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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 212151
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
251 PM PDT Sun Jul 21 2019

.SYNOPSIS...Dry weather will continue into Monday with
temperatures slightly above normal. A cold front and upper level
trough will bring showers and a chance of thunderstorms in the
Cascades on Tuesday. Strong onshore flow will prevail on Wednesday
then weaken on Thursday. Mild weather will continue into the
weekend. 

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Temperatures have warmed
into the 80s across parts of western WA this afternoon. We may 
see a few low 90s too in the interior, away from the sound. The 
coast is cooler and in the mid 60s to 70s with light onshore flow.
There is a thin strip of stratus clouds along the coast which 
should expand and push inland tonight, clipping the south sound by
morning.  

Monday will be slightly cooler (compared to today) as an upper 
level low spins over the NE Pacific. Highs in the interior will
reach the lower 80s with 60s to 70s along the coast in NW onshore
flow. A weak disturbance may trigger showers or t-storms, but most
likely over eastern WA where dynamics are favored (per the NAM
solution). 

A cold front and upper trough will then cross the area Tuesday and
Tuesday night for a chance of showers. Thunderstorms are possible
in the Cascades where heating and instability are the greatest.
We'll be under post-frontal onshore flow through Wednesday with
morning clouds expected. Temperatures will return to normal. 33

.LONG TERM /THURSDAY THROUGH SUNDAY/...Weak, flat ridging will
bring dry weather as we move toward the end of the week. Onshore 
flow will prevail through the period. Weak upper level disturbances
may clip the area Saturday or Sunday but there's little to no 
moisture attached. Dry and quiet weather may continue into early 
next week across western WA. 33

&&

.AVIATION...Not a cloud in the sky over western Washington TAF sites 
this afternoon. Only clouds showing up on vis imagery are some 
stratus hugging the northern coast, well north of HQM. Clear skies 
will rule thru the evening with marine stratus building a bit 
stronger offshore and along the coast tonight. This layer of stratus 
will push inland overnight, with confidence not high on exactly how 
far inland. Dropped ceilings at HQM and OLM late this evening and 
early Mon morning, respectively. Kept remaining sites VFR. Should 
see some more cirrus and perhaps some mid level clouds Monday, but 
VFR should prevail for all by the afternoon. A stronger marine push 
overnight Mon into Tues will likely drop ceilings down sub-VFR from 
Sound to coast. Winds generally W/NW under 10kts through sunset 
dropping to light and variable overnight and picking back up W/SW 
Mon afternoon.

KSEA...Clear skies, some passing cirrus on Monday. MVFR stratus 
early Monday is expected to remain south of the terminal across 
southern portions of Puget Sound. W/NW wind 5-10 kts through sunset, 
becoming light and variable overnight and picking back up W/SW near 
10kts Monday afternoon.

Kovacik

&&

.MARINE...Afternoon sfc analysis still argues that weak offshore 
flow is in place across western Washington as an expansive area of 
high pressure remains in place from the northern Rockies into the 
southern Canadian Prairies and into the northern Great Plains while 
low pressure resides well offshore the Washington Coast. This has 
allowed for calm conditions over the waters today, aside from some 
wind waves of generally 1-3 feet. This pattern will become disrupted 
late this afternoon and evening as a thermal trough builds north 
into Washington from Nevada and the offshore low inches closer 
towards the coast. These features will cause the flow to turn back 
onshore and help develop a stronger marine layer across the offshore 
waters and along the coast, eventually pushing inland some. In 
addition, it will cause a westerly push down the Strait of Juan de 
Fuca this evening and overnight, keeping a small craft advisory in 
place for central and eastern portions 5pm this evening thru 5am 
Monday. Winds will come down briefly in the Strait Monday morning 
before a stronger westerly push sets in by the afternoon. This will 
lead to another round of small craft winds for the central and 
eastern Strait and eventually into the Admiralty and Northern Inland 
Waters early Tuesday morning. 

Onshore flow will continue for much of the week, varying in 
intensity. The potential for gale force winds will exist Tuesday 
night into Wednesday for the central and eastern Strait.

Kovacik

&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Monday for Central U.S. 
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters 
     Strait Of Juan De Fuca.

&&

$$

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