NWS Portland, OR wrote its last area forecast discussion before the first event at Dec 5, 4:30 AM PST, 30 minutes earlier.
Area forecast discussion, Dec 5, 4:30 AM PST, 30 minutes before
Area Forecast Discussion
National Weather Service Portland OR
430 AM PST Tue Dec 5 2023
.SYNOPSIS...Moderate to heavy rain will continue pushing
southeast into the region this morning and will continue into
Wednesday morning. Rivers will be on the rise, with several of
those draining from the Willapa Hills and Coast Range likely to
flood sometime today into tonight. This will increase the risk
for river, small stream, and urban flooding through Wednesday
afternoon. Colder air moves inland behind the front Wednesday,
bringing snow showers to the Cascades. A colder weather system
Thursday will bring another push of lighter precipitation,
though moderate snow will be possible at times over the
Cascades. A brief break Friday before another warmer yet weaker
weather system impacts the region Saturday.
&&
.SHORT TERM...TUESDAY THROUGH WEDNESDAY...Satellite imagery and
surface observations indicate the surface cold front has
continued to slowly inch its way through the coastal waters and
along the PacNW coast over the past 8 hours. As of 3:30 AM PST,
the front has begun pushing inland along the SW Washington and
far NW Oregon coast with a clear frontal boundary stretching
from roughly the Puget Sound south into Tillamook County then
southwest into the Pacific Ocean. Water vapor satellite imagery
shows a vast amount of moisture streaming into the PacNW from
the Pacific Ocean along the front from the Pacific. The slow
movement of this strong atmospheric river as it approaches land
is as forecast. Guidance continues to indicate the front will
move very slowly through NW Oregon and SW Washington throughout
today and into early Wednesday morning, with a prolonged period
of moderate to heavy rain through this time. However, even now
uncertainty remains in exactly where the heaviest band of rain
will set up. This is down to how fast the surface front decides
to move and if and where the front ends up stalling, as
guidance has indicated it might, though the heaviest rain is
still expected along and northwest of a line stretching from far
western Lane county, through central Clackamas County, and west
of Hood River county. Many locations along and west of the front
have already picked up 0.5-1 inch of rain in the past 3 hours,
and the main push of moisture is yet to come. Rivers along the
NW Oregon and SW Washington coast are beginning to rise rapidly
and rivers across the region will continue to rise throughout
today and into Wednesday. Urban and river flooding is expected.
Please see the hydrology section below for more information on
river/urban flooding and forecast rainfall amounts.
Along with rain, gusty winds are occurring along and ahead of
the front, especially along the coast. Gusts up to 45 to 55 mph
have been observed and will continue into the morning hours for
wind prone areas along the coast and terrain. Elsewhere could
see gusts up to 25 to 35 mph along the front. A few downed
branches or power lines are possible in windier locations with
some larger tree branches and small trees falling due to
saturated soils.
Guidance shows a shortwave will eventually (finally) begin
pushing the front east Wednesday morning into the afternoon
hours. The HREF indicates rainrates could remain fairly steady
through Tuesday night/Wednesday morning with steadier rain
becoming showers sometime Wednesday morning, depending on
if/when the front stalls. Post-frontal thunderstorms are a
possibility as colder air pushes in behind the shortwave. Best
chances have shifted to later in the day and evening Wednesday
as the front will take its time moving east. As the colder air
moves in, snow levels will begin dropping over the Cascades
to around 4000-4500 ft Wednesday evening then to around
3000-3500 ft by Thursday morning, which will bring light snow
showers. -HEC
.LONG TERM...THURSDAY THROUGH MONDAY...Broad upper level troughing
expected to shift over the region on Thursday bringing another round
of widespread precipitation and cooler temperatures aloft. A
colder frontal system will move across the area on Thursday
causing snow levels fall to around 2500-3000 ft by Thursday
night. Ensemble 850 mb temperatures drop to around -2 to -4 deg
C which would support accumulating snow at Cascade passes. NBM
indicates a 70-90% chance of 12+ inches of snow at the
Willamette and Santiam Passes, with lower chances of 20-30% in
the northern Oregon Cascades.
Drier weather expected on Friday as appears likely to have a brief
break between weather system. The next frontal system then expected
to bring another round of precipitation on Friday. A plume of
moisture with increased IVT values likely to be associated with this
front, though there remains considerable uncertainty with exactly
where the Atmospheric River will take its aim. Latest models
show this will be directed somewhere along the Oregon coast.
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 on Sunday, likely
trending toward another break between systems. /DH/HEC
&&
.HYDROLOGY...
