Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 061818 AAC

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1017 AM PST Wed Dec 6 2023

Updated public and marine forecast discussion and WWA headlines...

.SYNOPSIS...Moderate to heavy rainfall continues across the
region through today as a stalled atmospheric river slowly
moves east through the region. As a result, river and urban
flooding concerns will continue through at least this evening.
A colder weather system will arrive Thursday, bringing
additional rain and Cascade snow. There will be a brief break in
the weather Friday before another warmer yet weaker weather
system arrives over the weekend. Dry weather expected to return
early next week.


.SHORT TERM...Wednesday through Friday...UPDATE: Current radar
imagery and observations as of 10 AM PST show moderate to locally
heavy rain across northwest Oregon and southwest Washington as a
stationary front remains draped over the area. Hourly rain rates
remain around 0.1-0.25 inches per hour, though occasionally higher
rain rates up to 0.4 inch per hour could be observed over the Coast
Range and Cascades due to orographic forcing. The highest rainfall
amounts have been from Salem to the Portland/Vancouver Metro Area,
where around 0.50-0.75 in of rain has fallen between 7-10 AM. Several
rivers remain over flood stage and there have been numerous reports
of urban flooding, including flooded roadways. Expect the threat for
river and urban flooding to continue through at least this evening as
this atmospheric river event continues. Please read the hydrology
section below for more details.

Model guidance suggests that by late this afternoon, another
stronger upper level shortwave trough will begin pushing the
front eastward at a faster pace. Once this happens, overall
rainfall activity will decrease and briefly become more showery
until the next system arrives Thursday. Post- frontal
thunderstorms are a possibility along the coast Wednesday night
into Thursday as colder air moves in aloft and creates a more
unstable atmosphere. Snow levels will also begin falling to
around 4000-4500 ft by Wednesday evening, though the showery
nature along with warm roads and surfaces means only a few
inches of snow will be likely to accumulate in the overnight

Broad upper level troughing is still expected to shift over the
region on Thursday, bringing another round of widespread
precipitation and cooler temperatures aloft. Rain amounts will
be much lower than previous days, though the convective nature
of this system may bring brief periods of scattered moderate
rain. Snow levels will continue to fall through the day to
around 2500-3000 ft by late Thursday night as another frontal
system moves through the area. Ensemble 850 mb temperatures
drop to around -2 to -4 deg C which would support accumulating
snow at Cascade passes. Accumulation may still be difficult to
significant amounts due to warm roads and surfaces. However,
elevations above 3500 ft still may see anywhere from 4-14
inches in 24-hours.       -Alviz/HEC/DH

.LONG TERM...Friday through Tuesday...Drier and colder weather
expected on Friday as ensemble guidance continues to indicate a
brief break between weather systems. The next frontal system
is slated to bring another round of precipitation on Saturday
into early Sunday. GEFS/EPS continue to indicate an 80-90%
probability of a plume of moisture with increased IVT values
likely to be associated with this front. However, uncertainty
still remains with exactly where the main plume of moisture with
this AR will fall as well as in the timing of heaviest rainfall,
though confidence is pretty high that this will not be as
impactful as previous ARs. Latest guidance has shifted the main
plume of moisture north into central and northern Oregon with
rainfall totals of 1-2.5 inches over the Coast Range and
Casacdes and 0.5-1.25 inches for inland lowlands. Snow levels
are expected to rise again on Saturday from around 3000 ft to
6000 ft.

WPC cluster analysis shows upper level ridging building over
the Pacific coast early next week bringing a couple of dry days
to the region. However, there are indications that another
frontal system could impact the region sometime mid to late
week, though uncertainty is high in precipitation amounts at
this time. -HEC


.HYDROLOGY...A surface front and associated atmospheric river
remains over the region, moving extremely slowly southeast.
Widespread rain, locally heavy at times over the Coast Range
and Cascades, will continue into this morning before the front
begins pushing east quicker later this afternoon. Additional
rounds of quicker moving light to moderate showers are expected
tonight through Thursday.

