Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
000
FXUS66 KSEW 300950
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
245 AM PDT Thu Mar 30 2017

.SYNOPSIS...An upper level trough over the region will shift
eastward tonight as a ridge of high pressures builds into the area.
Showers will diminish and drier conditions will prevail tonight
through Friday. A weak front will bring some light rain on Saturday
with a lingering trough providing a chance of showers on Sunday.
Somewhat drier conditions Monday could be followed by more rain
with another front by the middle of next week.

&&

.SHORT TERM...Shower activity continues to move onshore with
an upper level trough over the area this morning. Convergence zone
activity continues over northern King and Snohomish counties, but
onshore gradients are easing and this should wind down by later this
morning. The trough axis will be east of the Cascades by late
morning and showers will diminish through the afternoon. Low level
moisture will be slow to exit so expect a rather cloudy day ahead
with high temperatures 2 to 4 degrees below normal. By late tonight,
an upper level ridge axis will be pushing into the offshore waters.
This is a rather low amplitude, dirty ridge. Friday will be a dry
day for Western Washington albeit with a fair amount of high and mid
level clouds. With some partial sunshine temperatures will at least
be close to normal. Like a cruel April Fools joke, another system
arrives Friday night into Saturday for a damp beginning to the
weekend. QPF is not especially high with this system, but a little
light rain looks likely. The front exits by late in the day leaving
zonal flow in its wake.

.LONG TERM...Much of Western Washington could get away with a dry
day on Sunday, but the presence of a trough overhead is enough to
keep a chance of showers in the forecast. Both the GFS and Euro
bring more ridging to the area Monday into Tuesday. Current
forecasts have chance PoPs for that period, but those could be
trimmed back or eliminated if the models continue with that trend.
A return of more wet and unsettled conditions is possible by the
middle of next week as the models show a broad upper level trough
digging into our offshore waters with a series of systems moving
through. 27


&&

.AVIATION...Northwesterly flow aloft today on the backside of a
passing upper level trough becoming northerly Thursday. Gradients
loosening very gradually this early morning and will continue that
slow pace through the day...allowing for continued offshore flow but
with wind speeds slowly declining. Some locations may still
experience breezy conditions throughout the day today. The air mass
remains moist and slightly unstable and as such...showers will
persist into the remainder of the morning hours. As upper level high
pressure starts to make its way toward the area...conditions are
expected to stabilize late this afternoon and into this evening
allowing for diminishing shower activity.

Cigs remain a mix of VFR to MVFR conditions over W WA. Earlier
convergence zone over PAE has dissipated although sufficient low
level moisture remains in place there to keep cigs in MVFR range.
Elsewhere...MVFR cigs generally expected under shower activity. As
shower activity is expected to taper off this afternoon...should
also see cigs bounce back up into solid VFR range. Lingering low
level moisture may result in cigs deteriorating tonight...most
locations likely dipping back down into MVFR...although fog-prone
locations...such as OLM...may see fog develop late tonight. SMR

KSEA...Current obs and radar show not much in the way of showers
over the terminal at this time and likely to stay that way for the
next hour or two. Developing band of showers to the NW however seems
to be on track to potentially brush the terminal...so VCSH still
looks good. Cigs likely to remain MVFR for much of the morning
before rebounding to VFR by afternoon. Cigs may drop again into MVFR
tonight. SW winds 7-12 kts this morning tapering off to 5-10 kts
this afternoon and tonight. SMR

&&

.MARINE...Moderating onshore flow in wake of exiting trough.
Inherited headlines look good but did have to extend SCAs for the
Strait...N inland waters and Admiralty Inlet into at least late
tonight as onshore flow remains in place. Gradients will continue to
gradually slacken through the day allowing for a very slow
diminishing of winds over area waters...with most locations falling
out of SCA criteria at some point this afternoon or tonight. Lighter
winds are expected Friday as upper level ridging sets up over the
Pacific NW. Another frontal system is expected for Saturday. SMR

&&

.HYDROLOGY...After the recent heavy rain over the Olympics the
Skokomish river remains a little above flood stage. It will continue
to recede slowly this morning. Latest estimate is that it will drop
below flood stage around 9 AM. No other really strong and wet systems
are expected for the next week so flooding is not expected on area
rivers during the next 7 days.

The heavy rain did increase the threat of landslides across W WA
again. However, the light showers expected today will not have much
effect on soil saturation. The current statement highlighting the
landslide potential will be allowed to expire. Kam/27

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 PM PDT this afternoon 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 AM PDT this morning
     for Grays Harbor Bar.

     Small Craft Advisory until 5 AM PDT Friday for Admiralty Inlet-
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands.

     Small Craft Advisory until 11 PM PDT this evening for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 5 AM PDT early this morning for
     Puget Sound and Hood Canal.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.