Scientific Forecaster Discussion
fxus66 ksew 250421
Area forecast discussion
National Weather Service Seattle Washington
921 PM PDT Fri Mar 24 2017
Synopsis...showers tonight will slowly decrease on Saturday as an
upper level trough over western Washington moves off to the east.
After a brief dry spell Saturday night into early Sunday morning,
another system will bring rain to the area on Sunday. Showers will
follow the system into Monday. Another wet frontal system will
bring rain to the area Tuesday and Wednesday. Drier weather may
return later next week.
Short term...somewhat typical early Spring weather -maybe a bit
wetter and cooler than normal- is expected over western
Washington over the next few days.
Radars show abundant showers in the interior this evening moving
off to the northeast. Another area of showers from the Washington
coast westward is associated with the main body of the trough. The
trough will push inland overnight then off to the east early
Saturday. Area soundings early this evening show a marginally
unstable air mass with "skinny" cape values generally less than
200 j/kg; Lis across the area are just over zero. With the loss of
insoltion, expect the boundary layer to stabilize somewhat, so
the threat of thunderstorms across western Washington this evening
has pretty much come to an end. An update was made to take
thunder out of the forecast this evening
An eastward moving short wave ridge will give decreasing onshore
flow and gradual drying on Saturday, then a short dry period later
Saturday evening through early Sunday morning. Showers on Saturday
will increasingly become tied to the west slopes of the terrain
and a convergence zone that will likely stay north of King County
in the interior. Highs on Saturday will be in the lower to mid
The next system will move into the area later Sunday with its rain
shield moving in Sunday morning. Clouds and precipitation will
hold temperatures to round 50 on Sunday.
Post frontal showers are expected Sunday night and Monday. Breaks
in the clouds will allow temperatures to warm back into the mid
50s in most locations. Albrecht
Long term...from the previous long term discussion: Tuesday
earlier had appeared to be at least somewhat dry, but now both
Tuesday and Wednesday have wet weather systems. This is true of
both the Euro and GFS. Thursday will be showery as an upper trough
passes. Friday has a chance of being a dry day, though the GFS is
trying to bring another system in late in the day. Temperatures
will be around normal all four days. Burke
Aviation...an upper level trof of low pres will remain over the
region overnight for wly flow aloft. Low level sly flow will grdlly
diminish overnight. Expect areas of MVFR cigs overnight, with areas
of MVFR vsbys in heavier showers. The air mass was weakly stable.
Ksea...expect VFR conditions with ocnl MVFR cigs thru Saturday
morning. Sly winds will continue to slowly diminish overnight.
southerly flow will weaken overnight as weak high pressure builds
along the Washington coast. A 1021 mb high over far southwest Washington will result
in light onshore flow on Saturday. The next frontal system will move
onshore late Sunday, followed by another one on Tuesday. A stronger
system is possible on Wednesday.
Hydrology...river flooding is unlikely over the next 7 days.
Pz...Small Craft Advisory until 6 am PDT Saturday 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-
coastal waters from Point Grenville to Cape Shoalwater 10
to 60 nm.
Small Craft Advisory for hazardous seas from 6 am to 3 PM PDT
Saturday 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-coastal waters from Point
Grenville to Cape Shoalwater 10 to 60 nm.
Small Craft Advisory for rough bar until 6 am PDT Saturday for
Grays Harbor bar.
Small Craft Advisory until midnight PDT tonight for Admiralty
Inlet-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 out 10 nm-East Entrance U.S. Waters Strait of
Juan de Fuca-northern inland waters including the San Juan
Islands-Puget Sound and Hood Canal-West Entrance U.S.
Waters Strait of Juan de Fuca.
Small Craft Advisory for hazardous seas from midnight tonight to
3 PM PDT Saturday for 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 out 10 nm.