Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 160534
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
924 PM PST Sat Dec 15 2018

.SYNOPSIS...After off and on light rain Sunday and Monday, expect a
much wetter and windier storm system Monday night and Tuesday.

&&

.SHORT TERM...Tonight through Tuesday...Water vapor satellite imagery
this evening depicts a large shortwave trough over the Gulf of
Alaska. A strong surface low pressure is gradually filling at this
time, but likely still remains sub-965mb in the Gulf of Alaska. The
trailing front from this low pressure currently lies off the Pacific
Northwest coast this evening, but is already beginning to spread
light rain across northwest Oregon and southwest Washington. As a
stronger upper level jet enters the backside of the upper level
trough, the whole upper level trough observed in the Gulf of Alaska
will have a tendency to dig more south than east initially. This will
in turn result in the front currently found near 130W likely taking
at least the next 12, if not closer to 18 hours, to push eastward
into the I-5 corridor. As a result, expect periods of off and on light
rain through Sunday morning before a round of steadier rain pushes
across the region towards midday Sunday and Sunday afternoon.

It should be noted that the aforementioned surface low pressure in
the Gulf of Alaska created a large westerly swell that is currently
moving towards the region. The most powerful waves will likely arrive
along our coastline Sunday afternoon and evening so the High Surf
headlines look on track.

As the upper level trough currently found in northeast Pacific
finally pushes ashore Sunday evening, 500mb temperatures will drop to
~-29C, which may be enough to support a thunderstorm or two across
our waters. Modestly strong south-southwest low level flow should
allow some showers to push inland behind the front, but orographic
effects will be somewhat limited so snow amounts should remain well
below advisory criteria across the Cascades Sunday night despite snow
levels lowering to 4000-5000 feet.

Weak shortwave ridging should allow most of the area to dry out
briefly early Monday, but the next storm system will be quick on its
heals and likely begin to spread rain into the area Monday afternoon
and evening. A 160 kt WSW-ENE oriented jet and attendant atmospheric
river will then take aim at Oregon and likely result in one of the
wetter 24 hour periods of the winter season for the region.
Integrated water vapor transport values will likely exceed 750 kg/ms
per the GEFS, which would classify this as a strong atmospheric river
event. The strong southwesterly flow in this event will not be an
ideal direction for orographic enhancement across at least some of
the drainages of the Coast Range and Cascades, but given the IVT
values forecasted by the models, would not be surprised to see a
couple of our typically wetter sites in the Coast Range and Cascades
register around 2" of rain in a 6 hour period late Monday night or
Tuesday morning. In total 1-2" of rain seems like a fair bet for
many Willamette Valley locations late Monday through Tuesday and at
least, and this admittedly feels somewhat conservative, 2-5" of rain
across the higher terrain of the Coast Range and Cascades. In
addition, the southwesterly flow should allow some of the normally
rain shadowed portions of the east slopes of the Coast Range to
receive heavier rainfall totals, which should bring notable rises to
those rivers as well. It is still too early to say where the
heaviest rain will fall across our CWA given the inevitable wobbles
in the flow that the models simply cannot accurately resolve at this
point in time, will likely determine where the atmospheric river
will sit the longest.

Finally, models have been consistent in depicting strong southerly
winds develop offshore in advance of the front. These appear likely
to push onto at least our beaches and headlands Monday afternoon and
Monday night. Models have been suggesting pressure gradients may open
enough as the event unfolds to allow these stronger winds to push
into our coastal communities and maybe even the Coast Range late
Monday night and Tuesday so the current high wind watch for the
beaches and headlands may need to be expanded if this trend
continues. /Neuman


.LONG TERM...Tuesday night through Saturday...Previous discussion
follow...Weather pattern remains active later this week going into
the weekend. Tuesday night into Wednesday we will still be dealing
with the remnants of the atmospheric river that moved through Monday
night into Tuesday. While it tries to pass to our south Tuesday night
into early Wednesday, it stalls across the southern half of our area
as the front is weakening. The front also gets cut-off from its
tropical moisture feed by the next system out over the north-central
Pacific, which makes the front relatively anemic by Wednesday
morning. This system over the central Pacific actually ends up
picking up this dilapidated frontal boundary and merges it with its
warm front. The warm front then lifts north across our area during
the day on Wednesday for another round of steady, stratiform rain.

