which effectively stalled the front.
The upper level disturbance is apparent on water vapor satellite near
43n 130w at current. The most impressive precipitation totals so far
are in the S Washington and north or Coast Range and the S Washington Cascades...where
generally 1.5 to 3 inches have fallen so far. The focus of the
heavier rain has shifted slightly southward over the central or Coast
Range and north Willamette Valley over the past couple of hours.
Expect the heavy rain to push very slowly southeast through this
evening. The latest river forecasts do not indicate any river
flooding is likely to occur from this system. However...the coastal
rivers draining the Coast Range are showing sharp rises this
afternoon. Another concern is the potential for urban street flooding
in the Willamette Valley. Heavy rain is expected to continue well
into this evening. There are also plenty of leaves around...so
clogged storm drains could be a concern. Things should begin to quiet
down later this evening and overnight as the steadier rain moves into
the Lane County Cascades. However...expect plenty of Post frontal
showers remaining across the region through Thursday. A broad closed low
off of the British Columbia coast will spin a couple of shortwaves
toward the Pacific northwest overnight into tomorrow. This should serve to
enhance the shower activity and may even bring a few thundershowers
to the offshore waters along the north coast.
Expect showers to taper off Thursday night into early Friday as upper level
heights build upstream of ridging developing over The Rockies. Warmer
air aloft will begin to stabilize the atmosphere. However...model
agreement is fairly good that another period of light rain will
spread into the region from south to north Friday afternoon and evening
as a warm front lifts in ahead of another low pressure system. A
brief break from the precipitation is being modeled for most of the
region behind the warm front Friday night into early Sat. Confidence is
beginning to increase that the low pressure system will strengthen
rapidly offshore late Friday into early Sat...then push onshore along
the Washington coast late Sat. If the low tracks close enough to the Oregon
coast...like the GFS 12z is indicating...there could be potential for
very gusty winds through the Willamette Valley. However...there is
quite a bit of model spread in the exact timing and trajectory of the
low...which will have significant implications on how this system
impacts our forecast area.At this time...it appears that another wet
and windy period is likely Sat...but details remain uncertain. Pyle
Long term...Saturday night through Wednesday...an active weather
pattern will persist through the long term forecast with frontal
systems moving across the region every 24 to 48 hours. Models are in
good agreement that a front will bring rain and gusty winds Saturday
night into Sunday morning...with showers dwindling Sunday afternoon.
Snow levels may drop down to near or just below the Cascade passes
with the showers Sunday afternoon. A warm front ahead of the next
system should generate light precipitation Sunday night and Monday
before a cold front moves through Tuesday. The models are in less
agreement with the strength and position of the low with this
storm...but are in fair agreement on the timing of rain. Showers
will continue Tuesday night into Wednesday...then another front is
expected Wednesday afternoon or evening...with greater uncertainty
on the timing and strength tj.
Aviation...a mix of IFR...MVFR and VFR conditions will occur
through the evening hours as steady rain associated with a cold
front continues to reduce visibilities and occasionally produce
lower ceilings. As the lower atmosphere destabilizes
overnight behind the cold front and precipitation becomes more
showery from north to south...expect VFR conditions to follow
suit. Another developing weak low pressure system could temporarily
reduce flight conditions into MVFR across the north Thursday morning
into midday Thursday.
Kpdx and approaches...steady rain associated with a cold front will
continue to produce occasional IFR to MVFR conditions late this
afternoon and into early this evening. Conditions should trend
towards a mix of VFR and MVFR overnight and Thursday morning before
conditions likely turn VFR Thursday afternoon. /Neuman
Marine...a cold front continues to March southward across the
central Oregon waters and based on weather stations at Newport and
Florence...it appears to be exiting the waters. Will monitor
trends...but the Gale Warning for the inner central Oregon waters
appears about over despite short term models suggesting otherwise.
Will let evening shift take in latest observations before canceling
the Gale Warning for the central Oregon inner waters.
Further north...a sharp surface trough or weak low pressure has been
developing over our northwestern waters. It has deepened more than
anticipated and just started producing low end gale force wind gusts
of 35 knots at buoy 29. If winds have not peaked already at buoy
29...suspect they will in the next hour or two...before spreading
into the northern inner waters around the mouth of the Columbia
River. As a result...a Gale Warning for wind gusts to 35 knots was
issued for the northern waters. Confidence in the details is low.
For Thursday...seas will in general be on the decrease.
However...one model suggests another weak low pressure will
develop and move just north of the waters. This could result in
at least small craft winds of 25 to 30 knots across the northern
waters Thursday morning. Similar to todays winds...this may slow
the downward trend in seas.
Friday still looks relatively quiet across the waters as seas will
likely drop below 10 feet.
The big story will be this weekends low pressure system. Models
continue to vary on the details...but generally agree a developing
low pressure system...around 985mb...will move northward inside 130
west and make landfall somewhere between Vancouver Island and the north
Oregon coast. This will almost certainly produce gale force winds
across the waters on Saturday...with a strong possibility of low end
storm force gusts of 50 knots. Seas will likely rise sharply
Saturday afternoon and evening...and approach 20 feet late Saturday.
After a break in the weather late in the weekend and early next
week...models suggest another strong low pressure will make landfall
on the West Coast next Tuesday. Gale force to low end storm force
winds certainly appear possible once again with this storm system.
Washington...none. Pz...Gale Warning until 9 PM PDT this evening for coastal
waters from Cape Shoalwater Washington to Florence or out 10 nm.
Small Craft Advisory for winds until 3 am PDT Thursday for
waters from Cape Shoalwater to Florence or from 10 to 60 nm.
Small Craft Advisory for hazardous seas until 5 PM PDT Thursday
for waters from Cascade Head to Florence or from 10 to 60
Small Craft Advisory for rough Columbia River bar until 5 PM PDT
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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.