Scientific Forecaster Discussion
fxus66 ksew 231559 cca
Area forecast discussion...corrected
National Weather Service Seattle Washington
759 am PST Tue Jan 23 2018
Synopsis...the next front to reach the area will spread rain into
the area today with the precipitation continuing until the front
moves through the area Wednesday morning. Heavy snow is likely in
the mountains. A cool upper level trough will set up offshore later
Wednesday and remain through Thursday.
Short term...note: all routine forecasts will be late this morning
due to earlier Tsunami Watch concerns. As of 415 am, the Tsunami
Watch for Washington has been cancelled, and there are no tsunami
concerns in Washington.
Rain has reached the coast and is forecast to spread inland this
morning. It's likely to rain all day and all night, then taper
Wednesday. Snow will be heavy at times in the Cascades, with a
particularly heavy period from 18z this morning to 06z tonight. The
snow level will be 2000-3000 feet so the Highway passes will be
affected. Up to two feet will fall in both Snoqualmie and Stevens
passes during the 24 hours today and tonight. A Winter Storm Warning
is in effect for the Cascades, with an advisory for the Olympics.
The front will bring some wind as it moves inland today. Most areas
will have sub-advisory level wind, but in the San Juans and in the
Admiralty Inlet area there could be 20-35 mph wind, and an advisory
is in effect there.
What is left of the front will move through the area on Wednesday
keeping rain at times in the forecast. A couple of upper level
troughs moving through western Washington Wednesday night into
Thursday will keep precipitation in the forecast for the remainder
of the short term. There is some instability associated with these
upper troughs, so have added a slight chance of thunderstorms to the
coast Wednesday night through Thursday night. Felton/Burke
Long term...a pretty cold upper trough will move through the region
Friday. Overnight low temperatures will be mostly in the 30s, so
higher hills could see a dusting of snow in heavier showers Thursday
night and Friday morning. Showers will continue Friday.
A very wet weather system seems likely to impact the region at some
point Saturday through Monday. Timing and location of the heaviest
rain remains uncertain, but flooding on rivers beyond the most flood
prone ones is possible. Burke
Hydrology...the Skokomish River is below flood stage and is
receding slowly tonight. More rain today into Wednesday will push
the river back up to near flood stage on Wednesday and a Flood Watch
is in effect.
There is a potential for an atmospheric river over the area this
weekend or early next week for a period of significant rainfall. At
this time, the location of where the heaviest rain will occur was
still uncertain due to disagreement in the medium range models. The
main message is that the combination of rising snow levels and heavy
rainfall could lead to flooding on some rivers (besides the
skokomish river) late this weekend or early next week.
The USGS landslide threshold indices were near or at the thresholds
where landslides become probable. The additional rainfall this week
will only increase the risk of shallow landslides. Felton
Aviation...a warm front has started to enter the western half of
the County Warning Area this morning and is expected to move further east throughout
the day. Rain has already started along this feature and will spread
eastward through the morning. Cigs remain mostly VFR over the
area...with isolated MVFR around the south sound. Cigs expected to
deteriorate throughout the day with most taf sites dipping down into
MVFR by late this morning...with some isolated locations possibly
seeing IFR conditions. Southwesterly flow aloft is expected to
strengthen by late this morning and is not expected to start
relaxing somewhat until Wed morning.
Ksea...VFR cigs expected this morning. Warm frontal rain expected to
arrive by 15z. Cigs will start to lower thereafter, eventually
falling into the 020-030 range late morning. Reduced vsbys possible
at times from 18z-24z during periods of moderate rain. Otherwise,
low-level wind shear will reach criteria for inclusion in the taf
today owing to east-southeast surface wind of 8-13 kt and 2000-foot winds that
will be southerly around 35-40 kt for much of the day. Haner/smr
Marine...a warm front will move east through the waters
today with strong east to southeast pressure gradients. This will
allow southeast gales over the coastal waters, and over the inland
waters from Admiralty Inlet north. Strongest winds over the inland
waters will be focused from Admiralty Inlet up to Haro Strait.
Southeast gradients will relax this evening following the warm
front. All that being said...inherited headlines look good and will
remain in place.
From Tuesday night through Saturday, a low pressure trough will
pass about every 12 to 18 hours. Forecast winds could be
conservative during any single forecast period due to the effect
of averaging out so many small, compact, fast-moving features.
The strongest trough may come around Friday night. Haner/smr
Washington...Winter Storm Warning until 6 am PST Wednesday for Cascades of
Pierce and Lewis counties-Cascades of Snohomish and King
counties-Cascades of Whatcom and Skagit counties.
Flood Watch from late tonight through Thursday morning for Hood
Canal area-lower Chehalis valley area-Olympics.
Wind Advisory from 9 am this morning to 6 PM PST this evening
for Admiralty Inlet area-San Juan County.
Winter Weather Advisory until 6 am PST Wednesday for Olympics.
Pz...Small Craft Advisory for rough bar until 10 am PST Wednesday for
Grays Harbor bar.
Gale Warning until 3 PM PST this afternoon for coastal waters
from Cape Flattery to James Island 10 to 60 nm-coastal
waters from Cape Flattery to James Island out 10 nm-coastal
waters from James Island to Point Grenville 10 to 60 nm-
coastal waters from James Island to Point Grenville out 10
nm-coastal waters from Point Grenville to Cape Shoalwater
10 to 60 nm-coastal waters from Point Grenville to Cape
Shoalwater out 10 nm-West Entrance U.S. Waters Strait of
Juan de Fuca.
Small Craft Advisory until 6 am PST early this morning for East
Entrance U.S. Waters Strait of Juan de Fuca-northern inland
waters including the San Juan Islands.
Gale Warning until 6 PM PST this evening for central U.S. Waters
Strait of Juan de Fuca-East Entrance U.S. Waters Strait of
Juan de Fuca-northern inland waters including the San Juan
Small Craft Advisory until 10 am PST this morning for Admiralty
Gale Warning from 10 am this morning to 6 PM PST this evening
for Admiralty Inlet.
Small Craft Advisory from 10 am this morning to 4 PM PST this
afternoon for Puget Sound and Hood Canal.