Expected rainfall from 4 AM to 4 PM Wednesday ...
   ** S WA Coast/Willapa Hills0.25 to 0.5 inches
   ** OR Coast /Coast Range0.25 to 1 inches
   ** Inland I-5 Corridor0.25 to 1.25 inches
(includes Cowlitz, lower Columbia, Willamette Valley)
   ** South Washington Cascades/foothills0.5 to 1 inches
   ** Western Columbia Gorge0.5 to 1 inches
   ** North Oregon Cascades/foothills0.5 to 1.5 inches
   ** Lane County Cascades/foothills0.5 to 1 inches
   ** Mid-Columbia Gorge/Hood River Valley0.15 to 0.75 inch

Flooding continues along many of the rivers and streams that
drain from the Coast Range and Willapa Hills. Numerous flood
warnings remain in effect with minor to moderate flooding
occurring or expected. Landslides may occur, so anyone near
steep hillsides should remain alert for changes in conditions of
soil and vegetation. Flood Watch remains up for most of
southwest Washington and northwest Oregon through today.

On top of that, many of the smaller streams and creeks will remain
full into today, with some minor flooding expected. Areas with
poor drainage, such as railroad crossings and underpasses, as
well as areas with leaf-clogged drains, will see significant
ponding of water. An Urban and Small Stream Flood Advisory
remains in effect through this evening. -Rockey/HEC


.AVIATION...Moist, southwest flow associated with an atmospheric
river continues across the area through Wednesday. Predominately
MVFR conditions along the coast and inland, with occasional heavy
rain lowering conditions to IFR. MVFR cig probabilities remain
above 50% for northern and central Willamette Valley and increase
above 50% for the southern Willamette Valley after 14Z Wednesday
as the front pushes farther southeast. All sites have a 30-50% of
MVFR cigs persisting throughout the TAF period, as rain and
mountain obscurations are expected.

*Please note that the automated weather station at KONP (Newport) is
currently out of service. This site is not maintained by the
National Weather Service. Because of this, the TAF produced for KONP
is based on model data without the input of observations, and may
not be representative of current conditions.*

PDX AND APPROACHES...Rain will likely continue to maintain MVFR
VIS through Wednesday morning, while CIGS have begun to lower
from around 4000-6000 ft to 1500-2500 ft Wednesday morning. As
rain eases Wednesday afternoon, expect conditions to become a mix
of MVFR and low VFR around 20-22Z Wed. South to southeast light winds
Wed morning, increasing to 9-12 kts around 08z Thursday. /JH


.MARINE...A dynamic pattern will maintain active weather across
the waters through the weekend. Winds have eased and are expected
to become northerly as weak high pressure moves over the area on
Wednesday. The main impact across the coastal waters will be
hazardous seas through Wednesday due to a strong, long period
westerly swell. Seas of 16-18ft begin to gradually subside,
expecting to be below 15 ft late Wed afternoon to early evening.

The next frontal system is expected to approach from the west
late Wednesday and move across the coastal waters Wednesday night
into Thursday. Model guidance is showing seas building for a brief
period up to 17 ft in the region of strongest winds. Have extended
the current Hazardous Seas Warning to encompass this timeframe.

High pressure on Friday will bring a relatively brief break.
Another strong frontal system on Saturday will bring increasing
chances for Gales. -BMuhlestein



OR...Flood Watch until 4 PM PST this afternoon for Central Coast
     Range of Western Oregon-Central Oregon Coast-Central
     Willamette Valley-Coast Range of Northwest Oregon-Greater
     Portland Metro Area-Lower Columbia-North Oregon Coast.

WA...Flood Watch until 4 PM PST this afternoon for Greater Vancouver
     Area-I-5 Corridor in Cowlitz County-South Washington
     Cascade Foothills-South Washington Coast-Willapa Hills.

PZ...Hazardous Seas Warning until 1 PM PST Thursday for Coastal
     waters from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until 1 PM PST Thursday for Columbia River




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