Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 121702
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
1002 AM PDT Thu Apr 12 2018

.SYNOPSIS...Low pressure aloft currently over the region will
continue moving east. A frontal system will bring another round of
wet and locally windy weather to the area on Friday. This front
will exit the area Saturday morning. Expect cool and unsettled
conditions Sunday and Monday.

&&

.SHORT TERM...
There were scattered showers, more numerous over the southwest
half of the CWA, present this morning. There was also some
sunshine over the areas where there were breaks in the cloud
cover.

The upper trough axis will move over eastern WA late this afternoon.
Meanwhile a flat upper ridge will approach the Pacific Northwest
from offshore and move over the CWA this evening. Most of the area
will have showers at some point during the day, but precipitation
amounts generally will not amount to much. Overall, expect showers
with some sun breaks today. Highs will be in the upper 40s to mid
50s today or several degrees below normal.

Any break in precipitation from the upper ridge moving inland this
evening will be short lived, as a warm front immediately upstream
of it will spread steady precipitation inland late tonight and
Friday morning. Fairly significant precipitation amounts should
continue Friday and Friday night, as the frontal system will be
associated with the leading edge of a 140+ kt southwesterly jet
extending from well offshore into the Pacific Northwest. The
trailing cold front will drag across the area Saturday and
probably into Saturday night, gradually shifting southward. Models
have been somewhat inconsistent with regard to the cold front,
but the general model trend suggests it will be somewhat less wet
than previously thought.

Precipitation from this system will be heaviest over the Olympics.
We are forecasting generally 2.5 to 5 inches there during the
36-hour period from 11 pm tonight through 11 am Saturday, with
the snow level rising to around 4500 ft. The Cascades should
receive roughly 1.5 to 3 inches during that period, also with the
snow level rising to around 4500 ft; that should produce heavy
snow amounts at Paradise, and a winter storm watch will be issued
for the Cascades of Pierce and Lewis Counties. The interior
lowlands should receive comparatively modest rainfall amounts,
with the greater Seattle area receiving only around 0.50 inch.

The system will produce breezy to windy conditions as well on
Friday and Friday night. As is typical with a strong southeast
pressure gradient, the strongest winds will be at the coast and in
the north interior; winds in those areas could reach advisory
speeds of sustained 35 mph with gusts up to 50 mph. McDonnal/05

.LONG TERM FROM THE PREV DISCUSSION...
A fairly deep upper trough will move into the offshore waters on
Sunday and shift inland on Monday, for a period of cool and
showery weather. The snow level should be around 4000 ft on Sunday
and 3000 ft on Monday. The trough should move downstream Monday
night, with a minor upper ridge moving across the forecast area on
Tuesday. The ridge should move downstream by Wednesday, leaving
Western Washington in a region of weak flow as an upper low moves
into the offshore waters of northern California. While showers
can`t be ruled out on Tuesday or Wednesday, those two days look
like the best shot at "dryish" weather. McDonnal

&&

.AVIATION...An upper trough is moving inland and a weak upper
ridge will build over the area this evening. The air is moist and
unstable. At 8am there were showers along the coast and south of
Olympia and those will shift inland with areas of showers this
afternoon and evening. Rain will develop overnight tonight on the
coast, spreading inland by daybreak Friday as a new frontal system
arrives.

KSEA...Typical lower morning cigs will lift up, but also an area of
showers is moving into the area. Another frontal system arrives by
daybreak Friday with more rain.

&&

.MARINE...A trough along the Oregon and south Washington coast
will move ashore with increasing onshore flow this afternoon for
small craft advisory strength winds. Also ocean swell is in the 10-
14 ft range. Another vigorous frontal system will arrive Friday and
gales are possible over the Coastal Waters, with a good 20-30kt SSE
wind for most of the inland waters. The flow will ease on Saturday.
A cold front will move inland on Sunday, mostly to the south over
Oregon.

&&

.HYDROLOGY...
Additional rainfall during the next 24-48 hours will probably
cause the Skokomish River to rise above flood stage early
Saturday. The current forecast has the river rising above flood
stage early Saturday morning and then cresting at 17.5 feet
Saturday afternoon. Otherwise, river flooding is not expected the
next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from Friday afternoon through Saturday afternoon for
 the Skokomish River in Mason County.

     Winter Storm Watch from Friday morning through late Friday night
 for the Cascades in Pierce and Lewis Counties above 4500 feet.

PZ...Small Craft Advisory until 5 PM PDT Friday for 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 for rough bar until 5 PM PDT Friday for
     Grays Harbor Bar.

     Small Craft Advisory from 7 PM this evening to 5 PM PDT Friday
     for Coastal Waters From Cape Flattery To James Island 10 To
     60 Nm-Coastal Waters From Cape Flattery To James Island Out
     10 Nm.

     Small Craft Advisory for hazardous seas until 7 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.

     Small Craft Advisory from 5 PM this afternoon to 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.

     Small Craft Advisory from 5 PM this afternoon to 5 PM PDT Friday
     for Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory for hazardous seas until 5 AM PDT Friday
     for West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 1 PM this afternoon to 11 PM PDT
     Friday for Puget Sound and Hood Canal.

&&

$$

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