Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 202138
Area Forecast Discussion
National Weather Service Seattle WA
238 PM PDT Thu Oct 20 2016
.SYNOPSIS...A weak upper level ridge will move over Western
Washington tonight for a brief lull in precipitation. A convergence
zone may form over North Puget Sound. A weak warm front will bring
some light rain to the area on Friday. After another minor lull
Saturday, another cold front will reach the area late Saturday and
Saturday night. A large upper level low offshore will maintain moist
southwest flow aloft over the region and an active weather pattern
through early next week.
.SHORT TERM...RADAR shows this mornings cold front has moved
over the Cascade crest. Satellite imagery shows a mostly dry air
mass offshore out to 130W at 21Z/2 PM. The weak upper level
shortwave ridge will be moving over W WA tonight will help minimize
shower activity but there is still a slight chance of showers. Obs
are also showing W flow through the Strait of Juan de Fuca, so a
convergence zone is still possible this evening over Admiralty Inlet
and possibly down to the King-Snohomish county line. With all the
rain that has fallen over the area if a little clearing takes place
there could be patchy dense fog late tonight and Friday morning,
particularly over S Puget Sound and the SW interior. Tonight will be
a little cooler as well with lows mainly in the 40s.
Models still indicate that weak warm advection will develop tonight
offshore along roughly 135W ahead of a low headed toward the B.C.
coast. Unfortunately. there is little sign of that so far, but it is
still early. This warm front expected to reach W WA should be
forming by 06Z/11 PM tonight then bring some light rain to the NW
part of the area Friday morning. The rain should spread inland
during the afternoon with the best chances and amounts over the
northern half of W WA. The warm front should weaken over the
Cascades by Friday evening, so Friday night should be uneventful.
Models have been in fairly good agreement showing another upper
level trough approaching the region on Saturday. The details remain
messy so confidence remains only marginal. The GFS shows a little
weak warm advection ahead of the system which could brush W WA late
Saturday morning. The ECMWF does not have this weak warm advection.
The main cold front will probably move slowly across the area
Saturday evening while the associated upper level shortwave trough
and compact surface low curve sharply northward toward N Vancouver
The big upper level low W of B.C. near 52N/144W Saturday afternoon
will dig S to a position W of the WA coast near 47N/134W Sunday
afternoon. Moist SW flow aloft will continue. No significant small
scale features stand out at this point but the air mass will
probably be moist enough to keep a slight chance of rain.
.LONG TERM...The above mentioned upper level longwave low will
remain the main feature Monday through most of next week. Moist SW
flow aloft around the low will send a series of weak shortwaves
across W WA through the period. The models have not been all that
consistent with the smaller scale features so confidence in the
details is low. Confidence is still good that this will be a fairly
wet active period of weather. Kam
.AVIATION...Southwest flow aloft will prevail tonight and Friday.
A cold front will into Western Washington Friday afternoon. The
air mass is generally moist and stable.
Moisture will decrease across the area tonight in the wake of a
front. Partial clearing will allow for patchy fog Friday morning.
Clouds will increase across the area again Friday as the next
front moves onshore.
KSEA...The cloud forecast for tonight is tricky. Low clouds are
expected to scatter this evening but this might just allow lower
stratus to reform overnight with weak low level flow. There is
also a chance the current low clouds will never really scatter
and persist most of tonight. South winds 4-8 knots. Schneider
.MARINE...A series of fronts will move through the area over the
next several days.
The first in this series - a relatively weak front - will bring
small craft advisory southerly winds to the Coastal Waters and
West Entrance Strait on Friday.
A stronger front will bring small craft advisory winds to most
waters later Saturday or Sunday. Gales are also possible for the
Coastal Waters with this system although winds this strong are not
currently indicated in the forecast.
Another fairly vigorous frontal system will affect the waters
Sunday night and Monday, followed by yet another system on
.HYDROLOGY...The frontal system that moved across W WA last
night and this morning brought heavy rain to much of the area. The
cold front had moved over the Cascade crest around 2 PM and is no
longer a factor. Enough rain fell to push the Skokomish river near
Potlatch and the Skykomish River near Gold Bar above flood stage.
Flooding on the Snoqualmie river may also develop tonight. Other
than these areas no flooding is expected on area rivers the next 7
WA...Flood Watch through late tonight for Bellevue and Vicinity-
Cascades of Snohomish and King Counties-East Puget Sound
Lowlands-Everett and Vicinity-Seattle and Vicinity.
PZ...Small Craft Advisory from 6 AM to 3 PM PDT Friday 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-West Entrance U.S. Waters
Strait Of Juan De Fuca.
Small Craft Advisory until 5 PM PDT this afternoon for Central
U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
Waters Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at