Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 181118
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
318 AM PST Fri Jan 18 2019

.SYNOPSIS...A Pacific storm system will affect the area this
afternoon into Saturday morning with breezy winds and moderate
rainfall expected. These impacts will ease thru the day Saturday,
yielding generally dry conditions Sunday and Monday. By Tuesday,
the next round of rain is expected , which is expected to continue
into Wednesday. A high amplitude ridge of high pressure is
expected by the end of next week, which will make for a dry
forecast.

&&

.SHORT TERM...A departing shortwave trough can be readily seen in
water vapor imagery this morning, with axis roughly located along
Vancouver Island into the nearby Pacific. Associated sfc low is 
located in close proximity with a trough extending out ahead into 
nearby British Columbia and across the Cascade crest. 
Nonetheless, showers and some mountain snow exists across western 
Washington early this morning. These showers are expected to 
decrease somewhat as the morning continues, however, this will be 
temporary. A look upstream of the Pacific Northwest shows a broad 
area of cyclonic flow over a large portion of the north Pacific, 
with numerous shorter wave perturbations rotating about. One wave 
in particular will eject from the base of the larger scale trough 
later this morning and take aim for the central British Columbia 
coastline. As it does so, it will draw a river of atmospheric 
moisture towards the Pacific Northwest. This low is expected to 
deepen rapidly into a closed low by this afternoon/evening where 
another round of deep moisture will begin to move into the 
Washington coast. This sets western Washington up with another 
round of moderate rainfall late this afternoon through Saturday 
morning with QPF in the half inch (lowlands) to 1-2 inch range 
across the higher terrain. The exception will be the Olympics 
where locally higher amounts will be possible. The Flood Watch for
the Skokomish River in Mason County will remain valid given 
expected rainfall amounts.

In addition to the rain, wind can be expected again. Previous
guidance raised concern for high wind for portions of the area,
but not seeing winds quite as strong with the latest available
data. Gradients are less impressive than yesterday, and actually 
quite similar to Thursday's system, which would initially lean the
forecast towards Wind Advisory. Wind fields in the 850-700mb 
range still seem rather impressive at 40-50kts, but winds from 
sfc-925mb do not reflect what would resemble high winds. A close 
look at WRF 1km and 4km data suggest winds will be strongest along
the coast, eastern Strait, and north interior. For these reasons 
have upgraded High Wind Watch to a Wind Advisory for these areas 
beginning this afternoon and continuing thru Saturday morning. 
Winds will likely range 20 to 35 mph with gusts to 45 mph. Areas 
across Puget Sound could see gusts to 40 mph but think overall 
should remain just shy of advisory criteria. Day shift will have 
to keep an eye on this.

As for snow, have cancelled the Winter Weather Advisory for the
Cascades early this morning, as snow levels are rising up towards
3500 ft already with precip becoming more scattered. Several 
additional inches of snow are certainly possible, but are not 
expected to meet criteria levels. Snow levels will continue to 
rise rather significantly (4000-6000ft, maybe even 7000ft) with 
this next system so do not foresee much in the way of heavy snow 
accumulation at this time.

As the parent low moves well inland of central BC Saturday, rain
will taper to showers, winds will decrease, and snow levels will
fall back down. Not much in the way of sensible weather impacts
are expected, with the exception that the Skokomish River will
likely have made it into minor flood stage. The next in a series
of shortwaves will be ready to push onto the western US coast, but
still seems to be good consensus of it taking a southerly track
into OR/northern California. Aside from far southern zones, Sunday
may actually turn out to be a dry day.

Kovacik


.LONG TERM...Aforementioned trough on Sunday will continue to
venture inland inland across the western states with western
Washington is looking to remain dry through Monday as a broader 
scale ridge tries to build in from the Pacific. There still seems
to be a lot of moisture being pulled overtop of the ridge as 
several perturbations traverse the flow to its north. What would 
usually signify a break in the weather actually looks to hold the 
next round of rain. This looks to begin sometime Tuesday and 
continue through Wednesday. Ridge is finally able to build in 
better by Thursday as a deep trough develops across the eastern 
US. This still looks to make for a dry /tho likely foggy/ forecast
thru the end of next week.

Kovacik

&&

.AVIATION...Westerly flow aloft becoming southwesterly tonight. 
Low level onshore flow will turn back offshore later this morning 
through tonight as another frontal system moves into the area. 

Scattered-broken layer around 2000 feet with a broken to overcast 
layer around 4000 feet and another cloud layer in the 7000-8000 foot 
range early this morning. Isolated ceilings below 1000 feet over the 
Kitsap peninsula. Lower cloud layers dissipating 16z-20z with 
ceilings generally around 5000-6000 feet this afternoon through 
tonight. Rain spreading into the area from the southwest beginning 
along the coast around 21z and covering the entire area by 03z. 

