Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
1048 am PST Friday Dec 19 2014
Synopsis...scattered showers will continue today with snow showers
in the Cascades. Mild and very wet weather is expected this weekend
with an atmospheric river pattern resulting in heavy rain across much
of the region...especially mountainous terrain. There is the
potential for some flooding late this weekend into early next week.
Longer range model output is suggesting a colder air mass will settle
over the Pacific northwest late next week.
Morning update...upper trough looks like it has just passed the area this
morning and as such shower activity is lessening. Will see some
clearing this afternoon with scattered showers before clouds and then
stratiform precipitation from the next system begin to move into the area
early this evening.
There have not been any significant changes in the models this
morning and we appear to remain on track for a significant
atmospheric river event beginning tonight. Compared with
yesterday...it looks like models are showing a slightly later onset
of true westerly 850 mb flow...which would somewhat limit the
orographic enhancement during the beginning of the event...but that
should not change storm total quantitative precipitation forecast very much. Have issued a Flood
Watch for the entire forecast area based on forecasts of flood or
near flood stage at many river points in the area.
Models are indicating cold air will hang around the S Washington
Cascades a bit longer than elsewhere and SW flow tonight will favor
orographic enhancement there...so have issued a Winter Weather
Advisory for areas above 3500 feet. Could also see some snow
accumulation above 4000 feet around and east of Montana Hood into tomorrow
morning...but widespread amounts looks like they'll remain less than
6 inches during any given 6 hour period so have not issued anything for
that area at this time. Bowen
Short term (previous discussion)....today through Sunday...the main
weather feature today will be the cold upper trough. Latest
model guidance shows the bulk of the trough through the area by
midday. Snow levels drop to below the Cascades passes today...but the
majority of the quantitative precipitation forecast has already occurred. NAM model sounding for kast
would indicate the best potential for thunderstorms to be through the
morning hours. NAM sounding valid 17z today shows an Li of 0 with a
pretty hefty sweat index of 335...well above the generalized 250
sweat index benchmark used for elevated convective
confidence. The convective potential then diminishes in the afternoon
as the air mass aloft starts to stabilize.
The bigger story continues to be the atmospheric river event over the
weekend. Latest model runs are generally consistent with their
respective past runs with fairly decent continuity amongst the
individual models. The main forecast issues will be the focus of the
heaviest precipitation and the duration. GFS has been persistent in
directing the baroclinic band over extreme SW Washington and northern Oregon. The
NAM and European model (ecmwf) are fairly similar...but the European model (ecmwf) seems to focus the
heaviest quantitative precipitation forecast over the central part of the forecast area 12z sun
through 00z Monday. The 00z WRF-GFS 48-hour quantitative precipitation forecast totals valid 12z Sat
through 12z Monday show at least 3-6 inches for the Oregon Cascades and
2-4 inches for the Oregon Coast Range. It has lower amounts in the S
Washington Cascades and Willapa Hills. GFS and NAM have strong isentropic
lift along the 290k surface starting 12z Sat and continuing through
sun. 850 mb westerly flow of 40-50 knots into the Oregon Coast Range and
Cascades Sat through sun will result in a highly efficient orographic
rainfall component. Looking at the ensemble situational awareness
table...specifically the ensemble model climate quantitative precipitation forecast (or M climate
qpf)...it indicates a 99th percentile or maximum quantitative precipitation forecast event for a good
portion of the Oregon Cascades Sat afternoon through Sun morning.
This means that the gefs ensemble is forecasting an inordinate amount
of precipitation compared to the 1985-2012 winter ensemble model climate.
Would not be surprised to see 1.5 to 2.0 inches per six hours in the
Cascades for a 12-18 period. Fortunately...there is no low-elevation
snow and minimal snow at pass levels. However...latest Hydro guidance
suggests a few rivers will reach flood stage by sun or Monday. Refer to
the hydrology section for more details.
