Small Craft Advisory 87, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
930 AM PDT Wed Apr 12 2017

.SYNOPSIS...A weather system was moving through Western Washington
this morning. Another disturbance will move through the area tonight
and Thursday morning. Cool showery weather will continue through
Friday. Showers will gradually diminish Friday night and Saturday.
Sunday will be mostly sunny and warmer, before wet cool weather
returns around Monday and continues through much of next week.

&&

.SHORT TERM...Areas of rain will change to scattered showers and
breezy southerly winds will develop today. Showers will increase
tonight. What amounts to a cold front along the Oregon coast this
morning will move inland over Oregon later today, and the northern
portion of that--tilted back over Western Washington and extending
out across Vancouver Island--will push northeast tonight. Or you
could say that the disturbance is another shortwave trough rotating
about the upper low. But as a front, it shows up pretty well at
700mb where temps will cool from around -8c today to around -12c on
Thursday. 500mb vorticity shows these disturbances fine too--there
are five in all through the end of the week and the showers will wax
and wane with each. I think there will be only a few sunbreaks today-
-but on Thursday, with a cooler and more unstable air mass arriving,
there ought to be more sun between the showers.

Looking at 850mb temps and winds near the Cascades--the current snow
level forecast in the Cascades might be too low for this afternoon
and evening. The flow at 850mb backs from SW to SE and 850mb temps
warm a notch through this evening before dropping overnight as the
cold front moves through. The snow level might be closer to 6000'
this afternoon and evening, then fall to around 3000' Thurs morning.

.LONG TERM...From the 4am AFD...All forecast models eject the
remains of the offshore low northeastward across northwestern
Washington or southern British Columbia on Saturday and keep a
chance of showers across the area. Showers will likely come to an
end by sunset on Saturday.

Easter Sunday is looking rather nice across the region as a short
wave ridge develops and shifts northeast in between the departing
upper low and a new one that is developing off the northern
California coast. In fact, Easter Sunday is looking a lot like a
warmer version of yesterday (Tuesday) with sunny skies, offshore
north or northeast flow, and highs in the lower 60s in many
locations.

The nice April weather will come to an end Sunday night or Monday
as the upper low off northern California lifts northward into
western Washington. Following that low, a persistent upper low is
expected by all model guidance to develop offshore with difficult
to time short wave troughs rotating into the area for much of
next week (a return to cool and showery weather like we have seen
much of this spring). Albrecht

&&

.AVIATION FROM THE PREV DISCUSSION...The weak system moving into
the area this morning from the southwest has spread lower ceilings
and rain to the area overnight. Low level flow will transition to
southerly over the few hours and become weakly onshore this
afternoon. Ceilings have remained VFR despite the onset of rain from
KPAE southward. Expect ceilings to dip to MVFR levels as rain
continue to move into the area from the southwest this morning and
and improve this afternoon. The airmass will remain moist and
somewhat unstable today, with gusty southerly winds.

KSEA...Previous discussion: VFR conditions will transition to MVFR
conditions with the main band of precipitation over the next few
hours then improve this afternoon. Norther easterly winds 4 to 3 kt
will give way to gusty south winds late morning and into the
afternoon.

&&

.MARINE FROM THE PREV DISCUSSION...The surface low that has been
working north through the Oregon coastal waters overnight will move
into the Washington waters this morning. Expect northerly and
offshore flow to become southerly as the low works north along the
Washington coast with widespread small craft advisory conditions
most waters today. Southerly flow will persist Thursday becoming
southwesterly and weakly onshore Thursday evening into Friday.
Showers will remain over area waters through at least Friday. &&

.HYDROLOGY...river flooding is not expected over the next week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 11 AM PDT Thursday 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 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 until 5 AM PDT Thursday for Coastal Waters
     From James Island To Point Grenville 10 To 60 Nm.

     Small Craft Advisory until 6 PM PDT this evening for Admiralty
     Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands-Puget
     Sound and Hood Canal.

     Small Craft Advisory until 5 PM PDT this afternoon for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html