Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 271629
Area Forecast Discussion
National Weather Service Seattle WA
929 AM PDT Thu Apr 27 2017
.SYNOPSIS...A weak disturbance will move across the area this
afternoon for an increase in shower activity. The disturbance will
move east this evening, with high pressure following in its wake.
High pressure will prevail Friday into early Saturday. Another low
pressure system will bring a threat of precipitation to the region
There were a few showers over the CWA at this time. Also, some areas
were enjoying sunny skies at this time. Expect the sunny areas to
fill in some with clouds this morning in response to surface
Progs concur on one more embedded shortwave in the flow to swing
onshore today to enhance showers, particularly as daytime heating
kicks in this afternoon. Yet, not as many showers as on Wednesday
with sun breaks mixed in. Low level flow favors another Puget Sound
Convergence Zone to develop that should enhance the showers in the
central Sound into the Cascades as well.
The broad upper ridge should continue to build and provide a drying
trend tonight into Friday with more sunshine developing. High
temperatures should approach late April averages.
The next question is will the drier weather hang on into Saturday.
Again, guidance offers differing solutions in timing of the next
system. The Canadian and GFS bring some light rain to the NW
quadrant of Western Washington Saturday afternoon while the ECMWF
rushes the rain further inland. Will stick with the slower solution
given the higher amplitude upper ridge that develops Friday.
.LONG TERM...The next system swings onshore Saturday night with some
light rain turning to scattered showers for Sunday with most showers
in the mountains thanks to orographic flow. The showers should taper
off Sunday night as an upper ridge builds offshore.
Again, guidance is not consistent heading into early May. Ridging
aloft is anticipated. Yet with the warm advection, will there be any
accompanying light rain Monday into Tuesday despite the building
heights. In collaboration with NWS Portland and EC Vancouver, will
adjust the forecast to offer higher level clouds, a small threat of
light rain, and warming temperatures. By midweek, the upper ridge
should be more firmly in place for sunshine and perhaps the warmest
temperatures of the year thus far. Have those sunglasses handy.
.AVIATION...Moderate northwest flow aloft will weaken tonight and
become more northerly. Low level onshore flow will increase this
afternoon and evening. The air mass is moist and weakly unstable
with scattered showers. Most terminals have VFR ceilings but they
are MVFR around central Puget Sound. Some clear areas are apparent
on satellite imagery; these will likely fill in with a VFR cumulus
field by afternoon. VFR all areas this evening, then local MVFR
ceilings will develop overnight.
KSEA...Discussion above applies. Southwesterly wind 10-14 kt
switching to northerly around 22-24z. Winds should switch back to
southwesterly by late evening. Puget Sound convergence zone may
produce showers at the terminal later today and this evening. CHB
.MARINE...Onshore flow will increase this afternoon and evening.
Gale force winds are forecast in the strait and the warning is
already in effect. There is a small craft advisory in effect
everywhere else except Puget Sound. A PSCZ will probably switch
the winds in Puget Sound from southerly to northerly this
afternoon, at least over the northern part.
High pressure over the area will give light winds Friday into
Saturday morning. A front will bring increasing winds to the coastal
waters Saturday afternoon and to to the inland waters Saturday
night. Strong onshore flow Sunday will ease Monday. CHB
PZ...Gale Warning until midnight PDT tonight for Central U.S. Waters
Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
Juan De Fuca.
Small Craft Advisories are in effect for the remaining waters
except the Puget Sound/Hood Canal.
An illustrated version of this forecast discussion can be seen at