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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 211056
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
356 AM PDT Sun Apr 21 2019

.SYNOPSIS...Weak upper level ridge over Western Washington today
for dry weather but low level onshore flow will produce some
morning clouds. A front will reach the area Monday afternoon and
evening, then stall over Western Washington as it weakens
Tuesday. Dry weather Wednesday and Thursday before a weak front 
arrives late in the week. 

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...Satellite imagery shows a
cloud cover over most of the area early this morning. The cloud
free areas are mostly along the Strait of Juan de Fuca and over
the Northwest Interior. Temperatures at 3 am/10z were in the 40s.

Stratus will continue to fill in over the interior from about 
Seattle southward early this morning with weak low level onshore 
flow. This layer is fairly shallow with the tops around 1500 feet.
Another stratus layer around 4000-5000 feet that was near the 
Cascades last night has been slowly back building into the 
interior early this morning. The combination of the two will give 
cloudy skies this morning all the way to the Cascade crest over 
the southern half of the area and along the North Cascade 
foothills. The low level onshore flow is light and the flat upper 
level ridge over the area will strengthen a touch this afternoon 
as the next system approaches the area. This will help dissipate 
the stratus later this morning. Skies will still be mostly cloudy 
over the Northwest portion of the area this afternoon as the mid 
and high level cloud cover out ahead of the next system reaches 
Western Washington. Highs today will be in the mid 50s to mid 60s.

Cloud cover increasing overnight as the next system slowly moves
toward Western Washington. By early Monday morning the leading
edge of the precipitation will be close enough for a chance of
showers along the North Coast. The increasing cloud cover will 
keep lows in the 40s and lower 50s Monday morning.

Front continuing to drift southeast on Monday with the front near
the coast by late in the day. The front will be running into a
weak upper level ridge ( 500 mb heights over the area around 570 
dms ). This will help weaken the front as it arrives. Wet day
along the North Coast with shower chances increasing for the
remainder of the area as the day wears on. Highs will only be
in the 50s.

Front moving inland and stalling Monday evening. This looks to be
the time for the most widespread coverage of the shower activity.
Precipitation amounts will not be much, in the 0.10 to 0.25 inch 
range for the most part. Lows Tuesday morning in the mid 40s to
lower 50s. 

Tuesday looking like a gray and drippy day for Western Washington
with the front dissipating over the area and onshore flow at the
lower levels. Highs for the most part in the 50s. 

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...Extended models in good
agreement with the last gasp showers associated with the
dissipating front ending Tuesday evening. Surface ridging offshore
on Wednesday remaining through Thursday giving the area a couple
of dry days. Highs on Wednesday in the mid 50s to mid 60s. Big
temperature spread in the guidance on Thursday with the GFS going
for 71 in Seattle the the ECMWF 58. The GFS does turn the flow 
aloft southwesterly ahead of the next approaching system on 
Thursday but the low level flow remains onshore. On the flip side 
not much low level moisture around so with some sunshine would not
expect the highs to be below normal like the ECMWF guidance. Will
go for highs just a little above normal on Thursday, mostly in 
the lower to mid 60s. 

Model consistency goes out the window on Friday with the GFS
bringing a system up from the southwest while the ECMWF has a
system dropping down out of the northwest staying north of the
area. With so much variability in the solutions will go close to 
climatology and keep the slight chance pops in the forecast. 

Confidence in the models not very high for next weekend. Both
the GFS and the ECMWF on the 00z run have a cool trough moving 
into Western Washington Saturday for at least a chance of showers.  
The GFS is the warmer of the two but still has snow levels
dropping to below the passes. For now will trend the snow levels a
little lower but keep them a few hundred feet above the models
solutions, around 3000 feet. Felton

&&

AVIATION...Recent satellite imagery /3AM/ shows an area of lower 
clouds/stratus gradually expanding from near the central coast and
also along the Cascade foothills. Ceilings will be tricky going 
into the early morning hours but think all TAF sites near the 
Sound have the potential to drop to MVFR and even IFR this 
morning. Patchy fog is possible too, but think that may hang 
mostly around PWT. Given continued ridge influence in the upper 
levels, although low level moisture will likely remain trapped 
underneath, should see at least some scattering this afternoon 
with improvement back to VFR. With the next frontal system 
approaching for Monday, higher level clouds will also be on the 
increase later today. Winds generally south at less than 10kts.

KSEA...MVFR ceilings likely this morning improving back to VFR this 
afternoon. Winds from the south at 10kts or less.

Kovacik 

&&

.MARINE...Onshore flow will persist through the day today as a broad 
area of high pressure remains in place across the Pacific. The 
highest winds will remain confined to the central and eastern 
portions of the Strait of Juan de Fuca, where the small craft 
advisory is in effect. Observations at Race Rocks this morning do 
show sustained winds from the west between 21-24kts. Guidance does 
indicate the potential for the gradient to strengthen across the 
central Strait this afternoon and evening so for this morning 
forecast package, have opted to extend the advisory through 06z 
Monday. Elsewhere winds should remain fairly light at under 10kts.

The ridge across the Pacific will weaken on Monday as a frontal 
system approaches the area. Southerly winds across the coastal 
waters should increase and reach small craft through the day out 
ahead of the boundary. Winds across the interior waters /except the 
central Strait/ will also see an increase in wind speed towards 
20kts. At this time, no headlines have been issued for the interior 
waters but will need to be monitored for small craft potential.

No significant wave hazards are expected through Monday, though wave 
height will increase ahead of Monday's frontal boundary.

Kovacik

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 11 AM to 5 PM PDT Monday for Coastal 
     Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater Out 
     10 Nm.

     Small Craft Advisory from 5 AM to 5 PM PDT Monday for Coastal 
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To 
     60 Nm-Coastal Waters From James Island To Point Grenville 
     Out 10 Nm.

     Small Craft Advisory until 8 AM PDT this morning for East 
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 11 PM PDT this evening for Central 
     U.S. Waters Strait Of Juan De Fuca.

&&

$$

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