Heavy Rain in Oregon, January 18, 2019

Area Forecast Discussion
National Weather Service Portland OR
302 PM PST Fri Jan 18 2019

.SYNOPSIS...A warm front will lift across the region today, with
heavy rain at times this afternoon and tonight. Snow levels jump
way high today and tonight, then slowly lower over the weekend. Will
see additional rain and Cascade snow Saturday evening into Sunday.
Also some chance of light precipitation around the region to start
next week.

&&

.SHORT TERM...This afternoon through Monday...Radar and surface
observations this afternoon show rain has spread into the area as a
warm front lifts north. As the warm front moves through, it will drag
a moderately strong atmospheric river into the area, with moderate to
heavy rain expected later this afternoon through this evening. The
warm front will lift into Washington state this evening, but a cold
front will approach from the west, continuing rain overnight. Appears
that the cold front will push onshore late tonight, but likely hold
up to the south over southwest Oregon and parts of Lane County. So,
rain will turn to showers north of about a Florence to Salem to Mt
Hood line, but steady rain will continue to the south.

Upped rainfall amounts a little bit for this event, but still don't
expect much in the way of flooding. Dry antecedent conditions should
keep rivers below flood stage. Could see some ponding of water in
low-lying areas of roads or urban areas, but likely just to the level
of being a nuisance instead of presenting a serious threat to life or
property.

Snow levels will rise well above 6000 ft by this evening, so while
precip this afternoon may be falling as snow, it will quickly turn
over to rain in the Cascades. With heavy rain at times and overall
impressive rain totals over the next 18 hours, will keep as close of
an eye as possible on the large burn areas from the last couple of
years, particularly those in the southern half of the forecast area,
which hasn't seen this amount or intensity of rain yet this season.

Weak low pressure approaches from the southwest Saturday night,
moving northeast through Oregon. This low will briefly reinvigorate
moisture, again with the focus over the southwestern quadrant of
Oregon. It will also drag the leftover cold front/weak boundary out
of the area by Sunday afternoon. However, another low approaches from
the west just behind it on Sunday, bringing another shot of moisture,
again mostly to the same area through Sunday night.

The precipitation associated with both of these lows won't be nearly
as much as with the initial system Friday evening. However, by that
time snow levels will have dropped back below the passes, so could
see as much as a foot of snow at the passes between Saturday night
and Sunday, particularly south of Mt. Hood. Won't issue any winter
weather headlines for the Cascades at this point to try to avoid
confusion about when the snow will actually be falling (i.e. not with
this first system tonight), but will almost certainly need at least
an advisory and possibly more once the snow levels fall.

Unrelated to the rain and snow, will issue a Coastal Flood Advisory
for minor tidal overflow for the highest tide each day over the
weekend. Although the combined forecast tide and expected anomaly
will likely end up slightly under criteria, there have been reports
of minor tidal flooding in the usual locations around Tillamook
during high tide today. The highest tide occurs Sunday, but the
advisory may need to be extended for the Monday highest tide as well.

Colder air becomes more entrenched Monday, bringing snow levels down
even further, but by that time the moisture will have mostly dried
up. Models continue to indicate some chance for showers on Monday as
we remain under the influence of an upper trough, so have kept PoPs
around or below climatology for this time of year. Wouldn't be
surprised if Monday continues to trend drier, though. Bowen

.LONG TERM...Monday night through Friday...Models continue to show a
broad upper level ridge building into the Pacific NW next week. The
ridge axis looks to initially remain offshore early next week, with
northwesterly flow over the forecast area. Guidance continues to
show a shortwave disturbance riding over the top of the ridge early
next week. This should allow a warm front to move across the
forecast area on Tuesday followed by a weak cold front on Wednesday.
Snow levels will quickly rise to above 5000 ft by Tuesday afternoon
as the warm front swings inland which should minimize travel impacts
across the Cascades. The upper level ridge looks to finally
strengthen its grasp on the region late next week. This should allow
conditions to dry out on Thursday and Friday, with foggy conditions
likely developing in the interior valley during the overnight/early
morning hours. /64

