Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 161121
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
421 AM PDT Mon Oct 16 2017

.SYNOPSIS...An upper level ridge will retreat southward today,
allowing a cold front with plenty of rain and some wind. to slip
in from the north tonight and Tuesday. A break in the weather is
expected Tuesday evening. Then a strong warm front will arrive
late Tuesday night and Wednesday, accompanied by heavy rain and
windy conditions. A slow-moving cold front on Thursday will then
slip southward, bringing one more round of heavy rain before
tapering to showers under an upper trough for late Thursday and
Friday. Yet another strong warm front is forecast on Saturday,
with more heavy rain and wind possible, followed by weakening next
Sunday.

&&

.SHORT TERM...Today will be the last quiet, benign weather day for
the next week, so this is the time to finish those weather-
dependent outdoor projects and preparations for stormy fall
weather. An upper ridge will still exert some influence over the
Pacific Northwest today, though some rain is possible over the
northwest Olympic Peninsula and up near the Canadian border near
the periphery of the upper ridge.

The upper ridge will retreat southward into the Southwest U.S.
tonight and Tuesday. This will allow the corridor of strong jet
stream winds which is currently aimed at the B.C. coast to slip
southward into Western Washington. By late Tuesday, a surface cold
front will slip southward through the area. Preceding the front on
Tue AM, deep moisture, lift and W-SW 850 mb winds of 45-55 kt will
bring heavy rain to the windward side of the Olympics and
Cascades. Latest forecast calls for 24-hour rainfall of 2-4
inches of rain over the Olympics and North Cascades ending at 5 pm
Tue. The I-5 corridor should be under 1 inch of rain. In addition,
strong S-SE gradients will precede the front on Tue morning.
Expecting wind gusts of 30-40 mph along the I-5 corridor. Though
these values are sub-wind advisory, the first event of this
magnitude for the season can have greater impacts than would be
expected later in the month.

Once the cold front slips south into Southwest Washington on Tue
afternoon, a brief lull in the weather is expected on Tue evening
in the form of lighter wind and drier weather.

A strong, consolidated jet streak will start to nose into the Pac
NW and B.C. late Tue night and Wed. At first, this will push a
strong warm front across Western Washington. The front will be
accompanied by deep moisture, SW 850 mb winds of 50-60 kt, and
lots of warm advection lift. The result will be heavy rain. Heavy
rain will then continue into Thu morning, as a slow-moving cold
front spreads in from the NW. Over the course of 36 hours, 6-9
inches of rain is forecast for the Olympic Mtns, 4-6 inches for
the North Cascades, and 1-3 inches for the I-5 corridor. This is
when concerns for flooding begin. In addition, the warm front will
come with strong southeast gradients. Southeast-wind prone
locations such as the coast and Admiralty Inlet area may
experience wind gusts of 45+ mph. The North Cascades Hwy near
Rainy Pass could also get 5-8 inches of snow on Wed morning,
before changing to rain.

.LONG TERM...A slow-moving cold front will drag across Western
Washington on Thursday. It will be followed by an upper trough on
Thu night and Fri. The upper trough pattern will come with less
impactful showery weather, cooler temperatures and snow levels as
low as 3500 feet on Friday night.

Saturday could be the next day with impactful weather. Yet another
strong warm front will cross Western Washington. Similar to Wed,
the pattern support heavy rain and strong S-SE gradients and wind.
GFS ensemble members and the 00z ECMWF suggest that the
operational 00z GFS may be too far south with its precip bullseye
over Southwest Washington. Saturday should emerge as the next
focus of attention once we get past Thu morning.  Haner

&&

.AVIATION...Weak high pressure aloft will gradually flatten and
push east today and tonight. A vigorous surface cold front will
reach the coast tonight and push southeast through the interior late
tonight and early Tuesday morning. Strong southwest flow aloft
through late tonight. Air mass stable and somewhat moist in the
mid/upper levels. Becoming moist at all levels tonight into Tuesday.
Ahead of the front, rain will develop over the Olympic Peninsula
this morning spreading into the north interior this afternoon and
finally into the rest of the interior by this evening.

Areas of low level moisture and IFR conditions along the coast and
into the SW interior and far south Puget Sound during the morning.
Otherwise VFR skies with increasing mid/high clouds, then VFR
stratus with rain lowering into areas of MVFR later tonight/Tuesday
morning.

KSEA...A front will stay offshore today but some mid/high clouds
will gradually increase. VFR stratus with rain developing this
evening with periods of MVFR conditions as rain continues later
tonight/Tuesday morning. Southeast wind 3-6 kt becoming south 5-9 kt
by midday.

&&

.MARINE...A front will approach the coastal waters today with small
craft winds rising to 20 to 30 kt ahead of the system. The main cold
front will slowly move into the coastal waters tonight. Small craft
winds will probably develop over most inland waters by late today or
this evening through tonight. The front pushes through the inland
waters by early Tuesday morning. Winds may ease over most waters but
southwesterly small craft winds will probably persist in the Sound.

The next stronger Pacific frontal system is set to arrive Wednesday
and anticipate gale force winds over the coastal waters. Solid small
craft advisory winds are likely over the inland waters. Forecasts
should be monitored closely as the precise track of the front could
bring stronger or weaker winds than forecast, especially over the
inland waters.

Ocean swell will rapidly build Wednesday into Wednesday night to at
least 15 feet, peaking near 20-22 feet Thursday into Friday. dtm

&&

.HYDROLOGY...While some heavy rain is expected over mountain
headwaters on Tuesday, dry antecedent conditions and current low
river flows suggest river flooding will not be a concern on
Tuesday.

However, a 36-hour long atmospheric river event is shaping up from
Wed AM through Thu AM. Over the course of 36 hours, 6-9
inches of rain is forecast for the Olympic Mtns, 4-6 inches for
the North Cascades, 2-4 inches for the Central Cascades, and 1-3
inches for the I-5 corridor. Minor flooding will become possible
on rivers flowing off the Olympics and North Cascades. The flood-
prone Skokomish River is the most likely river to exceed flood
stage. Rivers will be rising late Wed with flood concerns peaking
Wed night and early Thu.

Rivers will recede late Thu and Fri in response to lighter precip
and a lowering of snow level to as low as 3500 feet on Fri night.

The weather pattern on Saturday looks like another floody
atmospheric pattern. This far in advance, there is lots of
variability in where guidance shows heaviest rain, but GFS
ensembles and the ECMWF favor northwest Washington. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 PM 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.

&&

$$

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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