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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
156 
FXUS66 KSEW 240326
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
826 PM PDT Wed May 23 2018

.SYNOPSIS...Onshore flow will prevail through the middle of next
week giving morning clouds and at least partial afternoon 
sunshine most days. Scattered thunderstorms along the Cascades
this evening will come to an end after sunset. 

&&

.SHORT TERM...Instability gave scattered thunderstorms to the
Cascades this afternoon and an isolated shower to the northern
Olympics. These showers and thunderstorms are dissipating this
evening with the loss in insolation and as weak upper level low
over central portions of Eastern Washington moves off to the
northeast. Low level flow is strongly onshore with pressure
gradients through the Strait over 4 mb and KOTH-KSEA at 5.9 mb.
The strong onshore flow means that stratus over the coastal waters
and along the western portions of the Olympic Peninsula will
rapidly stream eastward through the Chehalis Gap and through the
Strait of Juan de Fuca overnight. By the pre-dawn hours, stratus
is expected to spread east all the way into the Cascades. Webcams
along the coast show some patchy fog, so an update to include
patchy fog to the immediate coastal beaches was made for tonight
into early Thursday morning.

Thursday will be 8-10 degrees cooler than today with the morning 
stratus and the invasion of marine air into the interior. Current
forecast temperatures in the greater Seattle area may be 2-4
degrees too warm for Thursday afternoon given the increase in
marine air, but the current forecast reflects a mean of the models
and guidance and will be maintained until the full suite of 00Z
guidance comes in. The stratus will partially break during the 
afternoon hours over the interior.

The upper level pattern and resulting onshore flow will change
little Friday and Saturday resulting in a continuation of morning
clouds and partly cloudy afternoons with temperatures near
seasonal normals. Albrecht

.LONG TERM...From the previous long term discussion: A weak upper
ridge over the area this weekend will give way to an upper trough
next Tuesday and Wednesday. Low level onshore flow will continue.
The ECMWF shows a dying front reaching the area later Monday with
spotty light rain and then a few showers Tuesday and Wednesday. 
The GFS and GEM are essentially dry through the period. The 
forecast leans toward the dryer solutions for now which also 
matches neighboring offices. Temperatures should be seasonable 
through the period. Schneider

&&

.AVIATION...A weak upper low over central portions of Eastern
Washington will move northeast away from the area tonight.
Southwest flow aloft. The air mass is unstable at middle and 
upper levels and stable below 5000 feet as strong onshore flow 
ushers marine air inland overnight. The air mass will stabilize 
overnight.

Satellite imagery and area radars show instability showers and 
thunderstorms over the Cascades and an isolated shower over the 
northern Olympics this evening. This activity will rapidly
dissipate after sunset. Low level flow has become strongly onshore
this evening. Hence, widespread stratus with bases 009-012 over 
the western Olympic Peninsula and out into the coastal waters will
push rapidly inland and will engulf the entire area into the 
Cascades by 12Z. The stratus will give the area widespread MVFR
and local IFR conditions late tonight into Thursday morning.
Conditions are expected to improve after 19Z Thursday with stratus
lifting somewhat and partially burning off in the interior.
Albrecht

KSEA...Stratus will surge inland, reaching the terminal shortly
after midnight. Expect CIGS to fall to OVC010-015 by 10Z and
continue until about noon Thursday. Ceilings will lift and
conditions become VFR about 21Z Thursday. Southwest to south wind
5-8 knots. Albrecht

&&

.MARINE...Strong onshore flow this evening is giving gale
conditions to the central and eastern portions of the Strait and
small craft advisory conditions to portions of Admiralty Inlet and
the Northern Inland Waters adjacent to the Strait. The forecast 
for this evening was updated to increase winds to around W 40 
knots in the Central Strait based on observations and pressure
gradients and to include some patchy fog through Thursday morning
for the coastal waters and western portions of the Strait.

Expect winds and seas to gradually relax during the early morning
hours on Thursday as marine air surges inland. Then the pattern 
of high pressure offshore and lower pressure east of the Cascades 
will continue for the next several days. Expect solid small craft 
advisory conditions during the late afternoon and overnight hours 
through the eastern two thirds of the Strait and portions of the 
inland waters adjacent to the Strait each day. Friday evening 
appears to be the next best chance for possible gales at the 
eastern portions of the Strait as a weak upper level disturbance 
moves east across the area. Albrecht

&&

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

     Small Craft Advisory until 3 AM PDT Thursday for Northern Inland 
     Waters Including The San Juan Islands.

     Small Craft Advisory until midnight PDT tonight for Admiralty 
     Inlet.

&&

$$

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