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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 231620
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
920 AM PDT Tue Apr 23 2019

.UPDATE /TODAY/...Onshore flow will give mostly cloudy conditions
to the area today. Radars show some very light showers near the
western slopes of the Cascades this morning. These showers are
expected to increase somewhat this afternoon and evening as the
moist boundary layer is warmed as as inflow from the Pacific
converges around the terrain of Vancouver Island, the Olympic
Peninsula, and the Chehalis Gap and gets lifted up the Cascades.
Minor updates were made to today's forecast to refine cloud 
cover, shower chances, and precipitation amounts toward latest 
mesoscale model solutions. Otherwise, forecasts that show a
decrease in showers and clouds tonight and dry conditions
Wednesday through Thursday are on track. Albrecht

&&

.SYNOPSIS...Scattered showers will taper today as an upper trough
shifts inland. High pressure aloft will bring dry weather
Wednesday and Thursday. A weak upper trough will bring a slight
chance of showers on Friday. A stronger and wetter trough will
move through on Saturday followed by mainly dry weather Sunday and
Monday. 

&&

.AVIATION...A few scattered showers may linger today with a 
weak convergence zone potentially developing between the PAE to 
BLI area. Ceilings are low this morning, mostly in the MVFR 
category with some spotty IFR. Gradual improvement is expected 
this afternoon back to VFR. Ceilings could drop back to MVFR 
tonight but it does seem likely there will be more clearing come 
Wednesday. Winds from the S/SW in the 10-15kt range with a few 
gusts near 20kt possible. Winds then turn to the E/NE for terminals
near the South Sound tonight.

KSEA...MVFR cigs this morning with spotty periods of IFR possible. 
Cigs are expected to lift back to VFR this afternoon. Southerly
winds 10-15 knots, becoming more east-northeast tonight. 

Borth/Kovacik

&&

.MARINE...Onshore flow will increase across the coastal waters
today, followed by a rather strong push of westerly flow down the
Strait of Juan de Fuca. Have upgraded the Gale Watch starting at
18z to a Gale Warning for the central and eastern Strait of Juan 
de Fuca. Winds will likely bleed in to nearby Admiralty Inlet and 
Northern Inland Waters, so have extended the small craft craft
advisory to include those areas. Winds are expected to die down 
in the Strait on Wednesday but will need to watch Northern Inland
Waters and Coastal Waters for another round of SCA wind.

Borth/Kovacik

&&

.PREV DISCUSSION... /issued 237 AM PDT Tue Apr 23 2019/ 

SYNOPSIS...Scattered showers will taper today as an upper trough
shifts inland. High pressure aloft will bring dry weather
Wednesday and Thursday. A weak upper trough will bring a slight
chance of showers on Friday. A stronger and wetter trough will
move through on Saturday followed by mainly dry weather Sunday and
Monday. 

SHORT TERM /TODAY THROUGH THURSDAY/...A broad upper trough will
maintain a chance of showers today - mainly coast, mountains and 
in a loosely defined Puget Sound Convergence Zone this afternoon 
and evening. Highs will be near normal. 

An upper ridge will build over the area Wednesday and Thursday for
dry weather with highs warming a couple degrees each day.
Wednesday should be mostly sunny after areas of morning low 
clouds burn off. Mid and high clouds will be increasing again on 
Thursday as the next upper trough approaches. Schneider

LONG TERM /FRIDAY THROUGH MONDAY/...A weak upper trough will
cross the area on Friday. An isolated shower is possible but
mostly it looks like another dry day. A stronger upper trough will
drop down from the northwest on Saturday for a good chance of
showers and low snow levels, at least for late April. 

The Saturday upper trough will shift inland on Sunday with shower
chances ending. Another weak upper trough will develop over the 
area on Monday for a slight chance of showers again. Temperatures 
will be seasonable through the period. Schneider

AVIATION...A weak frontal boundary will continue to make eastward 
progress across the state this morning. This front was responsible 
for shower activity earlier in the day, much of which has ended. A 
few scattered showers may linger today with a potential convergence 
zone developing mainly in the PAE to BLI area. Ceilings are low this 
morning, mostly in the MVFR category with some spotty IFR. These 
ceilings will carry thru sunrise where then gradual improvement is 
expected thru the afternoon back to VFR. Ceilings could drop back to 
MVFR tonight but it does seem likely there will be more clearing 
come Wednesday. Winds from the S/SW in the 10-15kt range with a few 
gusts near 20kt possible. Winds then turn to the E/NE for at least 
terminals near the South Sound tonight.

KSEA...MVFR cigs this morning with spotty periods of IFR possible. 
Cig lift back to VFR this afternoon. Spotty showers linger in the 
vicinity. Winds S 10-15 with a few gusts before turning NE tonight. 

Kovacik

MARINE...A weak cold frontal boundary will continue to push through 
Washington State this morning. Southerly wind out ahead of the 
airmass boundary will shift to the W/NW in the front's wake. 
Overall, light gradients will yield fairly tranquil winds across all 
waters through sunrise.

During the morning hours following sunrise today, winds out the W/NW 
behind the front will increase across the coastal waters, followed 
eventually by a rather strong push of westerly flow down the Strait 
of Juan de Fuca. Have issued a small craft advisory for the central 
and eastern portions of the Strait beginning 18z. Winds will likely 
bleed in to nearby Admiralty Inlet and Northern Inland Waters. Have 
opted to go SCA for the Inlet but omitted headlines in Northern 
Inland Waters, as majority of these waters should remain well below 
21kts.

By this evening, guidance suggests gradients increase substantially 
within the Strait to the 3.6-3.9 mb range with continued strong 
westerly flow. Nonetheless, even the most aggressive guidance 
suggest winds only in the upper 20 kt range. Despite this 
discrepancy, gales could become possible across the central and 
eastern Strait tonight so have issued a Gale Watch and will let day 
shift make the call. Winds finally die down in Strait for Wednesday 
but will need to watch Northern Inland Waters and Coastal Waters for 
another round of SCA wind.

Kovacik

HYDROLOGY...River flooding is not expected for the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning until 3 AM PDT Wednesday for Central U.S. Waters 
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of 
     Juan De Fuca.

     Small Craft Advisory from 3 PM this afternoon to 3 AM PDT 
     Wednesday for Northern Inland Waters Including The San Juan 
     Islands.

     Small Craft Advisory until 5 AM PDT Wednesday for Admiralty 
     Inlet.

&&

$$

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