Heavy Rain in Oregon, October 13, 2016

Area Forecast Discussion
National Weather Service Portland Oregon
328 AM PDT Thu Oct 13 2016

.SYNOPSIS...Stormy weather has begun with rain spreading across the
area late last night. This first system brought some winds to the
Coast, but a much stronger system arrives today which will bring
strong winds to coastal areas and breezy conditions inland. Behind
this system on Friday, expect showery and breezy conditions to
continue across the area through the day. Another very windy and
quite wet system is on tap for Saturday, which could potentially be
stronger than the system we see today. Unsettled weather continues
into early next week.

&&

.SHORT TERM...(Today through Saturday)...Radar shows rain has spread
across the area this morning, and observation sites along the Coast
show gusts to 25 to 30 mph out of the south. This front stalls near
the Coast this morning, keeping steady rain through the morning
hours. This front is re-energized as a strong low pressure system
approaches from the southwest. This system is currently near 46N,
145W, and is tracking east right along with the models. 00Z NAM, GFS,
and ECMWF appear to have a good handle on the position of system at
initialization. As this system approaches the Coast, the models
appear to be honing in on a track with the Low sliding northeast up
the Coast, making landfall somewhere between the Olympic Peninsula
and south Victoria Island. The models diverge somewhat on how deep
the low pressure center will get. The NAM has the slightly deeper Low
with a central pressure of 977 around 126.5W, while the ECMWF is the
weakest with the surface low not quite closing off until it gets
north of our waters. The WRF GFS is probably a happy medium, with a
somewhat closed-off circulation around 125.5W off the Coast of
Astoria of 984 mb. Difference between the positions and intensities
of the lows may not impact rain forecast much, as models are still in
decent agreement for the highest rain rates to occur Thursday night.

Moderately high confidence of strong winds developing along the Coast
this afternoon and evening, increasing along the South Oregon Coast
later this morning, and spreading up the Oregon Coast in to
Washington during the afternoon. High Wind Warnings remain in effect
for all coastal areas as well as the Coast Range and Willapa Hills.
Strongest winds appear to be from mid-afternoon through early evening
along the Coast. With trees still heavy with leaves, any moderately
breezy conditions could wreak havoc on powerlines. Confidence is
lower for how strong of winds we will see in the interior lowlands.
Models suggest gusts to right around 40 mph for the Central
Willamette Valley northward, which is marginal wind advisory
criteria. Will definitely depend on how deep the system gets and how
close the Low gets to the Coast.

Models show a weaker system moving through on Friday, though the NAM
is the outlier with a slightly stronger Low than the open-wave weak
system suggested by the GFS and ECMWF. Really thinking right not that
we will see a break from the strong winds along the Coast on Friday,
though conditions will still be breezy with showers across the area.
There is a slight chance for some thunderstorms over the waters on
Friday associated with instability generated by warmer southerly
winds and cooler air moving in aloft as the upper-level trough
spreads over the waters.

The next significant storm incorporates the remains of Typhoon
Songda, which is currently around 42N, 175E.  As the remains of this
Typhoon enter the 180-200 kt Jet Stream over the Central Pacific, it
will get entrained into the storm track and become the system which
will impact the Pacific Northwest on Saturday.	Models continue to
vary significantly on the track of this low pressure system as it
approaches the PACNW. While the ECMWF continues to stick to its guns
with a track making landfall around mouth of the Columbia River, the
GFS is consistently showing the Low tracking north into South
Vancouver Island. While the ECMWF would certainly be the worst-case
scenario for Northwest Oregon and Southwest Washington, the strength
of the GFS track would still bring very strong winds to the Coast and
maybe even some strong winds to interior lowlands as well. Right now
forecasting a blend of the two, with a round of strong gusty winds
across the region, perhaps stronger than we see on Thursday. Again,
if we see the Low track closer to the Coast, winds expected will be
stronger and extend farther inland.

