Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 190252
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
752 PM PDT Wed Sep 18 2019

.SYNOPSIS...A weak upper ridge will build inland on Thursday. A 
weak system will slide down the back of the ridge Thursday night 
and Friday for a slight chance of showers. Rain will return 
Saturday night and Sunday as frontal system arrives.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...Radar indicates showers 
have all but ended across the area this evening with the loss of 
daytime heating. There might be a few showers in the Cascades for 
the next hour or two but that is about it. Any thunder threat that
might have existed has also ended. 

In addition, skies have cleared across much of the area this 
evening. The mostly clear skies will allow temps to fall into the 
mid 40s to lower 50s tonight which will feel a bit fall-like for 
a change. Some low clouds will probably reform Wednesday morning 
with lingering low level moisture from recent rains but weak 
northerly low level flow should be a limiting factor. There will 
also be patchy fog Thursday morning, especially in the usual 
south interior valleys.

A weak upper ridge will build over the area from the west on
Thursday for a dry day. Low clouds and fog will burn off in the 
morning but high clouds will be increasing in the afternoon for a 
partly sunny day overall. Temperatures will be slightly below 
normal. A weak system in the north/northwest flow aloft will 
bring a slight chance of showers Thursday night and Friday. An 
upper ridge will be over the area on Saturday but there will be a 
fair amount of clouds out ahead of the next frontal system. 
Schneider

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Previous  
discussion...Another wet Pacific frontal system will cross western
WA on Sunday. This system will bring widespread wetting rains to 
the area, similar to the system we saw this past Tuesday. Showers 
will taper off Sunday night as this system exits and high pressure
builds in from the west. Broad high pressure will set up over the
Pac NW early next week but the flow aloft is nearly zonal over 
western WA with the jet overhead. The models show weak systems 
passing through southern B.C. which may clip the area for a chance
of showers, but confidence is low. The timing is off in the 
models so will use a blend for now. 33

&&

.AVIATION...A weak upper ridge will move into the area from the 
northwest tonight and Thursday with northerly flow aloft. At the
surface, weak onshore flow will prevail with high pressure
offshore and lower pressure inland. The air mass is dry and
stable.

Areas of low clouds and patchy valley fog should form later
tonight in a weak onshore flow pattern. The low clouds and fog
should burn off Thursday morning but high clouds will be 
increasing by Thursday afternoon. 

KSEA...Mostly clear tonight. There might be a few hours of low
clouds Thursday morning but even this is not a sure thing. High
clouds will increase later Thursday. Northerly wind 3-6 knots will
become light tonight and then southerly Thursday morning. 
Schneider

&&

.MARINE...Relatively light onshore flow will prevail through
Friday with high pressure offshore and lower pressure inland. A
frontal system will approach from the west on Saturday and then
move through the area Saturday night and Sunday. Schneider

&&

.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 for hazardous seas until 11 PM PDT this 
     evening 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-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 for rough bar until 11 PM PDT this evening 
     for Grays Harbor Bar.

&&

$$

www.weather.gov/seattle