Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 232135
Area Forecast Discussion
National Weather Service Seattle WA
235 PM PDT Sun Apr 23 2017
.SYNOPSIS...Showers associated with an upper level disturbance
offshore will lift north through the area this evening. The upper
level low will move east across the area on Monday. Another front
will move across the region later Tuesday through Tuesday night.
Then an upper trough and low level onshore flow will give showers
to the area Wednesday through Thursday night. A ridge of high
pressure may give a period of drier weather to the area Friday
into Saturday. Temperatures will generally be near or slightly
below normal for late April.
.SHORT TERM...Radars show rain to the east of an upper low over
the Washington offshore waters spreading onto the Olympic
Peninsula early this afternoon, while precipitation is lighter and
more showery in the interior north of Seattle as we get some
downslope flow to the northeast of the Olympics. The showers will
continue to lift northward and fill in a bit later this afternoon
Right on the heels of the first disturbance moving into the
Pacific Northwest is a short wave trough centered along about
138W. A developing low centered near 45N 138W will race eastward
being steered by the upper level low to the northeast and a rather
strong jet along 40-45N that is aimed toward the south Oregon
coast. Models increasingly keep the steady rain associated with
this low along the southern edges of our forecast area while
places to the north see a more showery pattern with the upper low
that moves eastward in the wake of the surface feature and the jet
streak. Hence, highest POPS will be to the south, and areas from
about Seattle or so northward will see some partly sunny
conditions during the afternoon hours on Monday. Showers will
taper off Monday night as the upper low departs to the east and
the boundary layer stabilizes with nocturnal cooling.
On Tuesday, a negatively tilted frontal system, a warm occlusion,
will approach the area from the southwest spreading increasing
high clouds into the area. It appears that rain ahead of this
front may wait until the afternoon hours and precipitation may not
reach the interior until late in the afternoon. Will reduce POPS
to the chance category in the interior as a result and will word
the forecasts to say "mainly in the afternoon" in the interior
from Seattle northward.
Tuesday night and Wednesday will be cool and showery with onshore
flow as a broad low amplitude ridge develops well to our west and
troughing develops east of the Cascades.
Daytime temperatures will average a bit below normal for late
April through the period. Albrecht
.LONG TERM...Models are in general agreement Wednesday night and
Thursday in showing onshore flow and showery conditions
continuing. On Thursday night and Friday it appears that showers
will diminish as the onshore flow relaxes with the long-wave offshore
ridge pushing eastward a bit.
The ECMWF, GFS and a majority of their ensemble members generally
agree that the period Friday night and Saturday stands the highest
probability of being dry with temperatures finally get to, or
perhaps rising a bit above, the normal value for the tail end of
April. But inconsistencies start arising Saturday night as the
long wave ridge offshore temporarily retrogrades. The ECMWF and
GFS both show a short wave trough driving southeastward into the
area on the front side of the ridge by Sunday. Handing of this
short wave trough and others that may or may not follow is
becoming increasingly inconsistent in all model solutions.
.AVIATION...An upper level trough is swinging NE across W WA
this afternoon and should be over B.C. by 06Z. Rain has been
widespread across the coast and from Puget Sound southward this
afternoon and is finally spreading over the N interior. Rain will
continue through this evening with scattered showers overnight.
The precipitation will moisten the lower air mass so that MVFR and
low end VFR CIGS will predominate.
The trailing upper level low, still offshore, will approach the
coast tonight then move inland 18Z-00Z on Monday. This will keep
the air mass moist with scattered showers and MVFR or low end VFR
KSEA...Rain from an upper level trough crossing the area and
scattered showers tonight will keep the lower air mass moist. CIGS
are expected to remain in the BKN-OVC025-050 range. Surface winds
will remain southerly 5-12 KT. Kam
.MARINE...A weak surface ridge will across W WA tonight. Pressure
gradients will remain weak across the area tonight keeping winds
below small craft advisory levels.
A 1003 MB surface low offshore will approach the region tonight
then slow and weaken near the N Oregon coast Monday morning before
dissipating as it moves inland Monday afternoon. A weak surface
ridge following the low will increase the onshore flow Monday
evening with SCA W winds expected in the central and E Strait of
Juan de Fuca.
An occluded front will approach the coast Tuesday before moving
inland Tuesday evening. Onshore flow will develop behind the front
Tuesday night with SCA W winds expected in the central and E
strait. A surface ridge building offshore will increase the
onshore flow Wednesday night and Thursday. The UWWRF model kept
winds just below gale levels but westerly gales in the strait are
definitely possible during that period. Kam
PZ...Small craft advisory East Entrance and Admiralty Inlet until
5 PM this afternoon.
You can see an illustrated version of this discussion at