Flood Watch 1, NWS Portland, OR

Area Forecast Discussion...Updated
National Weather Service Portland OR
951 PM PST Sun Jan 10 2021

.SYNOPSIS...A change to a much wetter patterns begins Monday and
continues through Wednesday. An atmospheric river event is expected
to impact the forecast area beginning Monday night and continuing
through Wednesday morning. Snow levels will be well above the
Cascades passes the first half of next week. High pressure and
offshore low-level flow returns Wednesday night and Thursday. A weak
system may brush the area Friday with drier conditions possible next
weekend. Large surf and high tides will result in periods of
hazardous conditions for the beaches and low-lying coastal areas
today through at least Wednesday.

&&

.UPDATE...Dense fog has developed again across the central Willamette
Valley, prompting us to issue a Dense Fog Advisory, valid until 4 AM.
After that, an approaching front should allow for light S-SE flow
which should lift the fog a little bit. Rain is expected to spread
onshore early Monday morning, but there are some signs the front may
stall across NW Oregon, potentially keeping most of Lane County dry
for much of the day.  Weagle

.SHORT TERM...Tonight through Wednesday...Early afternoon water vapor
imagery revealed a deep low pressure area in the Gulf of Alaska with
an extensive baroclinic boundary along 130-135W longitude and
extending well into the central Pacific. Meanwhile, the upper level
front that went through the forecast area early this morning could be
seen moving across far eastern Washington and Oregon. KRTX dual-pol
doppler radar indicated minimal returns early this afternoon.

There will be a brief lull in the action tonight as a weak transitory
500 mb ridge moves over the area. However, hazardous conditions will
continue on the beaches through the evening due to high surf and
enhanced sneaker wave threat. There have been some impacts in the
Tillamook area earlier today due to strong high tides.

The start of a much wetter pattern begins Mon. The next frontal
system brings rain to the coast early Mon morning. Precip slowly
spreads inland during the day. However, models continue to stall the
frontal boundary Mon. It is possible the far southeast part of the
forecast area may see little to no precipitation Mon afternoon. By
12Z Tue the leading edge of a 150 kt jet is forecast to be just off
the coast for the start of an atmospheric river event. Details
regarding timing, precip focus areas and rainfall amounts are still
uncertain. However, confidence is increasing that there will be a
period of significant rainfall during the Tue through early Wed time
frame. The ECMWF continues to indicate a precipitable water plume of
1.0 to 1.25 inches along the coast 12Z Tue.

Several factors come into play Tue that will heighten hydrologic
concerns. The operational runs of the GEFS and ECMWF EPS Integrated
Water Vapor Transport (IVT) guidance indicate a strong AR event Tue,
with the EPS version showing this as a near extreme event. The
respective model means are at the low end of the strong category.
Rainfall in the Cascades and Oregon Coast Range will be further
enhanced by impressive 850 mb orographic lift. The 12Z NAM shows
50-75 kt WSW flow into the Coast Range Tue morning. This will result
in substantial orographic precipitation enhancement, with possible
hourly rainfall rates of one-half inch. Another contributing factor
will be high snow levels during the event. Expect snow levels to Tue
through Tue night to range from 6000 feet in the south Washington
Cascades to near 8000 feet in the Lane County Cascades. The ECMWF
Extreme Forecast Index (EFI) valid Tue is rather impressive, with a
large area of 0.6 to 0.7 EFI indices and up to an index of 0.8 in far
northwest Oregon.

Preliminary rainfall estimates from 12Z Tue through 00Z Thu are
generally 3 to 6 inches for the Cascades and Oregon Coast Range, and
1.5 to 4 inches elsewhere. Local amounts of 6 to 8 inches may be
achieved in the Cascades and Oregon Coast Range. These numbers are
subject to change and highly dependent on where the precipitation
focus resides. These preliminary amounts and rainfall rates will be a
concern to the recently-burned areas, such as the Echo Mountain
Complex near Lincoln City and the Beachie Creek, Riverside and
Holiday Farm Fires.

Another wrinkle in the 12Z model runs is the potential for fairly
strong surface low development Tue night. The 12Z ECMWF shows a 990
mb low center near KHQM 12Z Wed. The operational GFS is similar, but
a little faster with the timing. Several ECWMF ensemble members are
similar to the deterministic solution for surface low location, but
there are a few that show a much deeper low center. Thus, confidence
is growing that high wind criteria could be met for the coast Tue
night through Wed morning, with the potential for advisory-level wind
gusts in the Willamette Valley. There may also be a short period of
post-frontal southwest wind as the surface low races northeast.

Beach hazards will also continue through Wednesday, with a
combination of high surf and tidal overflow. The highest tides are
expected Mon and Tue afternoon. The forecast high-high tide at Toke
Point Tue afternoon is around 12.0 ft. Weishaar

.LONG TERM...Wednesday night through Sunday...The aforementioned
potential AR event likely runs through Wed evening. Once the surface
low races into southeast British Columbia Wed morning, the attendant
baroclinic boundary will push through the area fairly quickly.
Precipitatiion shuts down Wed night as high pressure moves over the
area. This trend is supported by the deterministic runs and 500 mb
cluster analyses. The majority of the ECWMF ensemble members indicate
dry conditions (no QPF) after 06Z Thu. Went a little lower than the
NBM POPS for Wed night, but would not be surprised if these continue
to lower in the next few runs.

