Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
252 PM PST Wednesday Dec 17 2014
Synopsis...weak low pressure will cross the area this evening with
showers lingering overnight. Expect fairly quiet weather to start
the day Thursday. A more organized front will bring gusty winds to
the coast with rain spreading inland Thursday night and Friday.
Rising snow levels will limit snow to the highest peaks. Colder air
behind the front will bring accumulating snow showers for the
Cascades passes Friday evening and Saturday. Mild and very wet
weather arrived this weekend. There is potential for an atmospheric
river and associated heavy rain to develop over the area affecting
rivers into next week.
Short term...tonight through Saturday...a weak and splitting trough
will send most of its energy south of our area tonight. Will see
showers taper off overnight as high pressure begins a transit across
the area. Will also see corresponding snow levels rise above the
passes and generally limit the amount of accumulations. The ridge
axis quickly passes east by tomorrow afternoon leading to a largely
dry day for the interior.
Another more cohesive trough and frontal system will dig offshore
with warm frontal rain touching the coast late Friday afternoon with
heavier amounts by early Friday evening. Timing could be earlier
depending on model of choice with the GFS perhaps a couple hours
faster than the European model (ecmwf) and the NAM slower yet. NAM typically does not
handle warm process rain very well so am putting less weight on it
to start with. As the rain comes in...a coastal jet develops
bringing fairly gusty winds to the coast and Coast Range but nothing
too out of the Ordinary. Not anticipating any highlights but
probably a run of the mill windy coastal storm. The associated cold
front come ashore around 12z Friday morning and crosses the Cascades
around 17z with colder air and lowering snow levels with and more so
behind it. Precipitation tapers off and becomes more showery in
nature behind the front. Expect light accumulating snow begins at
Willamette Pass around day break and lowers to near Government Pass
later that morning. The snow comes to be too little...too late for
much appreciable increase in the snow pack with perhaps a few inches
at the higher passes and less than an inch for Government Camp.
Higher peaks could see up to a foot...however. Any of the middle
elevation snow will be rather short-lived as snow levels immediately
begin rising in anticipation of the Pacific storm track becoming
centered over the area.
Main attention this shift has been be placed on the atmospheric
river event setting up to affect the region this weekend with
impacts lasting into next week. Raw GFS model run accumulated quantitative precipitation forecast
amounts exceed 12 inches for portions of the Cascades and 10 inches
for the Coast Range ending Monday morning. Will see the warm rains
begin affecting the Coast Range Saturday morning and push quickly
inland to the Cascades by Saturday afternoon. Most of the heavy rain
occurs beginning Saturday night. There would be deep westerly flow
from the surface up to the jet level thus expect this could be an
ideal orographic situation to extract a maximum amount of rain from
whatever comes ashore. There is general large scale support from the
European model (ecmwf) albeit with less quantitative precipitation forecast. Past experience shows following the
higher GFS quantitative precipitation forecast amounts is still not enough and that reality brings
significantly more rain. Assuming the current model runs stay
unchanged in the future...would expect multiple mainstem rivers
exceeding flood stage as soon as Sunday evening.
Have issued some outlook products today looking ahead to flooding
potential with the caveat that models have shifted the focus from
northern Washington/British Columbia south to our area during the
past 36 hours. Additional shifting is expected as well as timing
differences. Stay tuned as this situation evolves over time.
Long term...Saturday night through Wednesday...models maintain a
flattened ridge over the eastern Pacific for the weekend through
early next week. With a nearly zonal jet and abundant
moisture...expect plenty of rainfall to continue through Sunday. On
Monday and Tuesday...some weak ridging moves over the region but
showers likely continue in the moist onshore flow. An upper trough
drops south over the region by midweek with some rather cold air and
additional precipitation in store. While still some significant
uncertainty exists with respect to the timing of the precipitation vs cold
air and just how cold the air is...it does appear that a significant
surge of cold air may move into the region and therefore...decreased
temperatures accordingly for the Tuesday/Wednesday periods across the region.
It does appear that snow levels will drop below pass levels at
least...and perhaps down into the foothill elevations later
Wednesday into Thursday. Given the uncertainty...those planning
Holiday travel during this period should monitor the forecast.
Aviation...VFR conditions likely to continue through this
afternoon. Onshore winds this evening behind the weak low moving
onshore will allow for ceilings and visible to lower to IFR tonight with
the development of fog/low stratus. This should hold on through
Thursday morning...starting to lift back toward VFR around 15 to 18z
Thursday as winds start to increase out of the south. VFR will
continue through Thursday afternoon...though low-level wind shear
will be a concern at kttd and possibly kpdx as 20 knots easterly
winds from The Gorge undercut increasing southerly winds aloft.
Kpdx and approaches...VFR conditions to persist this
evening. Weak winds will could promote the development of fog/low
stratus after 06z Thursday...dropping ceilings and visible to IFR. This
should lift back to VFR by 18z Thursday. -McCoy
Marine...weak low pressure system moving northward along the
coast has brought winds up to Small Craft Advisory criteria this afternoon in our
southern waters. This is weakening and moving onshore...which
should allow winds to subside for a brief period this evening.
Winds will begin to increase across our waters Thursday morning
ahead of a cold front which will move through our waters during
the day Thursday. Ahead of this front...winds likely to ramp up to
gale force with a coastal jet fully developed by Thursday
evening and persist until the front moves onshore early Friday
Seas currently around 10-12 feet will drop to 9 feet later this
evening. Seas will begin to build again Thursday morning as winds
increase ahead of the cold front on Thursday. Seas expected to
build to the low-to-middle teens Thursday evening...subsiding to
10 to 12 feet early Friday morning. A dynamic fetch approaches early
Friday...bringing a large swell into our waters. This will cause
seas to rapidly build Friday morning...reaching 20 feet Friday
evening. The next strong front approaches on Saturday...with
gales possible...could build seas already 15 to 20 feet...to 20
to 25 feet Saturday. -McCoy
Pz...Small Craft Advisory for winds from 4 am to 1 PM PST Thursday
for coastal waters from Cape Shoalwater Washington to Cascade Head or out 60 nm.
Gale Warning from 1 PM Thursday to midnight PST Thursday night
for coastal waters from Cape Shoalwater Washington to Florence or out 60 nm.
Small Craft Advisory for hazardous seas until 9 PM PST this
evening for coastal waters from Cape Shoalwater Washington to Florence or out 60 nm.
Small Craft Advisory for winds until 1 PM PST Thursday for
coastal waters from Cascade Head or to Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar until 4 am PST Thursday.
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This discussion is for northwest Oregon and southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.