Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 191040 AAA
AFDPQR

Area Forecast Discussion...Updated
National Weather Service Portland OR
339 AM PDT Thu Oct 19 2017

Updated to add last paragraph to short term discussion.

.SYNOPSIS...A very wet cold front will gradually move southward
across western Oregon today. This front is the leading edge of a cool
upper level trough which will cause snow levels to lower to near the
Cascade passes tonight and Friday. Post-frontal showers will linger
through Friday, perhaps with a few thunderstorms. Another strong and
wet frontal system is expected to bring more heavy rain to the
forecast area this weekend. Drier weather is likely early next week
as a strong ridge of high pressure builds over the western United
States.

&&

.SHORT TERM...Today through Saturday...Impressive rainfall totals
across much of Pacific and Wahkiakum Counties overnight, as our slow
moving and very moist cold front slowly lumbered its way south of the
Columbia. Thus far these SW Washington counties have generally picked
up 2-4 inches of rain overnight, with significant rises on the Grays
River near Rosburg as a result. Thankfully the focus of the heaviest
rain has slowly shifted southward overnight, with 1-2 inches thus far
across much of Clatsop and northern Tillamook Counties. The leading
edge of the rain extends from a Portland to Newport line, slowly
advancing southeast with the front. It has been fairly breezy ahead
of the front, with south winds gusting 25-35 mph inland and 40-50 mph
along the coast. Decided to cancel the High Wind Warning for all
zones earlier in the shift, as winds appear to have peaked below
criteria.

Looking upstream, a strong vortmax near 45N between 135-140W is
approaching the Pac NW coast, being driven by a strong jet max to its
W-NW. This system will act as a kicker for the front, accelerating it
southeastward this afternoon and evening. This system will also push
the moisture axis southward into Northern California. By the time
that occurs, most of the Willamette Valley will have seen 0.50 to
1.00 inch of rain, with some spots over an inch. Coastal and higher
terrain locations will probably average 1.50 to 3.00 inches, with
local 2-4 inch amounts for our northern coast/higher terrain zones.
These are impressive totals, but probably not enough to cause
widespread hydro issues other than some problems with backed up storm
drains, especially in urban areas. Another area of concern is the
Eagle Creek burn scar in the Columbia Gorge, but the 2 inches or so
expected from this front will probably not be quite enough to cause
significant issues. Will need to monitor rain rates closely in the
Gorge, but burn scar impacts are still a bit of an unknown as the
soil becomes increasingly saturated and the environment becomes
increasingly different than it was during our last significant heavy
rain event back in late September.

Cool air pushing in behind the front will lower snow levels to around
4000 to 5000 feet tonight, based on GFS 850 mb temps around zero by
Friday morning. Snow showers will add up to a few inches tonight and
Friday at the higher ski resort elevations, but with the jet stream
focused to our south and only modest orographic flow at 850 mb, it
will be difficult to wring out 6 inches of snow in any given 12 hour
period. Therefore we will hold off on issuing any snow advisories at
the moment. For the lowlands, there will be plenty of showers, but
mainly of the air mass variety. With 500 mb temps starting off Friday
in the -28 to -30 deg C and lifted indices near zero or slightly
negative per the latest GFS, cannot rule out a few thunderstorms
Friday. However, temps aloft are expected to warm up during the
afternoon, which could make it somewhat difficult for thunder to
continue later into the afternoon and evening.

Warm advection and isentropic lift intensify Friday night and
Saturday as the Pacific jet stream takes aim again at the Pac NW.
Most models are hinting at a strong lobe of low pressure quickly
approaching the Pac NW coast Fri night before curving northward early
Sat. This system pushes a warm front onshore Sat morning and across
the Cascades by afternoon. Based on this evolution, expect rain to
increase throughout the forecast area late Fri night or early Sat.
Excellent isentropic lift of a very warm/moist air mass over the
cooler air in place will probably very efficient at generating
precipitation, meaning late Fri night/early Sat could be very wet
throughout the forecast area. Once the warm front pushes east of the
Cascades, snow levels will rise well above pass level Saturday
afternoon. 06z NAM shows a solid 40-50 kt W-SW flow at 850 mb across
the Coast Range and into the Cascades Saturday afternoon, which would
likely result in substantial QPF in the higher terrain with
considerable rain-shadowing for the inland valleys. Another 3 to 6
inches of rain are possible for the higher terrain from this system,
with 2 to 4 inches along the coast and 1 to 3 inches for the inland
valleys. This could be enough to start to cause hydro issues, and
brings up concerns for the Eagle Creek burn scar in the Gorge. Will
issue a hydrologic outlook based on all this shortly after getting
the morning forecast package out.

