Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 150331

Area Forecast Discussion
National Weather Service Seattle WA
831 PM PDT Sat Apr 14 2018

.SYNOPSIS...Rain and mountain snow may be heavy at times this
evening before tapering to showers behind a front. Showers or
areas of light rain will prevail behind the front later tonight
through Monday with cool daytime temperatures. A weak ridge builds
Tuesday, then split flow develops Wednesday with a decreasing
chance of showers. Drier and mild weather is expected by Thursday
and Friday. A system may bring some rain by Saturday.


.SHORT TERM...Strong, moist SW flow continues across Western WA
this evening bringing heavy rain and high-elevation snow. The bulk
of the moisture associated with the baroclinic band is over the SE
half of the CWA. Embedded in the flow are pockets of heavier
bands that were producing heavy rainfall rates, especially from
Seattle to Olympia. A Flood Advisory has been issued for the
potential of urban (water ponding on the roads) and small stream
flooding. Precip rates will taper off after midnight tonight as
the baroclinic band finally weakens and exits east. Daily precip
records were set at Sea-Tac, NWS Seattle and Olympia so far.

Winter Weather Advisories remain in effect for the west slopes of
the Cascades until 11 PM tonight, mainly above 4000-4500 feet.
Snoqualmie and Stevens Passes will see mainly rain with
temperatures in the mid 30s.

Previous discussion...Showers later tonight into Sunday morning
may fill in with areas of light rain by the afternoon and evening.
The trailing wave noted further offshore will try to rotate and
lift north, clipping Western Washington. The flow aloft will not
be as strong with the main lift associated with diffluence aloft
as the upper low approaches and becomes negative tilt. This system
is not likely to produce heavy enough rainfall to affect rivers
much, if at all. Some light snow amounts are possible in the
mountains, possibly in the passes but probably only a couple
inches or less.

The upper low shifts over the region Monday, then eastward on
Tuesday as a weak ridge builds overhead. Showers are still likely
on Monday, especially during the afternoon if any sunbreaks help
with diurnal instability. Models show a fair amount of moisture
so clouds and cool temperatures may limit the tops on the showers.
Will not add thunderstorms to the forecast given the lack of
surface based heating and lapse rates below 7C in the 850-500 mb

.LONG TERM...Models are trending drier by Wednesday as split flow
develops. The next system may bypass the area to the south
completely. By Thursday and Friday, the ECMWF keeps the Pacific
Northwest mostly dry between systems. Previous runs showed a
front arriving around Friday, but the latest 12z run puts it off
until Saturday. Might we get lucky with at least 2 consecutive dry
days on Thursday and Friday? Kept the forecast optimistic, showing
mostly dry and milder weather those two days. Highs may reach
near 60 around Puget Sound and other interior areas. A front will
probably bring rain back into the area by around Friday. Mercer


.AVIATION...Rain continues this evening with strong, moist SW
flow over the region. Precip rates will taper off after midnight
tonight as a cold front weakens and pushes east. Expect IFR to
MVFR cigs. The air mass will remain moist on Sunday as a trough
digs offshore. Expect more showers with MVFR conditions likely
persisting. 33

KSEA...Rain continue this evening but will let up after midnight
tonight. IFR to MVFR cigs will likely persist. Winds variable
becoming S after 07-09z then flipping to northerly 12-15z Sunday.


.MARINE...A slow moving cold front will weaken and push east of
the area after midnight tonight. Westerly flow will develop
through the Strait of Juan de Fuca for brief Small Craft Advisory
winds. Meanwhile, seas are still within 10-13 foot range and a
Small Craft Advisory for Hazardous Seas remains in effect for the
Coastal Waters and West Entrance. The flow will turn northerly on
Sunday as a weak cold front shifts east into Oregon. Onshore flow
will resume Sunday night as this system moves inland. Onshore
flow will continue into Monday. A stronger onshore push on Monday
night and Tuesday could bring gales to the Strait of Juan de
Fuca. High pressure will develop over the area Wednesday, giving
lighter winds. 33


.HYDROLOGY...Flooding will continue on the Skokomish river in
Mason county tonight.

A Flood Watch remains in effect for Thurston and Lewis Counties.
However, it appears that the Chehalis, Newaukum and Skookumchuck
will crest below flood stage if forecast rainfall amounts are on

Hydrologically significant rainfall will end after tonight, so any
further flooding is unlikely over the next seven days. 33


WA...Flood Watch from midnight PDT tonight through late Sunday night
     for Pierce and Lewis Counties

     Winter Weather Advisory until 11 PM PDT this evening for
     Cascades of Pierce and Lewis Counties-Cascades of Snohomish
     and King Counties-Cascades of Whatcom and Skagit Counties.

PZ...Small Craft Advisory for hazardous seas until 5 PM PDT Sunday
     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 for rough bar until 8 PM PDT Sunday for
     Grays Harbor Bar.

     Small Craft Advisory until 5 AM PDT Sunday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.


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