Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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154
FXUS66 KSEW 181605
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
905 AM PDT Sat Mar 18 2017

.SYNOPSIS...A cold front will move through the area this morning.
Showers this afternoon and evening will mainly be in the
mountains and in the Puget Sound Convergence Zone. High pressure
will build for dry days Sunday and through most of Monday. A weather
system will spread rain back into the area Monday night and Tuesday.
Unsettled weather will continue into next weekend.

&&

.SHORT TERM...The latest soggy mild front will move inland and the
showers behind the front will be mainly in the mountains and in the
PSCZ. After a mild rainy night in the mountains, the snow level will
drop this afternoon and evening. There probably wont be a lot of
showers left over this evening except perhaps in the PSCZ along
highway 2 heading up to Stevens Pass--and the snow level forecast of
2000` is probably too high for this evening in the PSCZ. I will take
a look at the UW models--maybe Index to Baring or Skykomish will get
a couple inches of snow in the PSCZ which could make highway 2
slushy for several hours this evening.

High pressure will build over the region Sunday for some sunshine.
Monday will also be dry--at least til about mid afternoon when
increasing clouds from the south bring a chance of rain to the
southwest portion of Wa, and then rain spreads north monday night in
southerly flow aloft.

.LONG TERM...Models in good agreement with the south southwesterly
flow aloft pushing weather systems into western Washington Monday
night through Tuesday night. Upper level trough moving through
Wednesday afternoon into Thursday morning with the ECMWF being the
slowest model with the trough passage. Models slightly different
with the splitting system for the end of the week with the ECMWF
faster and more consolidated with the system versus the slower
and weaker GFS. Either way will keep the chance pops in the
forecast for late in the period. Felton

&&

.AVIATION...Southwest flow aloft will become more westerly tonight
and Sunday as an upper level trough offshore moves slowly
eastward. At the surface, high pressure south and west of the area
will build into the area early Sunday morning then into southern
British Columbia late Sunday into Monday.

Onshore flow is giving variable but mainly MVFR conditions to the
area this morning. Expect lower clouds and showers to become
increasingly confined to the west slopes of the terrain and
convergence zones as the day progresses. Lowest ceilings by early
this afternoon away from the mountains will be in a Puget Sound
Convergence Zone that is now forming north of Seattle and around
KPAE that will briefly push southward to around KBFI then lift
back toward KPAE in the afternoon. Confidence in the forecast of
the exact position of the convergence zone is generally low.

While conditions will improve this evening as the air mass dries
and onshore flow decreases, expect areas of fog to form south of
KSEA and into the interior of southwestern Washington in light
pressure gradients after 10Z tonight and continuing until midday
Sunday. Albrecht

KSEA...Showers continue at the terminal this morning and MVFR
ceilings remain around the terminal area. Expect conditions to
generally improve this afternoon as showers become confined to a
convergence zone to the north of the terminal. The convergence
zone at 1530Z was between KPAE and KBFI. It will likely slip down
to near KBFI by about 18Z for 2-3 hours before it moves back to
its position between KPAE and KBFI. There is about a 25 percent
chance that the convergence zone will slide to over or just south
of the KSEA terminal for a couple of hours between 18Z and 20Z
-disrupting the traffic pattern. Winds will be southwest 10-14 KT
but may briefly go light northeasterly or easterly for 2-3 hours
if the convergence zone moves over or very near the terminal until
21Z. Conditions will become mainly VFR after 21Z. Albrecht

&&

.MARINE...High pressure well to the southwest of the waters will
build into Western Washington late tonight and early Sunday then
into southern portions of British Columbia later Sunday into
Monday. Moderate to strong onshore flow today will rapidly
decrease this evening with light pressure gradients Sunday
morning. Moderate offshore flow will develop later Sunday into
Monday.

A gale warning is in effect for the central and eastern portions
of the Strait of Juan de Fuca today, while small craft advisories
are in effect for the other waters. Expect warnings and advisories
to end late this afternoon through this evening. Small craft
advisories may be required for northerly flow in Puget Sound and
easterly flow at the West Entrance to the Strait of Juan de Fuca,
and possibly other coastal zones, later Sunday into Monday as the
moderate offshore flow develops.

A series of weak fronts will move through the waters from the
south and southwest starting Monday night and continuing through
much of next week. Albrecht

&&

.HYDROLOGY...The Nisqually will rise today as water is released, and
other rivers are rising today after the rainy night. The Skokomish
river in Mason county is over flood stage. The White River at
R Street continues to hover around flood stage. The threat of
landslides
will remain elevated.

&&

.CLIMATE...The February and March precipitation for Seattle is
13.89 inches through 1 am this morning. This is the 4th wettest
February and March in over 120 years of records. The three higher
years are 2014 15.55 inches, 1972 14.85 inches and 1950 with 14.14
inches.

Since February 1st in Seattle there has been measurable
precipitation on 36 days. 40 out of the 45 days have had 70
percent or greater cloud cover. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Warnings Skokomish river in Mason county and the
     White River at R Street King and Pierce Counties.

PZ...Small Craft Advisory until 9 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 midnight PDT tonight
     for Grays Harbor Bar.

     Gale Warning until 6 PM PDT this evening for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 8 PM PDT this evening for Admiralty
     Inlet-Northern Inland Waters Including The San Juan Islands-
     Puget Sound and Hood Canal-West Entrance U.S. Waters Strait
     Of Juan De Fuca.

&&

$$

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