Gale Warning 71, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
803 PM PDT Mon Oct 21 2019

.UPDATE /THIS EVENING/...Forecast largely on track this evening so
no major changes needed. Widespread moderate to locally heavy
rainfall continues across pretty much the entire area, save for
the far southern tier of counties. This activity will continue to
slowly shift south tonight and push into Oregon by Tuesday
morning. Precipitation amounts range from around a tenth to two
tenths in the Seattle metro area (due to prolonged rain shadow
much of the day) to over 4 inches across portions of the Olympic
Peninsula. North and Central Cascades have seen around 1 to 2
inches so far. Latest hi-res guidance continues to suggest that
bands of heavier precipitation will focus across the central
Cascades, especially near the Snoqualmie basin, over the next 4
to 5 hours. Have issued a few river flood warnings so far and will
be keeping a close eye on the hydro situation overnight. Updated
details can be found in the hydro section below as well as updates
for the aviation and marine sections. CEO

&&

.SYNOPSIS...A wet frontal system will bring widespread rain, heavy
at times, to the region tonight into tomorrow morning. Rain
chances will gradually decrease from north to south Tuesday as the
system sags further south and out of the area. High pressure aloft
will then start to build into the region from the west, making way
for drier weather Wednesday and Thursday. Rain chances return
Friday as a quick system drops down from British Columbia.

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY/...Current radar shows
widespread rain across northern portions of the coast and Sound at
this hour. Recent observations indicate that some spots along the
northern coast have already received an inch of rain or greater
over the past 6 hours. As of 3 PM, Quillayute has reported 1.45
inches of rain over the past  6 hours, while Forks has received
1.05 inches. This wet system will continue to make its way
southward through the region today. Areas just east of central
Puget Sound will remain rain shadowed at times, resulting in
lighter rainfall accumulations across the area. However, with
ample moisture being pooled into the region and strong
southwesterly flow in the low levels, can expect a 2-3 inches of
rain across portions of the central Cascades, with a few local
spots up to 4 to 5 inches possible. Rivers will continue to be
monitored closely over the next several hours as rainfall
accumulations could lead to sharp rises in river levels.

Rainfall will continue to make its way southward Tuesday morning
and will gradually taper from north to south. With an increase of
onshore flow, could see the development of a weak convergence zone
across portions of Snohomish and King counties in the early
morning hours. Rain should primarily be confined to the Cascades
by early afternoon and should taper completely by the evening
hours. High temperatures will be in the upper 50s to low 60s
across the region.

An upper level ridge will then build into the region from the
west on Wednesday and Thursday. This will give western Washington
a good break from the series of systems that have impacted the
region over the past week. Drier weather and increasing heights
over the region will allow for temperatures to increase a few
degrees each day, reaching the low 60s by Thursday.

.LONG TERM /FRIDAY THROUGH MONDAY/...An upper level trough will
drop down from British Columbia on Friday, bringing another shot
of rain to the region. Current guidance indicates that the system
will move through quickly and will make way for a high amplitude
upper ridge building over the northeastern Pacific. This ridge
will keep conditions dry for western Washington through the
weekend and into early next week. 14

&&

.AVIATION... Widespread area of moderate to heavy rainfall
continues to slowly push south across the area this evening. This
activity will continue into tonight before pushing south into
Oregon Tuesday morning. Ceilings and visibilities are all over the
place...ranging from MVFR to LIFR and this will continue to be the
case through most of the night. As rainfall tapers Tuesday morning
into afternoon, will likely see some gradual improvement. Breezy
southerly winds at the surface will continue overnight 10 to 15
kts with a few gusts of 20 to 30 kts. Lighter winds expected by
Tuesday afternoon and evening, shifting to more northerly.

KSEA...Conditions will likely continue to bounce around this
evening into the overnight as moderate rain continues. MVFR will
be the most likely category but will see periods of IFR,
especially in heavier showers. Improvement likely Tuesday morning
into the afternoon. Southerly wind 10-15kts with some gusts near
20kts into the overnight hours, with a shift to more westerly flow
Tuesday afternoon. CEO

&&

.MARINE...Breezy winds, generally from the south, continue across
all waters this evening, with the exception of the central
Strait. As such a Small Craft Advisory for wind remains in effect.
This is in response to the passage of a warm front across the
region. Following behind the warm front tonight will be a cold
front, which will shift winds more westerly as it passes through
western Washington from north to south overnight into Tuesday
morning. Given strong gradients associated with this frontal
system as a whole, expecting a rather strong westerly push of wind
down the Strait of Juan de Fuca late tonight and early Tuesday.
Right now, a small craft advisory for the central and eastern
Strait covers this scenario, but this will need to be monitored
for potential gale force winds. The best chance for gale force
winds will be in a brief window in the 09z-18z timeframe on
Tuesday. Westerly winds will likely bleed into the adjacent waters
of Admiralty and Northern Inland Waters as well.

Winds appear to relax Tuesday night into Wednesday as western
Washington finally catches a break from the persistent series of
storms off the Pacific.

Kovacik/CEO

&&

.HYDROLOGY...Generally 2 to 4 inches of rain is likely over the west
slopes of the Olympics and the central Cascades through Tuesday as
this system continues to sag southward. The heaviest rainfall is
expected over the central Cascades as the moisture band hangs over
that area longer and local rainfall accumulations of over 5 inches
are possible. Confidence is a little bit higher that heaviest
precipitation will occur in the central Cascades this evening. Less
rain is expected over the north and south Cascades but these areas
will also need to be monitored. Snow levels will remain between 7000-
9000 feet today. This combination could be enough to push some
rivers to flood stage tonight or Tuesday. Flood warnings are already
out for the Tolt and Snoqualmie Rivers.

This will be the first rain event of the season with the potential
of river flooding. The first high river flood flows of the season
can be tricky to forecast because flood waters can get deposited in
areas along the river that have been dry since the summer.
14/Bower

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch through late Tuesday night for Admiralty Inlet Area-
     Bellevue and Vicinity-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties-East Puget
     Sound Lowlands-Eastern Strait of Juan de Fuca-Everett and
     Vicinity-Hood Canal Area-Lower Chehalis Valley Area-North
     Coast-Olympics-Seattle and Vicinity-Western Strait of Juan
     De Fuca-Western Whatcom County.

PZ...Small Craft Advisory until 5 AM PDT Tuesday 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 11 PM PDT Tuesday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 2 AM PDT Wednesday for 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 from 11 PM this evening to 11 PM PDT
     Tuesday for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 5 PM PDT Tuesday for Admiralty Inlet.

     Small Craft Advisory until 2 PM PDT Tuesday for Puget Sound and
     Hood Canal.

&&

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