Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 262203
Area Forecast Discussion
National Weather Service Seattle WA
303 PM PDT Wed Apr 26 2017
.SYNOPSIS...Low pressure aloft will keep conditions unsettled
through Thursday. Expect drying on Friday as high pressure aloft
moves over the region. Another low pressure system will bring a
threat of precipitation to the area this weekend.
There were scattered showers, more numerous over the south half of
the CWA, this afternoon. Some of the showers were heavy, producing a
quarter to half inch of rain in an hour or less.
The primary Puget Sound Convergence Zone (PSCZ) appeared to be
located from roughly Leland to Arlington. A weaker convergence zone
was located over central Kitsap County across northern King County.
The primary PSCZ is expected to remain quasi-stationary through
tonight. It may slip as far south as northern King County, possibly
merging with the weaker convergence zone presently there. If this
occurs, there could be intense rainfall rates (at least an inch per
hour) within the merged convergence zones. Therefore, will need to
keep an eye on this. The PSCZ will weaken and shift north, back into
southern Snohomish County, by Thursday morning in response to an
approaching upper level trough.
Expect some increase in the shower activity Thursday afternoon due
to an upper level trough moving across the region. This will also
cause the PSCZ to redevelop over King County Thursday afternoon. The
PSCZ is anticipated to move into Pierce County late Thursday
afternoon or early Thursday evening before eventually dissipating.
The upper level trough will move into the intermountain west late
Thursday night or early Friday. An upper level ridge of high
pressure will follow in its wake. There will be some lingering
moisture and instability for a low threat of showers over mainly the
interior on Friday; although, the threat will become confined to
the Cascades in the afternoon.
High pressure aloft will prevail over the region Friday night
through Saturday morning for dry weather. Confidence in the
forecast for Saturday was not very high. The ECMWF and GFS solutions
have flip-flopped. The ECMWF solution was now faster than the GFS
in bringing precipitation back into the area. For now, decided to
stick with the previous forecast, which now reflects the GFS
Confidence in the forecast for this period was not high due to lack
of consistency in the medium range solutions. Expect a chance of
showers during the first part of the period. An upper level ridge
is anticipated to bring dry and warmer weather to the region the
middle of next week. Temperatures should be near or slightly above
normal by the end of the period.
.AVIATION...Moderate northwesterly flow aloft will continue
tonight, then weaken Thursday. At the surface, strong onshore
flow will continue tonight and moderate Thursday. The air mass is
moist and weakly unstable with scattered showers. Showers will
decrease later tonight. The Puget Sound convergence zone has
formed and will shuttle between KPAE and KSEA this evening. A
wind shift to the northwest will occur over north Puget Sound this
afternoon and could cause southwest winds at KBFI/KSEA to switch to
northwest for a few hours late in the afternoon and early
evening. Mostly VFR ceilings this evening except MVFR in showers.
Ceilings will fall to MVFR overnight.
KSEA...Ceilings are VFR 3-4k ft with lower scattered decks. Southwest
wind 10-15 KT will become northerly 5-10 KT around 00Z. CHB
.MARINE...Strong onshore flow will give gale force westerlies in
the strait tonight. Winds will ease late tonight but will generally
remain moderately onshore Thursday and Friday. Small craft advisories
are in effect over all waters, with gale warnings in the central and
east strait. Small craft advisory strength winds are likely to
continue over most waters Thursday into Friday. There is a chance
that westerlies could again increase to gale force in the strait
High pressure over the area will give light winds Saturday. A
front will reach the area Saturday night, with strong onshore flow
returning on Sunday. CHB
PZ...Gale Warning until 3 AM PDT Thursday 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.
An illustrated version of this forecast discussion can be seen at