National Weather Service National Headquarters

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

-- Discussion containing changed information from previous version are highlighted. --
952
FXUS66 KSEW 182129
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
230 PM PDT Sun Apr 18 2021

.SYNOPSIS...Onshore flow will trigger a gradual cooling trend
starting Monday. As an upper level ridge remains in place, dry
conditions will persist although systems passing near the area will
allow for an increase in clouds over the first half of the week. A
pattern change is still expected for the end of the week with a
return to cooler and wetter conditions.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Clear skies remain in
place over much of W WA this afternoon. Temps around the area not
deviating too much from where they were 24 hours ago with some
locations running a degree or two cooler, others a degree or two
warmer. Coastal locations proving to be an exception though running
anywhere from 9 to 18 degrees cooler thanks to the change to onshore
flow. Not seeing much of an eastward push just yet, but that will be
discussed in a bit.

Upper level ridging remains the main headline for the short term
period, keeping dry conditions in place. The aforementioned
transition to onshore flow at the surface will result in a more
widespread impact on temperatures Monday as temps fall to around 60
along the coast and into the upper 60s to around 70 for the interior
lowlands.

A shortwave trough evolves into a small closed low Monday night into
Tuesday but is not expected to make much of an impact to W WA given
both its sharp SE trajectory taking it south of the area as well as
the fact there is very little moisture associated with it. The
largest expected impact will be an increase in cloud cover over the
western half of the CWA into Tuesday morning. Upper level ridging
rebuilds over the nearshore coastal waters, clearing these clouds
out Tuesday afternoon although the gradual cooling will remain in
place as temps will be be a degree or two cooler than Monday. This
ridge stays in place Wednesday, however deterministic models show an
interesting feature...although the details differ. The GFS points to
a weak upper level low coming down the Pacific ridge just off the WA
coast but no real moisture with it while the ECMWF shows a similar
feature but this passing to the north of the area with potential
impacts east of the Cascades. Ensembles register a bit of a blip to
represent the prospects of these solutions however, proving them to
be more outliers and not really warranting inclusion in the forecast
at this time.  18

.LONG TERM /THURSDAY THROUGH SATURDAY/...Models seem to have backed
off on the timing for the general pattern change over W WA a little
bit. Deterministic models hinted at this 24 hours ago and ensembles
look to be following that trend for now, keeping upper level riding
in place for Thursday. The ridge finally starts to break down
Thursday evening as a system moving south over the ridge may bring
some showers to the northern two-thirds of the Cascades. The ridge
continues to diminish and move eastward throughout the day Friday,
setting W WA up for an upper level low to bring rain back to the
area for the upcoming weekend.

Temps during this time will continue to cool, with mid 60s expected
for the interior lowlands...upper 50s along the coast...for both
Thursday and Friday and further cooling on Saturday...mid 50s along
the coast, only around 60 for the interior lowlands.  18

&&

.AVIATION...Increasing northwesterly flow this afternoon, persisting
through the period. VFR through the period at all TAF sites, with
the exception of KHQM where MVFR to IFR stratus will build in over
the terminal later this evening through Monday morning. CIGs
generally between 500 and 1000 feet here in this time period.
Prevailing winds generally AOB 10 kts at all sites during the day
Monday.


KSEA...VFR through the period. Northwesterly 4 to 7 kts through the
remainder of the afternoon, becoming northerly this evening. Winds
will become southeasterly between 12 and 15Z Monday morning, with
speeds remaining below 10 kts.

Pullin

&&

.MARINE...With the exception of spotty advisory level winds across
the Northern Inland waters and in the west entrance and adjacent
near-shore waters this evening through Monday morning, quiet weather
and calm seas will prevail through much of the week. Predominantly
offshore flow will transition to onshore late Monday into Tuesday
and will persist through the end of the period.

Pullin

&&

.HYDROLOGY...No river flooding expected over the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 11 PM this evening to 11 AM PDT Monday
     for Coastal Waters From Cape Flattery To James Island Out
     10 Nm-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 8 PM this evening to 11 AM PDT Monday
     for Northern Inland Waters Including The San Juan Islands.

&&

$$

www.weather.gov/seattle