Heavy rain is slowly moving southeast through the region
Tuesday morning a cold front enters the region. Moderate to
heavy rain will impact the region into Wednesday morning.
Rainfall rates of 0.25 to 0.33"/hr are expected with the
potential for brief periods of 0.33 to 0.50 "/hr over the Coast
Range, Willapa Hills, and south Washington Cascades.
Rainfall for 4 AM Tuesday - 4 AM Wednesday:
** Coastal areas 1 to 4 inches
** Coast Range/Willapa Hills 2 to 6 inches
** Inland I-5 Corridor 1 to 3 inches
(includes Cowlitz, lower Columbia, Willamette Valley)
** South Washington Cascades/foothills 2 to 6 inches
** Western Columbia Gorge 2 to 4 inches
** North Oregon Cascades/foothills 1 to 3 inches
** Lane/Linn County Cascades/foothills 0.25 to 0.5 inches
** Mid-Columbia Gorge/Hood River Valley 0.5 to 2 inches
With all this new rainfall, many rivers will be on the rise again,
with those that drain from the Coast Range and Willapa Hills
being of most concern reaching flood stage on Wednesday. This
includes the Grays, Naselle, Nehalem, Nestucca, Wilson, and
Siletz. With higher seas at the bays/estuaries, any river may
see high water levels due to backing up of water during high
tides. Current Flood Watch remains in effect through Wednesday
evening.
On the other hand, many of the smaller streams and creeks will rise
quickly today into Wednesday, with some 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 Small Stream and Urban Flood
Advisory is in effect through Wednesday evening. -Rockey/HEC
&&
.AVIATION...12Z TAFs: Widespread VFR inland, with occasional MVFR
conditions from rain showers. With increasing rain, expect inland
CIGS to slowly drop to widespread MVFR through this morning.
Coastal areas will remain under MVFR conditions with areas of IFR
in heavier rain, lasting through the TAF period. Expect winds
gusting 30 to 40 kts through Tuesday morning, with strongest along
the headlands and open beaches. As often the case, wind gusts
around 45 to 60 kts likely farther aloft, mainly above about 3000
ft along the coast and over the Cascades through Tuesday morning.
Not much change through Tue afternoon, with widespread MVFR and
rain for nearly all of the region. Expect higher terrain to be
obscured in clouds and precip through Tue afternoon.
KPDX AND APPROACHES...VFR with occasional MVFR and rain throughout
the afternoon. Surface wind slowly turns more southerly Tuesday
morning, bringing some potential of low level wind shear, however
confidence remains low, thus have excluded from the TAF. /JH
&&
.MARINE...The atmospheric river (AR) that is the main weather
maker for the next 24-48 hours is positioned over northern
Washington. It has slowly shifted southward overnight into
Tuesday morning, which will increase winds and seas. Winds
currently are around 35 kt at buoy 46050, with similar speeds in
the nearshore waters based on coastline observations. Will see
these winds persist through Tuesday morning, then ease behind the
AR. The gale warning remains in effect through Tuesday morning for
winds.
Conditions will become showery behind the system. Each shower
will bring periods of breezy winds. Seas will remain around 12-15
ft through the next several days with a brief lull late tonight.
However, a dropping low pressure system from the Gulf of Alaska
will coincide with the post frontal environment causing winds to
shift to the west. The westerly wind, when combined with the
westerly long range swell, will once again cause seas to build to
around 18 ft by late Tuesday/early Wednesday. These seas will
coincide with a long period of 15-18 seconds. Will experience
hazardous seas behind the front.
In the long term, yet another system approaches from the north,
but this time a cold front. This cold front will cause winds to
intensify once more but seas will be unaffected.
-JH
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Flood Watch through Wednesday afternoon for Cascade Foothills
in Lane County-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-Northern Oregon Cascade Foothills-South
Willamette Valley.
WA...Flood Watch through Wednesday afternoon for Greater Vancouver
Area-I-5 Corridor in Cowlitz County-South Washington
Cascade Foothills-South Washington Coast-Willapa Hills.
PZ...Gale Warning until 10 AM PST this morning for Coastal waters
from Cape Shoalwater WA to Florence OR out 60 NM.
Hazardous Seas Warning from 10 AM this morning to 4 PM PST
Wednesday for Coastal waters from Cape Shoalwater WA to
Florence OR out 60 NM.
Gale Warning until 8 AM PST this morning for Columbia River Bar.
Small Craft Advisory from 8 AM this morning to 4 PM PST
Wednesday for Columbia River Bar.
&&
www.weather.gov/portland
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