Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 211552
Area Forecast Discussion
National Weather Service Seattle WA
852 AM PDT WED SEP 21 2016
.SYNOPSIS...An upper trough will gradually shift south of the
region the next couple days. The Western Washington lowlands will
have mostly sunny weather Wednesday and Thursday, with isolated to
scattered afternoon and evening showers over the mountains. A
weak front could bring some rain Friday, then a strong ridge will
bring dry and warmer weather Saturday through Monday.
.SHORT TERM...Temperatures were chilly this morning with a few
spots dropping into the upper 30s, like Shelton and Bremerton. The
visible satellite shows patchy fog and low clouds in the interior
and along the Chehalis river. The low clouds will burn off late
this morning for mostly sunny skies. There is an upper level
trough over the Pac NW with isolated showers popping up in the
mountains late this afternoon and evening. Current forecast is on
Thursday is looking like a dry day as the upper level trough
shifts south and east, while a ridge nudges in from the west.
We`ll likely see patchy low clouds and fog again during the
morning. Temperatures will remain near or slightly below normal
with highs in the 60s.
A weak, splitting front will push inland on Friday bringing rain
to the coast...with a chance of rain in the interior. Showers will
mainly affect the Cascades in the afternoon. 33
.LONG TERM...Previous discussion...All the global models
including the GFS/ECWMF/and Canadian continue to show a strong
ridge building north over the region Friday night through the
weekend. This will lift the flow well to the north into B.C.
keeping Washington dry. There will likely be some areas of late
night and morning fog to contend with, but otherwise mostly sunny
skies. Temperatures will warm considerably, well into the 70s and
possibly low 80s. Some models show thermally induced low pressure
along the Oregon coast possibly reaching the Washington coast by
Sunday which could induce some offshore flow and give a further
boost in temperatures. The current extended forecast indicates
mostly 70s to around 80 which seems reasonable at this point.
Models show the ridge shifting inland on Tuesday with southwest flow
aloft over Washington. Some models show a weak system clipping the
area to the north but it does not appear significant enough to
mention precipitation yet. Most likely, it will induce onshore flow
and result in increased marine clouds and cooler temperatures.
.AVIATION...A weak upper level trough over Western Washington will
move slowly south to Oregon. Flow aloft will remain light northeast
today, with northerly low level flow. The air mass is mostly dry and
There is some IFR fog and stratus around Puget Sound this morning.
It should evaporate by 18Z or so. The upper level low could spawn a
few mountain showers this afternoon and evening. Otherwise
conditions will be VFR today and this evening. Patchy fog and
stratus will likely return overnight.
KSEA...IFR conditions will turn VFR by 18Z or so. Northeasterly
winds 4-8 KT will become northerly 6-12 KT around midday. CHB
.MARINE...Light northerly winds this morning will strengthen this
afternoon especially over the coastal waters, and become more
onshore this evening as higher pressure builds over the offshore
waters of Oregon and Washington. Mesoscale models continue to
highlight small craft advisory criteria winds over the central and
southern coastal zones late this afternoon and evening. Onshore flow
will develop further Thursday. Models show a front reaching the
coastal waters Friday with widespread small craft advisory southerly
winds becoming northwesterly late in the afternoon and evening.
Surface gradients and winds continue to look lighter through the
weekend as high pressure develops over the region. CHB
PZ...Small Craft Advisory from 2 PM this afternoon to 2 AM PDT
Thursday for 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.
You can see an illustrated version of this discussion at