KSEA...Ceilings 2000-3000 feet improving to around 5000 feet by 18z. 
Ceilings remaining in the 5000-6000 foot range through the evening 
hours with light rain beginning late afternoon. Southerly wind 10-15 
knots with gusts to 20-25 knots until 16z. Winds becoming 
southeasterly 5-10 knots in the afternoon. Felton

&&

.MARINE...Winds and seas that developed in response to southerly
winds associated with a trough that lifted north across the waters
overnight are now subsiding.

Attention now is focused on a developing 988 mb low that is
centered near 41N 141W. This low is forecast to move west of the
offshore waters and deepen to near 970 mb as it passes by to our
west. The low will then move into Haida Gwaii late tonight or
early Saturday morning. The strong front associated with this low
will pass through the waters tonight. Gale warnings are now in
effect for the coastal waters, the eastern two-thirds of the
Strait of Juan de Fuca, Admiralty Inlet, and the Northern Inland
waters. Small craft advisories have been issued for Puget Sound
and Hood Canal and for the west entrance to the strait. 

Seas over the coastal waters are now predicted to be lower than
previously thought, and are not expected to reach 20 ft on the
beaches. So at this point, there appears to be no need for high
surf advisories with this system along the coast and the threat
for any flooding around high tide is decreasing. 

However, some locations along the shorelines of the northern 
inland waters, specifically the coastline of Western Whatcom
County around Sandy Point and Birch Bay could see some coastal
flooding around the two high tides on Saturday. Tides and tidal
anomalies stay just below the level that usually produces flooding
during calm conditions, but strong southwest winds over the Strait
of Georgia from about the San Juan Islands to the shores of
Bellingham Bay may result in 4-6 ft waves that would crash onto
and over seawalls and other protective coastal structures around 
the high tides. These areas had already seen flooding and damage 
from waves on two previous occasions this winter, and this system 
has the potential to exacerbate the problems. A coastal flood 
watch has been issued for Western Whatcom County on Saturday. 

The weather will be considerably calmer Sunday and Monday, with 
another system possibly arriving Tuesday. Albrecht

&&

.HYDROLOGY...Last night's system is tapering to showers early this
morning, with coverage expected to wane thru the morning, albeit
only temporarily. Another system will bring moderate rainfall
across the area later this afternoon through the first part of
Saturday. QPF generally progged at less than an inch in the
lowlands to an inch or so in the Cascades. The Olympics may see 
the most with generally 1-2 inches expected with locally higher 
amounts. The rain in the Olympics will likely carry the Skokomish
into minor flood by this evening or overnight tonight. The Flood 
Watch will remain valid to represent this potential. No other 
river concerns are expected. Rain will taper thru the day Saturday
with mainly dry conditions to carry us all the way through 
Monday. Next chance for widespread rain looks to be Tuesday and 
Wednesday.

Kovacik


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from this afternoon through late Saturday night for 
     Hood Canal Area-Lower Chehalis Valley Area-Olympics.

     Wind Advisory until 4 AM PST early this morning for Admiralty 
     Inlet Area-Central Coast-Everett and Vicinity-North Coast-
     San Juan County-Western Skagit County-Western Whatcom 
     County.

     Wind Advisory from 6 PM this evening to 6 AM PST Saturday for 
     Admiralty Inlet Area-Eastern Strait of Juan de Fuca-San 
     Juan County-Western Skagit County-Western Whatcom County.

     Coastal Flood Watch from late tonight through Saturday afternoon 
     for Western Whatcom County.

     Wind Advisory from 2 PM this afternoon to 6 AM PST Saturday for 
     Central Coast-North Coast.

PZ...Small Craft Advisory for rough bar until 6 PM PST Saturday for 
     Grays Harbor Bar.

     Small Craft Advisory until noon PST today 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.

     Gale Warning from noon today to 6 AM PST Saturday 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.

     Small Craft Advisory until 1 PM PST this afternoon for East 
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 1 PM this afternoon to 6 AM PST Saturday for 
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PST this evening for Admiralty 
     Inlet-Northern Inland Waters Including The San Juan Islands.

     Gale Warning from 6 PM this evening to 6 AM PST Saturday for 
     Admiralty Inlet-Northern Inland Waters Including The San 
     Juan Islands.

     Small Craft Advisory until 3 AM PST Sunday for West Entrance 
     U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from noon today to 2 AM PST Saturday for Central 
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PST Saturday for Puget Sound and 
     Hood Canal.

&&

$$

www.weather.gov/seattle