One last item of concern for the Saturday...winds, particularly
inland. After the warm front moves through late morning or midday
Saturday, the latest GFS/ECMWF/NAM all open up the pressure gradients
considerably for gusty southerly flow both along the coast and
inland. Depending on the magnitude of south-to-north pressure
gradients, peak gusts could be anywhere between 30-50 mph inland and
40-60 mph along the coast and in the higher terrain. The higher end
of these values would warrant a Wind Advisory for the inland valleys,
and the coast could touch warning criteria.  Weagle

.LONG TERM...No Changes. Previous discussion follows...
Saturday night through Wednesday....Strong moist zonal flow will set
up over the region Saturday night through Sunday. Snow levels will
rise above 8,000 feet. Still some uncertainty as to whether or not
the heaviest rain will set up over western Oregon or western
Washington. Have kept PoPs high, but forecast details such as amount,
intensity, and duration are unclear.

Expect to see some rises on area rivers, especially in the coastal
basins, and if the heavy rain sets up over urban areas, some
localized urban flooding issues could develop. Also, locations with
significant burn scars should be aware of possible flash flooding and
debris flows.

Models are suggesting, short wave ridging will develop over the
Pacific NW early next week, which should bring at least a day or two
of dry weather. The Euro model suggests a weak front could push into
the region during the middle of next week while the GFS remains dry
and mild. Given the uncertainty, have trended PoPs towards
climatology. Bishop

&&

.AVIATION...Leading edge of rain band behind a cold front
is along a Pacific City to Portland line. This band will slowly
slide south today bringing moderate rain with MVFR to possibly
IFR cigs and vis. Conditions look to improve after 20/02z. Plan
for a return to VFR inland with MVFR at the coast.

KPDX AND APPROACHES...MVFR cigs and vis expected as a rain band
slides across the area today. Heaviest rain arrives
around the morning push bringing some possibility of IFR
Cigs/Vsbys. Conditions starting to improve after 22Z with VFR
expected after 20/02Z. South wind gusting to 15 to 25 kt range
likely to persist through 20/00Z.

&&

.MARINE...Cold front over the central Oregon coastal waters
sliding south. Winds at buoy 46050 off Newport gusting 35-37 kt
past few hours. Peak winds at Newport Jetty was 44 kt. Winds
will be easing over the next few hours with gales ending by 5 AM
or so. Winds for the remainder of the day and evening will be
gusting 20 to 25 kt as a trough moves into the waters. An
unstable air mass later today brings a slight chance of
thunderstorms through Fri.

Seas will be around 16 ft in the area of gales, then
subside some as the winds ease. But will be ramping up to 20 to
22 ft this afternoon as a longer period northwest swell arrives.
Expect coastal areas above 20 ft around 2 PM and peaking late
afternoon/early evening then slowly subside. Seas will remain in
the 15 to 18 ft through at least late Sat. In addition to the
Small Craft Advisory for Hazardous seas, there is a High Surf
Advisory for the coastal beaches this afternoon through Fri
morning. Additionally, expect particularly hazardous bar
conditions along the length of the coast. Another storm system
capable of producing at least Gale Force wind gusts appears on
track for Friday night and Saturday. /mh /JBonk

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory from 1 PM this afternoon to 10 AM PDT Friday
     for Central Oregon Coast-North Oregon Coast.

WA...High Surf Advisory from 1 PM this afternoon to 10 AM PDT Friday
     for South Washington Coast.

PZ...Small Craft Advisory for winds until 9 PM PDT this evening for
     Coastal waters from Cape Shoalwater WA to Cascade Head OR
     out 10 NM-Waters from Cape Shoalwater WA to Cascade Head
     OR from 10 to 60 NM.

     Small Craft Advisory for hazardous seas until 11 PM PDT Friday
     for Coastal waters from Cape Shoalwater WA to Cascade Head
     OR out 10 NM-Waters from Cape Shoalwater WA to Cascade
     Head OR from 10 to 60 NM.

     Gale Warning until 5 AM PDT early this morning for Coastal
     waters from Cascade Head to Florence OR out 10 NM-Waters
     from Cascade Head to Florence OR from 10 to 60 NM.

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

&&

$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the forecast area.



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