Flood in Washington, November 24 to 25, 2016

Area Forecast Discussion
National Weather Service Portland Oregon
1017 AM PST Thu Nov 24 2016

.SYNOPSIS...A strong frontal boundary moving into Southwest
Washington and far Northwest Oregon this morning then become nearly
stationary. A secondary low pressure area develops offshore along the
front this afternoon, reaching the North Washington coast Thursday
night. The frontal boundary weakens late Thursday night and Friday
morning with the main energy over Southwest Oregon. A cold upper
trough settles over the forecast area this weekend. The general
progressive pattern continues next week.

&&

.SHORT TERM...Today through Saturday...A cold front was just easing
onto the south WA and north OR coast this morning, in the process of
stalling out there today. Front has been active this morning, with
thunderstorms contiuing along the front mainly off the central OR
coast, a band of heavy rain over the north OR coast and Coast Range
extending nne into sw WA, and gusty winds along the OR part of the
coast. Winds overall have been generally topping out in the 45 to 55
mph range at lower elevations on the coast this morning, and with
the front expected to weaken some today, will go ahead and bring
this warning to an end with the morning forecast issuance. Will
retain a slight chance for thunderstorms on the coast and out over
the coastal waters across the southern part of the forecast area
today with the front stalling there. Will bump up qpf amounts from
the Willapa Hills to the north Oregon Coast Range this morning in
response to where heaviest rain is currently falling. Models
indicate heaviest rain as seen in water vapor transport fields
shifting only slightly further south this afternoon, leaving the
Coast Range in heavy rain later today. Rainfall rates appear on
track to be as high as 1.5 to 2 inches per 6 hours there. Southwest
flow and the proximity of the front still favor south WA Cascades
for highest snowfall potential today into tonight, so will keep
headlines there the same as in previous forecast. Offshore flow
component has kept snow levels locally al ittle lower in the upper
Hood River Valley this morning, with temps in the middle to
upper30sin  Snow levels in the upper Hood River Valley have dropped
to as low as 2500 ft this morning as temps dropped into the mid to
upper 30s above Parkdale.

This morning will also need to address the high wind watch up for
the coast this evening. At this time models have generally been
indicating a marginal high wind event at best for the coast, with
12z nam still indicating a sub 1000mb closed low expected to track
nne off the coast this evening. 12z gfs has returned to a more open
wave like the 00z ec, but still shows marginal high wind potential
ahead of the front this evening. As most models seem to suggest low
level wind fields no stronger than this mornings system, and the gfs
going to a more open wave solution, prefer to drop the watch at this
time, and limit gusts to 50 to 55 mph.

Remainder of short term discussion unchanged...The 06Z GFS has the
heaviest rain this afternoon over the North Oregon coast basins,
then slowly shifts the focus south late this afternoon and tonight.
By Friday morning the front has weakened with the primary dynamics
over far SW Oregon and NW California. But, the front never goes
completely away. The GFS, and to a certain extent the ECMWF, lift
the frontal remains north Fri evening and Fri night as a colder
upper level trough pushes toward the coast.

Snow levels will vary quite a bit across the Cascades today and
tonight. Cold air will likely be stubborn to scour out across the
south Washington Cascades especially near Mt Adams where 10-20 inches
of snow appear likely above 3500' and somewhere between 15-30 inches
above 4500'. Warm air streaming northeastward ahead of the front
should result in snow levels rising to around 4500 feet this
afternoon across the Oregon Cascades. This process will be slowest to
occur near Mt Hood so the current Winter Wx Advisory looks on track.
The good news is that this should bring a period of improved travel
conditions for those traveling across the Cascade passes today.

The cold upper trough Saturday will keep snow levels below the
passes. QPF will not be all that impressive, but snow will continue
to accumulate above the passes. Weishaar

.LONG TERM...(Saturday night through Wednesday)...No Changes...
Previous Discussion from Wednesday afternoon follows...The active
weather pattern looks to continue through much of the long term
forecast as a series of storm systems move across the Pac NW. Models
continue to show an upper level trough moving across the forecast
area Saturday night and Sunday which will maintain showers over the
region. Snow levels look to lower to around 2500 ft on Sunday, with
an additional 3 to 6 inches possible at the Cascade passes. If you
are planning on traveling over the Cascades, prepare for hazardous
road conditions.

