River Flood Warning 15, NWS Portland, OR

Area Forecast Discussion
National Weather Service Portland OR
1107 AM PST Mon Jan 12 2021

.SYNOPSIS...A quasi stationary front is expected to produce periods
of moderate to heavy rain through Wednesday for the potential for
river, rural, and urban flooding. Strong winds are expected along
the coast today and tonight, with breezy conditions for the
Willamette Valley this evening. Large surf and high tides will
result in hazardous conditions for the beaches and low-lying coastal
areas through Wednesday. Dry weather on Thursday will allow rivers
to recede before the next weaker front that is expected Thursday
night.

&&

.SHORT TERM...No changes since the 420 AM discussion. Today through
Thursday...Rain with periods of breezy winds are the main weather
concerns today through Wednesday coupled with high surf and minor
tidal overflow flooding. A quasi-stationary front fueled by an
atmospheric river of moisture will continue producing moderate to
heavy rain across the region through early Wednesday morning. Lows
moving along the front will result in periods of stronger winds and
enhanced orographic rain. The first low is occurring early this
morning when wind gusts of 45-55 mph have been observed. Rain was
enhanced with this low with the focus of heaviest rain early this
morning is over extreme SW Washington and NW Oregon.
Rain gages have reported 6 hour rainfall totals between 1.5 and 2
inches across the higher terrain, and at least one station in the
Willapa Hills (WA) reported a half of an inch of rain between 1 and
2 AM. The front is making its way inland this morning and expect
this heavy rain to extend inland to the Cascades and further South
into Oregon this morning through this afternoon.

The next low will produce even stronger winds tonight into early
Wednesday, and have more of a southern focus for the winds and heavy
rain. A high wind warning has been issued for the coast from mid
morning today through midnight tonight to account for the periods of
expected strong winds. There will be a period of gusty winds inland
this afternoon and evening with gusts of 40-45 mph possible for the
Willamette Valley. Models are in good consensus on the front moving
inland Wednesday with rain rapidly ending in its wake Wednesday
afternoon and evening. Forecast rainfall totals from this morning
through Wednesday morning are 3 to 7 inches across the higher
terrain and 1 to 2 inches for the interior lowlands. See the
HYDROLOGY discussion below for hydrologic concerns related to this
atmospheric river event.

Beach hazards will continue through Wednesday, with a combination of
tidal overflow and sneaker wave potential Tuesday and Wednesday, and
a bonus high surf on Tuesday. The forecast total high tide at Toke
Point is 12 feet Tuesday afternoon, and 11 feet Wednesday afternoon.
Higher river levels may contribute to minor coastal flooding around
these high tides.

Colder air will fill in behind the front on Wednesday with snow
levels lowering to around 3000 feet. There is a rapid decrease in
moisture flux when the snow levels lower, and do not expect much
snow production. Wednesday night will be relatively cooler with
minimum temperatures in the mid to upper 30s. Patchy valley fog is
likely late Wednesday night as high pressure builds over the area
and skies start to clear. The high pressure will result in dry
weather Wednesday night and Thursday. East winds will develop early
Thursday morning and increase in the afternoon as the next front
approaches. A Dalles to Troutdale pressure gradient of 5 to 7 mph
could produce 25 to 25 mph winds through the Columbia Gorge and east
Portland metro area.  ~TJ

.LONG TERM...No changes since the 420 AM discussion. Thursday night
through Monday...Another front is expected to move onshore Thursday
evening. Precipitable water values are expected to be around an inch
with this front, and models are forecasting IVT between 250 and 500
kg/ms indicating the possibility for a weak atmospheric river. The
jet dynamics are weaker with this front, and do not anticipate high
rainfall production regardless of the available moisture. The front
should move through the area rather quickly and over by Friday
afternoon with rainfall totals likely remaining below an inch even
across the terrain favored areas. Snow levels will be at or above
5000 feet and do not anticipate much if any snow from this front.
High pressure rebuilds Friday night and Saturday for a brief period
of dry weather. in the wake of this front. Models are in good
agreement on another non impressive front Saturday night through
Sunday followed by dry weather next Monday.
~TJ

&&

.HYDROLOGY...No changes since the 420 AM discussion. Mainstem rivers
along the SW Washington and NW Oregon coasts are rising this morning
in response to the heavy rain overnight. The only river at this
point forecast to rise above flood stage is the Grays River at
Rosburg, in which a Flood Warning has been issued for. Other coastal
rivers are currently forecast to rise above baneful, but just shy of
flood stage including the Willapa River at Willapa, the Naselle
River, the Wilson River at Tillamook, the Nehalem River at Foss, The
Siletz river at Siletz, and the Siuslaw River at Mapleton. The
inland rivers will likely see rises later today as the heavy rain
expands across the area, and expect there will be more flood
warnings issued later today.

Besides Mainstem rivers, Small streams and creeks will also rise,
and possibly flood today, and minor urban flooding in poorly drained
areas are possible. Will continue the Flood Watch for the same areas
to account for all of the flooding concerns. The saturated soils
will also likely lead to land slides. ~TJ

&&

.AVIATION...A moist southwest onshore flow ahead of a cold front
will continue to bring widespread MVFR conditions with rain to the
region today. The southern half of the Willamette Valley will likely
continue to see areas of VFR conditions mixed in through 02Z, while
coastal areas see occasional IFR conditions in that same time
period. A cold front moving southeast across the region tonight will
allow the rain to taper off behind the front after 03Z-09Z. Expect
gusty southerly winds to temporarily pick up as the front moves
through between 02Z and 08Z along the coast, and from 05Z-09Z
inland. MVFR ceilings will still be widespread behind the front
after 07Z-09Z, but some areas will see improvement to VFR category.

