Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 291035
Area Forecast Discussion
National Weather Service Seattle WA
335 AM PDT Wed Mar 29 2017
.SYNOPSIS...A frontal system will move through Western Washington
today, bringing more rain and locally windy weather. An upper
trough will follow tonight, with decreasing showers and some sun
breaks Thursday. An upper ridge will move 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...The next frontal system is just offshore early this
morning. Its 1009 mb surface low is centered about 120 nm off the
Washington coast, with a warm front that extends southeast into
Western Oregon and a cold front that trails southwest. Aside from
parts of the north interior, precipitation is already widespread
across Western Washington ahead of the warm front. Southeast winds
are gusting to 35 to 35 mph on the north coast and in the north
interior. Temperatures were in the mid to upper 40s at 2 am.
The warm front will lift northeast across Western Washington
during the next few hours. The cold front will follow midday or
early afternoon, and the steady rain will turn to showers. The
surface low will track northeast across the southern part of
Vancouver Island today, bottoming out only around 1004 mb; however
models agree that strong southwest winds -- probably 40 to 50 kt
in the layer 2000 to 6000 ft aloft -- will produce a 1006 mb near
the northeast corner of the Olympic Peninsula. That should produce
breezy to windy conditions across much of the Western Washington
interior today, including the Seattle area.
Cool moist onshore flow will follow the front tonight, with an
upper trough moving through the forecast area late tonight.
Showers will fall mainly in along the west slopes of the
mountains and the Puget Sound convergence zone, with scattered
We can expect precipitation amounts of 1 to 2.5 inches during the
next 24 hours in the Cascades and Olympics, with up to an inch of
rain in the lowlands. The snow level is around 3000 to 4000 ft
early this morning; it will rise to 5000 to 6000 ft this morning
as the warm front moves through, then fall to around 3500 ft late
tonight. So most of the spots in the mountains that people can
reach by car could have snow early this morning, switching to rain
during the day and back to snow tonight. Mt Baker probably has
the best shot for precipitation to fall as snow throughout the
Northwest flow will take over on Thursday behind the upper trough.
Showers will decrease during the day and be pretty much confined
to the Cascades and maybe the weakening convergence zone by
afternoon, and most of the Western Washington lowlands should have
a partly sunny afternoon or at least some sun breaks. It will be
rather cool though with highs mainly 50 to 55 -- similar to today,
and the snow level will be near 3000 ft.
A positively tilted upper ridge will lean across the forecast area
Friday, and it should be a day of dry weather. However, models
seem to be trending toward slightly faster timing with the passage
of the ridge. And it looks rather with dirty, with some upper
cloud cover probably moving into the area ahead of an approaching
weak frontal system. In fact, models agree fairly well now that
that weak system will bring a chance of rain to the area by late
Friday night, and I`ve made that change to the forecast. McDonnal
.LONG TERM...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...An upper level trof offshore will approach the area
during the day today. Strong wly flow aloft will cont. A cdfnt will
move across the area this afternoon. Low level sly flow will become
wly behind the front. Fairly wdsprd MVFR CIGs/VSBYs will improve to
areas of MVFR CIGs this afternoon. The mtns will be obscd this
morning, and partially obscd this afternoon.
KSEA...CIGs will likely bounce between VFR and MVFR categories
thru this morning. Ely winds will become sly 10-15 kt by midday.
A cold front will move across the coastal waters this morning and
the interior waters this afternoon. Expect moderately strong onshore
or westerly flow to develop behind the front. Onshore flow will
persist on Thursday before weakening Thursday night.
.HYDROLOGY...Up to 4.5 inches of rain fell over the wetter
portions of the Olympic Peninsula during the past 24 hours, which
drove the Skokomish River above flood stage Wednesday evening. The
Skokomish near Potlatch appears to be near its crest early this
morning, though additional rain over the next few hours could
drive it a bit higher. It should fall slowly later today and
tonight, probably dropping below flood stage late tonight.
Other rivers will see rises, but no other river is forecast to
reach flood stage. Aside from the current flooding on the
Skokomish, flooding is not expected on our rivers during the next
The threat of landslides has increased somewhat due to rainfall
over the past 24 hours and expected through Wednesday. A special
weather statement is out to cover this elevated risk. McDonnal
WA...Winter Weather Advisory until 10 AM PDT this morning for
Cascades of Whatcom and Skagit Counties.
PZ...Small Craft Advisory until 6 PM PDT this evening for Admiralty
Inlet-Coastal Waters From Point Grenville To Cape
Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
To Cape Shoalwater Out 10 Nm-Puget Sound and Hood Canal-
West Entrance U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory for rough bar until 6 PM PDT this evening
for Grays Harbor Bar.
Small Craft Advisory from 8 AM this morning to 6 PM PDT this
evening for Coastal Waters From Cape Flattery To James
Island 10 To 60 Nm-Coastal Waters From James Island To
Point Grenville 10 To 60 Nm.
Gale Warning until 8 AM PDT this morning 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.
Small Craft Advisory for hazardous seas from 8 AM this morning
to 6 PM PDT this evening for Coastal Waters From Cape
Flattery To James Island Out 10 Nm-Coastal Waters From
James Island To Point Grenville Out 10 Nm.
Gale Watch from this evening through late tonight for Central
U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
Waters Strait Of Juan De Fuca.
Gale Warning until 9 AM PDT this morning for East Entrance U.S.
Waters Strait Of Juan De Fuca-Northern Inland Waters
Including The San Juan Islands.
Small Craft Advisory until 9 AM PDT this morning for Central
U.S. Waters Strait Of Juan De Fuca.
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