Models then show a weak upper level shortwave dropping southeastward
Sunday night and Monday. This system looks to bring another round of
rain to the area late Sunday/early Monday, with cool northwesterly
flow keeping showers going on Monday. High pressure looks to build
across the area Monday night and Tuesday bring with it a period of
quieter weather and raising snow levels to around 4500 ft. However,
this bring will be short as models show another upper level trough
dropping south out of the Gulf of Alaska Tuesday night and Wednesday
which could bring another round of heavy rain and Cascade snow, with
locally windy conditions. /64

&&

.AVIATION...Predominantly MVFR at the Coast this morning, with
moderate rain reducing vis as well as MVFR cigs. Inland has stayed
predominantly VFR so far, but this should change as heavier rain
moves into northern terminals. Through the day expect northern
terminals to stay predominantly MVFR as the front hangs over a
line from KONP to KTTD through tonight. KSLE and KEUG will see
some rain today as well, but should remain predominantly VFR
until after midnight tonight. Winds will be breezy out of the
south, with strongest winds along the coast, gusting up to 45 kt
at KONP and 35 to 40 kt at KAST. In the Willamette Valley, expect
gusts to 25 to 35 kt through this afternoon and evening, with a
bit of a lull midday. Predominantly MVFR conditions overnight
through at least early Friday morning.

KPDX AND APPROACHES...VFR right now, but heavier rain about to
move in should reduce vis to MVFR by 1830Z. Predominantly MVFR
conditions through the rest of today, with cigs lowering to
around 2500 ft by 00Z and MVFR vis with moderate rain.  -McCoy

&&

.MARINE...Line of thunderstorms off the Coast of Florence should
slowly move onshore and weaken by noon today. Winds have decreased
behind the front that moved through this morning, with winds
generally back down below 34 kt. Seas are still up around 21 ft at
Buoy 50 with winds gusting to 40 kt likely due to the line of
thunderstorms. Still expect a lull in the winds before winds
start to ramp back up this afternoon, though shorter for the
waters south of Cascade Head.

The next system will start to ramp up winds south to north this
afternoon with another round of widespread Gales expected through
around midnight tonight. There is a chance for a few gusts to 50
kt south of Cascade Head, but right now not thinking they will be
frequent enough to warrant a storm warning. Will evaluate further
as this system approaches as the models have not done well with
the track of this system. Seas with this system will again get
back up to around 20 to 23 ft, maybe even as high as 25 ft in the
waters south of Cascade Head if winds are stronger than currently
anticipated. Seas will be primarily wind-driven, so as the front
moves through, expect seas to start coming down fairly quickly
behind the front.

Winds and seas subside through the day on Friday into the weekend.
Probably will still see small craft advisory winds through most
of Friday, though winds should drop below 20 kt by Friday evening.
Seas should drop back down into the mid-teens by Friday evening as
well. Weaker fronts move across the waters over the weekend, but
generally seas expected to drop to around 9 ft by Saturday night
or Sunday, and winds stay fairly benign for this time of year.
-McCoy

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 4 AM PST Friday for Northern
     Oregon Cascades.

     Flood Watch through Friday evening for Central Coast Range of
     Western Oregon-Central Oregon Coast-Central Willamette
     Valley-Coast Range of Northwest Oregon-Greater Portland
     Metro Area-North Oregon Coast-South Willamette Valley.

WA...Winter Storm Warning until 4 AM PST Friday for South Washington
     Cascades.

     Flood Watch through Friday evening for South Washington Coast.

PZ...Gale Warning until 10 AM PST this morning for Coastal Waters
     from Cape Shoalwater WA to Florence OR out 60 nm.

     Gale Warning from 1 PM this afternoon to midnight PST tonight
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 10 AM
     PST 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.