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Forecast Discussion for Seattle/Tacoma, WA

000
FXUS66 KSEW 291713
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
930 AM PDT Wed Mar 29 2017...CORRECTED TYPO

.SYNOPSIS...A frontal system will move through Western Washington
today, bringing more rain, mountain snow and locally windy weather.
An upper trough will follow tonight, with decreasing showers and
some sun breaks Thursday. High pressure will build across the area
for dry weather Friday. A weak system should bring some light rain
to the area over the weekend, with a chance of showers early next
week.

&&

.SHORT TERM...Water vapor satellite imagery shows the low pressure
circulation tracking northeast into central Vancouver Island this
morning. Latest MSAS analysis indicates the 1005 mb low is a couple
millibars deeper than models had forecast but will probably not have
a huge different on winds forecast to pick up over Wrn wa today.
Strong southwest flow aloft will allow an embedded meso low to form
over the north Kitsap Peninsula over the next few hours. This will
cause breezy winds to become locally windy, especially over
central/north Puget Sound and Admiralty inlet. Southerly winds could
reach 15 to 30 mph with gusts of 35 or 40 mph for a few hours around
midday. Saturated soil will allow trees to topple easily WITH some
isolated power outages are possiblE. This will not be a significant
wind event and speeds still look to remain sub-advisory.

More rain will fall today, then transition to showers tonight. See
the hydrology section below regarding impact on the rivers. The
lowlands could see a half inch to locally 1 inch will  maintain a
further elevated landslide risk across all the lowlands. A special
weather statement is in effect to cover this threat. The tricky part
of the near term forecast is snow levels and mountain snow
accumulation. Models were indicating a much more rapid rise in snow
levels overnight but that has yet to occur. In fact, it is still
snowing down some lower passes such as Snoqualmie. Snow levels are
still expected to gradually rise from south to north, but a
significant jump will probably not occur until the occluded front
moves through this afternoon. It snowed at all passes overnight with
several inches, heaviest further north toward Stevens pass and
higher elevation spots like Mount Baker. In fact, over 20 inches
already fell at Mount Baker with another foot possible through this
evening if a change over to rain does not occur. Decided to upgrade
to a warning for the north Cascades based on the lack of warmer air
reaching northern areas. Snow is finally beginning change to rain at
lower passes and southern most Cascades in Pierce and Lewis county.
With snow still falling and snow covered roads on mountain web cams,
decided to issue a winter weather advisory through 1 PM pst. A few
spots mainly above 4000 feet could still pick several more inches
before snow levels finally shoot up to 6000 feet this afternoon.

Post-frontal onshore flow will bring cooler air and falling snow
level back to the area tonight. However, moisture will be decreasing
with mostly showers around Wrn Wa. Orographics may help squeeze out
a few inches tonight in the mountains above 3000 feet but probably
not advisory amounts given the lack of moisture.

A trough moves through on Thursday with showers decreasing, although
a residual convergence zone could bring a somewhat better chance of
measurable rain to the Snohomish and King county areas. Sun breaks
are likely in the afternoon as well.

High pressure builds Thursday night and Friday for a period of dry
and warmer weather. Highs on Friday could reach close to 60 around
greater Puget Sound and the southwest interior. Sea-Tac has yet to
reach 60 this year.

.LONG TERM...From previous discussion...The frontal system discussed
just above will weaken as it moves across Western Washington
Saturday, for a chance of rain. An upper trough will follow it on
Sunday for a chance of showers. Then a small upper ridge will
progress across the area Monday for a partly sunny and probably some
spotty light showers.
A weak upper trough will keep a chance of showers in the forecast
again Tuesday. Although a chance of precipitation will be
mentioned each day Saturday through Tuesday, this looks like a
period of rather benign weather and precipitation -- where it does
fall -- won`t amount to much. McDonnal

&&

AVIATION...Southwesterly flow aloft today will become more southerly
tonight as an upper level trough approaches the area. Flow will turn
northwest late tonight after the trough passes. At the surface, a
front will move across the area this afternoon. Moderate low level
southerly flow will become strong westerly behind the front. The air
mass is moist and stable.

Widespread IFR ceilings and visibilities will improve to MVFR or even
VFR this afternoon. The mountains will be obscured.

KSEA...Mostly MVFR today, with some improvement from low end to high
end MVFR this afternoon and evening. Southeast wind 10-15 KT will
become southwest 15g25 KT by 21Z, then ease tonight. CHB

&&

.MARINE...A cold front will move across the coastal waters this
morning and the interior waters this afternoon. Moderate to strong
onshore flow will develop behind the front. Onshore flow will persist
on Thursday, then weaken Thursday night. A ridge of high pres will
give lighter winds on Friday. Another front will arrive Saturday. CHB

&&

.HYDROLOGY...Heaviest rain amounts the past 36 hours are over the
Olympics with up to of 4.5 inches along the south and west facing
slopes. The Cascades have picked up 1 to 3 inches over this same
period. An additional 1 to 3 inches of rain may fall through this
evening below the snow level which will range from 3000 to 6000 feet
due to a front moving through the area later today. This amount of
rain will cause rises on rivers in the Cascades but flooding is not
expected in any of the cascade basins at this time.

The Skokomish river surpassed flood stage last evening near Potlatch
and appears to be near its crest this morning, though additional rain
over the next few hours could drive it a bit higher. It should fall
slowly tonight, then probably drop below flood stage by early
Thursday morning. Only minor flooding is expected. The Bogachiel
river running off the Olympics is the only other river that may
approach the flood stage. However, current trends are lining up well
with the forecast indicating very little chance of flooding on that
river. Forecasts should continue to be monitored closely, and always
refer to the current flood statement for the most current
information

Otherwise, flooding is not expected on our rivers during the next wee

The threat of landslides has increased further due to rainfall over
the past 24 to 36 hours hours. More rain today and tonight will keep
the landslide risk particularly high the next 24 hours. A special
weather statement is out to cover this elevated risk. Mercer

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 1 PM PDT this afternoon for
     Cascades of Pierce and Lewis Counties-Cascades of Snohomish
     and King Counties.

     Winter Storm Warning until midnight PDT tonight for Cascades of
     Whatcom and Skagit Counties.

PZ...Small Craft Advisory until 5 PM PDT Thursday for Admiralty Inlet-
     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-Northern Inland
     Waters Including The San Juan Islands-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory for rough bar until 11 AM PDT Thursday for
     Grays Harbor Bar.

     Gale Warning until 11 PM PDT this evening for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Gale Warning until 6 PM PDT this evening for Puget Sound and
     Hood Canal.


&&

$$

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NWS Seattle/Tacoma, WA (SEW) Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather