Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 181137
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
437 AM PDT Wed Oct 18 2017

.SYNOPSIS...A strong warm front today will bring heavy rain and
locally strong winds to the area. A cold front on Thursday
morning will push the heavy rain out of the area, but it will be
followed by numerous showers later Thursday into Friday night.
Another strong front will pass across the Pacific Northwest on
Saturday and Saturday night. Drier weather may develop early next
week as a ridge of high pressure strengthens to our south.

&&

.SHORT TERM...A strong warm front will enter Western Washington
this morning, hanging up this afternoon over northwest Washington
before slowly sinking back south tonight and Thursday morning as a
slow-moving cold front. The two concerns will be wind and rain.

Regarding wind, southeast pressure gradients will be picking up
this morning. The 00z GFS forecasts PDX-BLI gradient to peak at
+11.4 mb at 21z (2 pm), OLM-BLI of +5.9 mb at 21z, OTH-SEA at
+9.9 mb at 00z (5 pm), and AST-UIL at +8.3 mb at 21z (2 pm).
Strongest wind should be in southeast-wind prone locations such as
the coast and from the Everett area north to Bellingham and the
San Juans. In these places, wind gusts around 45-55 mph are
possible, peaking early this afternoon. Would not normally go so
far as to issue a High Wind Warning for this range of wind speeds,
but the first event of this magnitude for the season usually comes
with more tree and power-outage related impacts than after the
first event has pruned the weakest tree branches. For the Seattle
metro area south to Olympia, gusts around 35-45 mph are possible,
peaking late this afternoon around the time of afternoon commute.
Have expanded the Wind Advisory to include the Southwest Interior
zone, which includes Olympia and Centralia. The process of easing
wind will occur from northwest to southeast this evening.

Regarding rain, some intense rainfall rates in excess of 0.5" per
hour are expected from late morning into early evening, most
notably over the windward Olympics and North Cascades. SW 850 mb
winds of 55-65 kt today and Precipitable Water values approaching
1.5" along the coast around mid-day plus strong warm advection
lift will support the heavy rain. Over the next 24 hours, 4-7+
inches of rain are forecast for the windward Olympics, 3-5 inches
for the North Cascades, and 1-4 inches for the Central Cascades.
Heaviest rain over the lowlands will come late this afternoon and
early this evening, contributing to 24-hour rain amounts of 1-2
inches over the interior lowlands (outside the rain shadow near
Port Townsend). At the least, this will cause a sloppy evening
commute. Urban flooding is possible in poorly drained areas.

Once the cold front moves south of the forecast area late tonight,
winds rain will taper to numerous showers. An upper trough with
cold air aloft and instability will support convective shower
activity all the way through Friday afternoon. Could even be a few
claps of thunder along the coast and over the coastal waters on
Thu and Thu night. Snow levels will gradually fall on Thursday and
Friday, bottoming out in the 3000-3500 foot range on Friday
evening. Stevens Pass could start picking up some snow
accumulation by Thursday night. The current Paradise (Mt.
Rainier) snowfall forecast could necessitate an eventual Winter
Weather Advisory on Thu ngt and Fri. Haner

.LONG TERM...Another strong warm front will spread across Western
Washington on Saturday. S-SE gradients will again pick up,
supporting windy conditions, especially in southeast-wind prone
locations such as the coast and from Everett northward.
Hydrologically significant rainfall is likely later Saturday and
Saturday night, though the exact location of the heavy rainfall
axis is still uncertain. Rainfall will gradually ease on Sunday
night and Monday.

An upper ridge should bulge northward into the Pac NW around
Tuesday, bringing a change to dry weather with seasonably mild
temperatures.  Haner

&&

.AVIATION...A vigorous frontal system will move through the area
today. West flow aloft will become southwest tonight. High clouds
early this morning will give way to low clouds this morning as
the front moves onshore.

KSEA...High clouds early. MVFR low clouds and rain will develop
this morning as a front arrives. Southerly wind 5-10 knots will
rise to 10-20 knots later today. Schneider

&&

.MARINE...A vigorous frontal system will bring gales to most
waters today. A period of onshore flow will follow the front
Thursday and Friday. Another strong front will move through the
area Saturday. West swell 20-24 feet are expected to affect the
Coastal Waters later Thursday through Friday morning. Schneider

&&

.HYDROLOGY...Heavy rainfall is expected today and tonight,
especially over southwest facing slopes of the Olympics and North
Cascades. Precipitation amounts of 4-7+ inches can be expected in
the Olympics, while the North Cascades should see 3-5 inches. A
large part of the interior lowlands will exceed 1 inch, with a few
places exceeding 2 inches. The heavy rainfall rates are
accompanied by high snow levels well over 7000 feet. Hydro models
do not explicitly show flooding on area rivers. However, river
forecasts are close enough (within a foot) on the Skokomish,
Nooksack, Stillaguamish, and Skagit that Flood Watches will
continue from this afternoon through midday Thursday for
Snohomish, Skagit, Whatcom and Mason counties. Lowland locations
may see urban and small stream flooding due to the combination of
high rain rates and drainage issues due to leaves clogging drains.

Rivers will recede later Thursday through Friday night as snow
levels drop to around 3500 feet and precipitation becomes more
showery.

Hydrologically significant precip is expected Saturday and
Saturday night. Rivers could quickly rise later Saturday into
Sunday. Rivers will be starting from a higher point; minor
flooding is possible on some rivers.

Conditions will dry out starting later Sunday or Monday of next
week allowing all rivers to recede. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory from 11 AM this morning to 11 PM PDT this evening
     for Bellevue and Vicinity-Bremerton and Vicinity-Hood Canal
     Area-Seattle and Vicinity-Southwest Interior-Tacoma Area.

     Flood Watch from noon PDT today through Thursday morning for
     Bellevue and Vicinity-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties-East Puget
     Sound Lowlands-Everett and Vicinity-Hood Canal Area-Lower
     Chehalis Valley Area-Olympics-Western Skagit County-Western
     Whatcom County.

     High Wind Warning from 9 AM this morning to 9 PM PDT this
     evening for Admiralty Inlet Area-Central Coast-Everett and
     Vicinity-North Coast-San Juan County-Western Skagit County-
     Western Whatcom County.

PZ...Small Craft Advisory for rough bar from noon today to 9 PM PDT
     this evening for Grays Harbor Bar.

     Gale Warning until 9 PM PDT this evening 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 until 6 PM PDT this evening for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 11 PM PDT this evening for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 5 PM PDT this afternoon for Central
     U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 11 AM this morning to 9 PM PDT this evening
     for Puget Sound and Hood Canal.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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