Models diverge once we get to late Wednesday into Thursday. The GFS
lifts this warm front completely north of our area for a dry day on
Thursday. However, the ECMWF doesn`t quite clear the northern half
of our area, then moves the cold front through quicker which spreads
the rain across our area again by Thursday evening. Have went with a
blend of the two which is somewhat reflected in the NBM, but also
did some manual tweaking. As of now, the bulk of the moisture with
this late Thursday/Friday system makes landfall north of our area,
then moves quickly southeastward across our area. This is not a very
heavy rain/flooding scenario for our area as the front never stalls.
Still would be a good idea to monitor this time period in case the
track of the low changes.

Unfortunately, with how warm these systems are, neither the rain on
Wednesday or the next round Thursday night into Friday will drop
much snow in the Cascades.  As we get later into Friday and into
Saturday, we should be between systems, which may mean a break in
the rain. -McCoy

&&

.AVIATION...Rain spreading into the area this evening with VFR
conditions. Front approaching tonight will bring gradually
lowering clouds. Increasing south to southeast wind...especially
along the coast this evening through Sunday morning. MVFR
conditions will probably develop during the morning along the
coast and late morning inland areas with good chances for
VFR returning Sunday evening as rain decreases.

PDX AND APPROACHES...Light rain with VFR conditions expected
through Sunday morning then MVFR cigs becoming more likely after
20Z Sun.

&&

.MARINE...Active marine weather for the next week...especially
with larger seas and surf conditions.

Gale force winds over the outer marine zones since this afternoon
with solid advisory winds nearer to shore. Models are in good
agreement that winds will be in the strong gale category, just
below storm force over the outer waters later tonight and early
Sunday morning. This first front moves through rather slowly and
weakens some in the north by early sunday morning and weakens
further as it moves across the central coastal waters of Oregon
Sunday afternoon. Winds will diminish to small craft advisory
criteria Sunday night and Monday.

A second potentially stronger system arrives Monday and Monday
evening with strong gales 40 kt with gusts to 50 kt for both
outer and inner waters. There is a chance we could see storm
force winds briefly early Monday evening just as the front
passes.

Seas are beginning to come up ahead of the first front with 14 to
15 footers over the outer waters. Seas will increase overnight
tonight getting up to 20 to 25 feet over the other waters around
midnight and 18 to 20 feet over the inter waters after 3 am and
Front weakens and wind sea diminishes a bit Sunday morning but
the westerly swell is increasing for overall little chance in
combined seas during the day.

Very energetic long period seas arriving quickly Sunday
afternoon after 3pm rising up to 28 feet over the outer waters
and 22 to 24 feet over the inner waters. There is a lot of energy
in these seas and we will see dangerous high surf along all of
the south Washington and Oregon coast. Have issued a high surf
advisory for initial long period swell but going to high surf
warning after 3pm Sunday through Sunday night when the strong
westerly swell arrives. Seas will remain high...above 20 feet
through much of the early part of the week. Schneider

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory from 4 AM to 3 PM PST Sunday for Central
     Oregon Coast-North Oregon Coast.

     High Wind Watch from Monday afternoon through late Monday night
     for Central Oregon Coast-North Oregon Coast.

     High Surf Warning from 3 PM Sunday to 6 AM PST Monday for
     Central Oregon Coast-North Oregon Coast.

WA...High Surf Advisory from 4 AM to 3 PM PST Sunday for South
     Washington Coast.

     High Wind Watch from Monday afternoon through late Monday night
     for South Washington Coast.

     High Surf Warning from 3 PM Sunday to 6 AM PST Monday for South
     Washington Coast.

PZ...Gale Warning until 4 PM PST Sunday for Waters from Cape
     Shoalwater WA to Florence OR from 10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 6 PM
     PST Sunday.

     Small Craft Advisory until midnight PST tonight for Coastal
     Waters from Cape Shoalwater WA to Florence OR out 10 NM.

     Gale Warning from midnight tonight to 4 PM PST Sunday for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     10 NM.

&&


$$

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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.


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