&&

.AVIATION...Warm front across SW Oregon early this afternoon.
MVFR coverage starting to increase as of 21Z, especially along
the coast. The warm front is expected to lift into SW Washington
by early evening. Flight conditions will continue to lower
through the early evening as the warm front migrates north.
Coastal areas likely to become IFR by 00Z Sat and remain there
through 12Z Sat. Some improvement expected after 12Z. Inland
areas to be a mix of IFR and MVFR through Sat morning. South to
Southwest wind 25-35 kt with gusts to 45 kt occurs along the
coast late this afternoon through tonight. LLWS possible for
inland TAF sites late this afternoon through the evening,
especially near the Gorge. Finally, mountains and higher terrain
will be obscured through at least Sat morning.

KPDX AND APPROACHES...VFR conditions at 21Z. Warm front expected
to reach the terminal and vicinity by 00Z Sat, with conditions
lowering to MVFR. Areas of IFR possible 00Z to 15Z Sat,
especially in the Tualatin Valley. LLWS potential late this
afternoon through the evening with E-SE surface wind and
increasing S-SW wind aloft. Weishaar

&&

.MARINE...A 980 mb low pres center was noted near 46N 137W at
21Z. 19Z ASCAT pass showed wind speeds primarily 30 kt or less,
with some pockets of 35-40 kt over the south Oregon offshore
waters. The low will continue to deepen and move north to northeast
through tonight, with a minimum low pres around 970 mb 06Z Sat.
The low eventually reaches Haida Gwaii early Sat morning. Will
maintain current gale warning. Models were a little too strong
with the previous system so have trended wind speeds down just a
little from the earlier forecast. Still looking for max gusts to
around 45 kt late this afternoon through this evening. The 12Z
NAM would suggest possible storm force gusts over PZZ275 late
this afternoon with 50-55 kt 975 mb wind speeds. Gale warning
runs through 14Z Sat, but may be able to end it a little earlier.
Wind speeds diminish late tonight through Sat, falling below 15
kt over the central zones and to 20 kt over the north zones by
Sat afternoon.

Another low pressure system moves across the waters on Sunday,
though the exact track of this low is uncertain, with differences
between the various forecast models. This will be a weaker system,
with 20-25 kt gusts possible over PZZ275 Sun afternoon. Weak high
pres develops Sun night and Mon, followed by the next frontal
system Mon night and Tue. GFS and ECMWF in general agreement on
this system. Will indicate 30 kt gusts across the north zones
Tue, but gales not out of the question. High pres returns for
the latter half of next week.

Seas have eased to around 15 ft early this afternoon. Max seas
expected to be 17-20 ft late tonight through Sat, with the
highest seas over the outer waters. Initially this evening, there
will be a primary west swell around 10 ft with a significant
south wind wave component. Overnight, the primary west swell
builds as the wind wave portion diminishes. Wave heights fall
to the lower teens by Sun morning and then ease to near 10 ft by
late afternoon. Seas expected to reach the mid teens Tue night
and Wed. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Coastal Flood Advisory from 10 AM to 1 PM PST Saturday for
     North Oregon Coast.

     Coastal Flood Advisory from 11 AM to 2 PM PST Sunday for North
     Oregon Coast.

WA...Coastal Flood Advisory from 10 AM to 1 PM PST Saturday for
     South Washington Coast.

     Coastal Flood Advisory from 11 AM to 2 PM PST Sunday for South
     Washington Coast.

PZ...Gale Warning until 6 AM PST Saturday for Coastal Waters from
     Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 4 PM
     PST Saturday.

&&

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 10 AM PST this morning for Coastal
     Waters from Cape Shoalwater WA to Florence OR out 60 NM.

     Gale Warning until 6 AM PST Saturday for Coastal Waters from
     Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 6 AM
     PST Saturday.

&&

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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.