For detailed regional Pac NW weather information, go online to:
http://weather.gov/zse

KPDX AND APPROACHES...MVFR conditions are likely to remain prevalent
over the next 24 hrs. A cold front expected to move east across the
region between 05Z and 09Z tonight will temporarily bring gustier
south winds, before the winds turn to the southwest and taper off
behind the front. The front will also bring an end to the persistent
rain, while also introducing at least a chance of seeing temporary
improvement to the VFR flight category after 09Z
&&

.MARINE...No changes since the 420 AM discussion. A front will
slowly shift southeastward towards the region today into tonight.
Weak waves riding along this front will temporarily enhance and
decrease winds periodically across the waters through this evening.
A coastal jet will likely enhance winds within 10-20 NM of the coast
by at least 5-10 kt when compared to winds farther offshore during
this time. A period of storm force wind gusts are ongoing early this
morning, but should temporarily decrease later this morning. There
may be another spike in the winds around midday to early this
afternoon before yet another lull in the winds occurs. However,
there are some suggestions that a final push of winds, potentially
the strongest of the bunch, will occur across the central waters of
Oregon and potentially all the way up to the Columbia River near the
coast. As a result, opted to just extend the Storm Warning early
this morning through this evening to cover this potential hazard.
Otherwise, expect seas to generally be in the 20-25 ft range through
tonight as a longer period westerly swell and wind waves/fresh swell
keep seas elevated.

High pressure will then begin to shift across the waters Wednesday
and Thursday and allow seas to subside below 10 ft late Wednesday
night or Thursday. However, the next front appears poised to impact
the waters Thursday night or early Friday with a round of Small
Craft Advisory level wind gusts. This should result in seas climbing
back into the teens Friday into next weekend. An even weaker front
will be possible over the weekend before high pressure shifts more
squarely over the region early next week.  /Neuman

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Flood Watch through Wednesday morning for Cascade Foothills in
     Lane County-Cascades in Lane County-Central Coast Range of
     Western Oregon-Central Columbia River Gorge-Central Oregon
     Coast-Central Willamette Valley-Coast Range of Northwest
     Oregon-Greater Portland Metro Area-Lower Columbia-North
     Oregon Coast-Northern Oregon Cascade Foothills-Northern
     Oregon Cascades-South Willamette Valley-Western Columbia
     River Gorge.

     High Surf Advisory until 5 PM PST this afternoon for Central
     Oregon Coast-North Oregon Coast.

     Coastal Flood Advisory until 4 PM PST Wednesday for Central
     Oregon Coast-North Oregon Coast.

     High Wind Warning from 10 AM this morning to 2 AM PST Wednesday
     for Central Oregon Coast.

     High Wind Warning from 10 AM this morning to midnight PST
     tonight for North Oregon Coast.

WA...Flood Watch through Wednesday morning for Central Columbia
     River Gorge-Greater Vancouver Area-I-5 Corridor in Cowlitz
     County-South Washington Cascade Foothills-South Washington
     Cascades-South Washington Coast-Western Columbia River
     Gorge-Willapa Hills.

     High Surf Advisory until 5 PM PST this afternoon for South
     Washington Coast.

     Coastal Flood Advisory until 4 PM PST Wednesday for South
     Washington Coast.

PZ...Gale Warning until 10 PM PST this evening for Waters from Cape
     Shoalwater WA to Cascade Head OR from 10 to 60 NM.

     Storm Warning until midnight PST tonight for Coastal waters
     from Cape Shoalwater WA to Cascade Head OR out 10 NM-
     coastal waters from Cascade Head OR to Florence OR out 60
     NM.

     Small Craft Advisory until 3 PM PST Wednesday for Columbia
     River Bar.
&&

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PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE PORTLAND OR
943 AM PST TUE JAN 12 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0930 AM     HEAVY RAIN       RYDERWOOD               46.37N 123.05W
01/12/2021  M2.55 INCH       COWLITZ            WA   PUBLIC

            2.55 INCHES IN LAST 12 HOURS WITH 3.75
            INCHES OVER THE LAST 24 HOURS.

0840 AM     HEAVY RAIN       1 ESE WELCHES           45.33N 121.95W
01/12/2021  M2.00 INCH       CLACKAMAS          OR   TRAINED SPOTTER

0809 AM     HEAVY RAIN       1 NE LAKE OSWEGO        45.42N 122.69W
01/12/2021  M1.25 INCH       CLACKAMAS          OR   TRAINED SPOTTER

            RAINFALL SINCE 7PM ON THE 11TH.

0734 AM     HEAVY RAIN       1 NW SUMMIT             44.64N 123.58W
01/12/2021  M1.31 INCH       BENTON             OR   TRAINED SPOTTER

            RAINFALL SINCE 9PM ON THE 11TH.

0657 AM     HEAVY RAIN       2 W MCMINNVILLE         45.21N 123.23W
01/12/2021  M1.42 INCH       YAMHILL            OR   TRAINED SPOTTER

            RAINFALL SINCE MIDNIGHT.

&&