Do not anticipate high wind highlights for the coastal areas but it
will be rather blustery. The NAM indicates a kast-koth gradient of
around 10 mb by late Sat afternoon and in the evening. The benchmark
gradient is around 12 mb. Expect 45-55 miles per hour gusts along the beaches
and headlands Sat through Sat night.
In addition...those planning to be along the coast at area beaches
should closely monitor the conditions this weekend. Rapidly building
seas will bring the potential for dangerous high surf along the
beaches as early as today. Long period swells may reach shore and
surprise individuals who are unaware. Anyone on the beaches should
exercise extreme caution and avoid the surf zone as large waves may
wash over beaches and jetties with little warning. Some potential for
minor tidal overflow is possible for Saturday and Sunday...especially
around high tides. This threat could be enhanced by high flows on
rivers in response to the rainfall this weekend. Weishaar
Long term...Sunday night through Friday...models agree the weekend
atmospheric river event will wind down Sunday night into Monday.
Upper ridging amplifies Monday...but this will likely be a dirty ridge
as the GFS and European model (ecmwf) hold on to some quantitative precipitation forecast...especially across the northern
half of the forecast area. Have adjusted probability of precipitation to reflect this
thinking. A weak warm front may brush the northern zones Monday night and Tuesday
for some additional precipitation. The GFS and European model (ecmwf) then bring a colder
upper level trough across the area Wednesday. The European model (ecmwf) drops 1000-500 mb
thickness values to near 530 dm 00z Thursday with a decent amount of quantitative precipitation forecast.
Thus...the Cascades and ski areas may finally get a decent shot of
snow. Colder north flow will follow and allow temperatures to cool
well below normal as conditions dry out late week. Weishaar
Hydrology...situation setting up for a memorable hydrologic event
mainly for later this weekend and heading into next week. Strong and
deep moist westerly flow will begin crossing the pacnw coast come
Saturday morning. A very long fetch of moisture is stretching across
the Pacific with origins west to the South China Sea. This moist air
mass will be pushed east by a 150 to 170 knots jet and westerly 25 to
50 knots lower level winds. As such...this continues to give the
appearance of a classic high precipitation atmospheric river event
where the deep moist flow will spend a significant amount of time
on a nearly perpendicular course against the Coast Range with the
Cascades taking a bit higher of the impact. As of the morning model
runs and recent nwrfc updates...it appears multiple rivers have
potential of exceeding flood stage. Additionally...this event has
sufficient depth to bring significant rains to the north Oregon and
South Washington Cascades where rivers will also see potential for
notable flooding. Have issued Flood Watch for the entire forecast
area. Keep in mind that details will always be in flux under these
scenarios as subtle north/south shifts cold make dramatic differences
in which basins receive extended periods of moderate to heavy rain.
Please continue to follow the latest forecasts and impacts as
conditions warrant. River forecasts are available on our web Page at:
http://water.Weather.Gov/ahps2/index.Php?Wfo=pqr (all lower case).
Aviation...a showery airmass will lead to predominantly VFR
conditions through this evening. A warm front spreading widespread
light to moderate rain into the area overnight should result in
a deterioration at all taf sites after 06z Saturday. Expect
widespread high end IFR to mainly MVFR conditions between 09z
and 18z Saturday.
Kpdx and approaches...an unstable showery airmass will lead to
predominantly VFR conditions through at least 06z Saturday. A warm
front will spread steady rain into the terminal between 09z and
18z Saturday. Expect a gradual deterioration to mainly MVFR
conditions during this time. /Neuman
Marine...the main weather story in the immediate short term is
that seas currently in the 10 to 12 feet range will nearly double in
height today...rising into the 20 to 23 feet range with dominant
periods near 18 seconds. Any small craft vessels are urged to come
onshore immediately as higher seas are beginning to arrive. Buoy
89 about 70 miles offshore is already reporting seas near 15 feet
at 8 am...and will likely exceed 20 feet by early afternoon.