We should see a lot of rain today through Saturday, perhaps as
much as 3 to 6 inches coastal mountains, and generally 2 to 4 inches
for the coast and Cascades. Should see 1 to 3 inches in the valleys.
At this point, there appear to be enough breaks in between the
systems to prevent river flooding. But with leaves falling off trees
now, expect areas of low-lying and other poor drainage areas, and
some urban flooding that could be exacerbated by leaf-clogged storm
drains. 						-McCoy

.LONG TERM...(Saturday night through Wednesday)...No Changes.
Previous discussion follows...Still going to have windy conditions
Saturday night with the strong low pressure system on Saturday. Will
see winds dropping becoming less of a concern on Sunday, while we
continue to see post-frontal showers through the day and into Monday.
Another low pressure system seems in store for Late Monday into
Tuesday. The most recent run of the ECMWF appears to have turned it
into more of an open wave system which would be much weaker, while
the GFS continues to show another sub 960-mb low pressure system
within 130W moving into the Washington Coast. This Tuesday system
appears to move through much quicker which would lead to a lesser
threat for winds, maybe only a short period of moderately strong
winds along the Coast and a breezy period inland. The faster movement
would also mean not as much rainfall with this system compared to the
past Thursday or Saturday systems. Because the track and intensity of
this system are uncertain, we can't be confident in the details at
this time. Going past Tuesday next week, the weather seems to settle
down, with both the ECMWF and GFS suggesting broad ridging building
over the area. This means we may get a well-needed break from the
active weather at least on Wednesday and possibly through the end of
the week.     -McCoy

&&

.AVIATION...Conditions currently VFR inland and MVFR along the
coast, but expect inland areas to lower to MVFR for at least a few
hours Thursday morning with some possible pockets of IFR along the
coast. Expect a return to mix of MVFR and VFR by Thursday
afternoon with continuing rain. Could see occasional IFR cig or
vis with any heavier rain later Thursday. Expect windy conditions
out of the south at the coast starting around 20Z with breezy
conditions inland. Will be keeping an eye on potential for low
level wind shear Thursday afternoon as winds above the surface
will pick up before surface winds.

KPDX AND APPROACHES...VFR conditions, lowering to MVFR during the
morning hours. May see occasional MVFR cig or vis during the
afternoon and evening hours with RA. Also possible low level wind
shear during the afternoon hours before surface winds pick up.
Bowen

&&

.MARINE...Buoys currently showing gale gusts, which should
decrease briefly this morning before strengthening quickly up to
storm force out of the south this afternoon. Winds ease a bit
with a secondary low Friday but still may reach gale force at
times. The next very strong system is expected Saturday with
a possibility of hurricane force wind gusts, particularly in the
waters off the central coast. Will hold off on issuing a Hurricane
Force Wind Watch for now, but mostly to avoid confusion with
today's system. There is no doubt Saturday's system will bring
very dangerous conditions to the Oregon and SW Washington waters.

Expect seas to reach ~25 ft for a brief time late this afternoon
and evening. Seas then drop into the teens Friday with the
decrease in winds. The storm on Saturday has the potential to
push seas above 30 ft and if a dynamic fetch situation develops,
seas could go higher, up close to 40 ft.

Considerably lighter westerly winds should spread into the waters
Sunday and Monday, which will allow seas to subside well into the
teens. However, it looks like a third surface low pressure system
will bring another round of gale to storm force winds late Monday
night or Tuesday, possibly pushing seas up above 20 ft once again.
Bowen

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 2 PM this afternoon to midnight PDT
     tonight for Central Coast Range of Western Oregon-Central
     Oregon Coast-Coast Range of Northwest Oregon-North Oregon
     Coast.

     High Surf Advisory from 4 PM this afternoon to 3 AM PDT Friday
     for Central Oregon Coast-North Oregon Coast.

WA...High Wind Warning from 2 PM this afternoon to midnight PDT
     tonight for South Washington Coast-Willapa Hills.

     High Surf Advisory from 4 PM this afternoon to 3 AM PDT Friday
     for South Washington Coast.

PZ...Storm Warning from 2 PM this afternoon to 2 AM PDT Friday for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 nm.

     Gale Warning until 2 PM PDT this afternoon for Coastal Waters
     from Cape Shoalwater WA to Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar from 11 AM
     this morning to 5 PM PDT Friday.

&&

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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.