Another weak front pushes across the area Thu night or Fri. The
operational GFS is at least 12 hours faster than the ECMWF. However,
the total mean 500 mb cluster analysis valid Fri shows fairly strong
ridging centered from central California to the northern Rockies.
Thus, would tend to favor the slower ECWMF solution. There is
reasonable confidence that the change to a drier pattern will
continue next weekend. All in all, high confidence exists that mild
conditions will persist through the extended period with daytime
temperatures 5-10 degrees above average. Do not see any indication of
low-elevation snow or unseasonably cold air through the next 10 days.
Weishaar

&&

.AVIATION...Wide range of flying conditions across the forecast
area at the moment. IFR/LIFR cigs persist in portions of the
Willamette Valley, while easterly flow keeps KPDX/KTTD VFR or
high-end MVFR. A few showers persist near the western Columbia
Gorge; these should gradually taper off over the next few hours.
Outside of the eastern PDX metro, it appears east- southeasterly
flow will not be sufficient to prevent IFR/LIFR cigs from
settling back down or redeveloping over the central and portions
of the southern Willamette Valley. Next frontal system is
expected to spread rain onshore around 12z, reaching most inland
areas by 18z. As rain develops, expect mostly MVFR cigs/vsbys
Monday midday and afternoon.

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

KPDX AND APPROACHES...VFR or high MVFR conditions prevail this
evening. Easterly breezes persist at the surface, and may result
in some LLWS as our next front approaches later tonight/Mon
morning. Will hold off on including this in the TAF for now,
mainly due to uncertainty in the altitude where the shear will
occur. Rain is expected develop by 15z, eventually resulting in
MVFR as rain increases throughout the day Monday. Weagle

&&

.MARINE...Details continue to evolve and unfortunately are
consistently inconsistent. Feel more confident about the next 24
hours as a modest cold front will move across the waters
tonight. Expect winds will primarily gust 30-35 kt ahead of and
with the front tonight. However, there may be occasional gusts
35-40 kt across the northern waters late tonight and early Monday
morning. Seas will slowly subside toward the mid teens this
evening and then be held in place through Monday afternoon as
winds increase the fresh swell component against a slowly
decaying longer distance swell.

We are still looking at a stronger wind event Monday night and
Tuesday morning, and models are all showing a similar split
parent low center...much weaker than this time yesterday at 960
at 970 mb and a little further away near 155 W. However, the
major models are now showing a secondary surface 980-990 mb low
swinging northeast Monday night with the closest approach running
about 180 nm off the Columbia mouth at some time between sunset
and midnight Monday evening. That much spread in timing and
strength does not lend toward great confidence in the details
once again. Will hold off on upgrading the Gale Watch with a plan
that the evening and/or overnight shift will get a glance at a
more consolidated picture of the surface pressure pattern for
Monday night. The strongest solutions do bring potential for
gusts 45-50 kts for a few hours ahead of and with the associated
frontal passage around or after midnight Monday night, depending
on the eventual timing. Seas will also increase Monday night
bringing an initial shot at 20-25 ft seas from wind waves/fresh
swell, only to be maintained through much of daylight Tuesday
from a trailing dynamic fetch swell associated with the low
centers.

New this morning, was the major models both picking up on another
surface low riding northeast along the SW flow aloft for Tuesday
night and Wednesday. This also has potential for gusts around 50
kt and would potentially be even a little stronger than Monday
night's winds. Seas would return to the 20-25 ft range Wednesday
morning after a brief stint below 20 ft Tuesday night. Ramped
winds up but took a blended approach given this new development.

Winds and seas take a bit of a breather thereafter through
Thursday but will still remain rather active with SCA level winds
and seas still likely.

Finally, these enhanced seas Monday through at least Wednesday
will make tidal overflow likely as the incoming swell will be in
phase with the peak of high tides. Furthermore, this sequence of
storms will likely bring heavy river flows somewhere along the
coast, so have issued a Coastal Flood Advisory for our coastal
land zones Monday through Wednesday. Main concerns will be around
the time of the higher high tides occurring midday and the
afternoons. /JBonk

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Coastal Flood Advisory from 8 AM Monday to 4 PM PST Wednesday
     for Central Oregon Coast-North Oregon Coast.

     Dense Fog Advisory until 4 AM PST Monday for Central Willamette
     Valley.

WA...Coastal Flood Advisory from 8 AM Monday to 4 PM PST Wednesday
     for South Washington Coast.

PZ...Gale Watch from Monday evening through Tuesday morning for
     coastal waters from Cape Shoalwater WA to Florence OR out
     60 NM.

     Small Craft Advisory until 6 PM PST Monday for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until 7 PM PST Monday for Columbia River
     Bar.

&&

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