People should avoid walking and climbing on jetties along the
Oregon and Washington coasts through Saturday...but especially
this afternoon and evening. There is considerable concern that
sneaker waves will pose a risk to the general public walking along
the beaches this afternoon and evening. This general pattern of a
large incoming swell often allows unwary beach goers to be caught
off guard by waves that run much farther up the beach than waves
over the previous 15 to 30 minutes. Under similar scenarios in the
past...waves have been known to knock unwary people and pets off
their feet before the receding water pulls them back into the surf
zone. In addition...these higher waves can cause Driftwood that
superficially appears locked in place along the beach to become
Mobile and roll over an unsuspecting person as the wave pushes
ashore and then recedes. This happens nearly every year along the
Oregon coast...and this is an ideal setup for these large sneaker
waves to do just this. Please be cognizant of this when recreating
on beaches today and tonight.
The next frontal storm system is still on track to impact the
waters late tonight and Saturday. Expect winds to gradually
increase today and tonight...with solid gale force winds of 30 to
35 knots with gusts of 40 to 45 knots expected across the waters by
Saturday morning. There is a chance gusts may approach 50 knots
within 20 nm of the coast as a short lived coastal jet
develops...especially along the central Oregon coast. While the
aforementioned large westerly swell will be slowly tapering
Saturday...this renewed shot of winds across the waters...will
likely keep seas flirting with 20 feet for much of Saturday. Expect
the highest winds to taper around midday or early Saturday
afternoon. However...additional gusts of 35 knots may linger into
Saturday evening as another weak surge of winds enters the waters.
Opted to extend the Gale Warning into Saturday evening to cover
this period...but confidence is considerably lower that these 35
knots wind gusts will occur late Saturday afternoon and evening.
Again...the primary window for solid gale force winds will be
Saturday morning and confidence is high.
Westerly winds will continue into Sunday...and keep seas well into
the teens through Sunday night. Seas may temporarily bottom out
near 10 feet early next week before a weak front...and accompanying
larger swell slides into the waters towards midweek.
Will continue to investigate whether or not a coastal Flood
Advisory for minor tidal overflow flooding will be necessary for
the South Washington coast centered around the high tide expected
to occur at 11 am Saturday. Given the Willapa and Naselle river flows
will be well below flood stage...and high tide near Toke Point is
expected to flirt with 11 feet with a tidal anomaly near 1 feet...we
are leaning towards not issuing a coastal Flood Advisory for
Saturday. However...a similar tide height will occur around midday
Sunday. By this time...flows on the Willapa and Naselle rivers
will be considerably higher so there is an increased concern that
minor tidal overflow flooding will occur along low lying roads
around Raymond...Naselle and the Willapa Bay in general in
southwest Washington. Given widespread coastal flooding is not
expected...we are opting to not issue a coastal Flood Watch...and
will likely wait to issue the advisory until we gather more ground
truth data and Sunday approaches. The peak time of concern on
Sunday will be from approximately 9 am to 3 PM. /Neuman
Pqr watches/warnings/advisories... or...Flood Watch from Saturday
afternoon through Monday afternoon 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 metropolitan area-lower Columbia-north Oregon coast-
northern Oregon Cascade foothills-northern Oregon Cascades-
south Willamette Valley-Upper Hood River Valley-western
Columbia River gorge.
High surf advisory until 4 PM PST Saturday for central Oregon
coast-north Oregon coast.
Washington...Flood Watch from Saturday afternoon through Monday afternoon 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 4 PM PST Saturday for South Washington
Winter Weather Advisory from 4 PM this afternoon to 1 PM PST
Saturday for South Washington Cascades.
Pz...Small Craft Advisory for winds until 1 am PST Saturday for
coastal waters from Cape Shoalwater Washington to Florence or out 60 nm.
Gale Warning from 1 am to 9 PM PST Saturday for coastal waters
from Cape Shoalwater Washington to Florence or out 60 nm.
Small Craft Advisory for hazardous seas until 1 am PST Saturday
for coastal waters from Cape Shoalwater Washington to Florence or out 60
Small Craft Advisory for rough Columbia River bar until 4 PM PST
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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.