Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
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

000
FXUS66 KSEW 211611
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
911 AM PDT Tue Mar 21 2017

.SYNOPSIS...A series of disturbances embedded in southwesterly
flow aloft will move through the area today through Friday.
Heaviest precipitation is expected Thursday night into Friday. The
best chance for a break in the wet weather for the interior will
be on Thursday. Showers will diminish over the weekend as the
upper level flow becomes westerly.

&&

.SHORT TERM...A series of disturbances are rotating around an
upper low centered near 45N 134W. One weak disturbance that
brought some locally moderate rain for a time this morning is now
moving from the northern interior zones into southwestern portions
of British Columbia. A second disturbance can be seen extending
westward from north of Klamath Falls Oregon into a surface
low just to the northeast of the upper center. This second
disturbance to the south of the area will lift northward through
Western Washington this afternoon. The air mass will become
somewhat unstable later this afternoon with that second
disturbance, mainly in the southwestern interior. However, models
are in disagreement as to how unstable it will get. The normally
more convective 12Z NAM shows fairly low convective temperatures
in the lower 50s this afternoon but anemic CAPE values less than
200 J/kg and not much shear. The 06Z GFS was showed more shear and
cold advection aloft with CAPE values 500-700 J/kg -but higher
convective temperatures in the lower 60s. It appears that the best
chance for thunderstorm activity will be later in the afternoon
here in our portion of Western Washington, but the main area of
convection will be down in Oregon. Temperatures today will likely
hold in the mid 50s most areas.

The offshore trough will eject east-northeastward late tonight and
across Western Washington midday Wednesday. Shower activity will
decrease for a short period this evening between systems, then a
more organized band of showers will move through the area late
tonight into Wednesday morning. Lows tonight with plenty of cloud
cover will hold in the 40s. We can expect showers with some breaks
in the clouds Wednesday afternoon as flow aloft interacts with the
Olympics. The mesolow to the northeast of the Olympics that will
be responsible for some breaks in the clouds in the interior zones
will also give windy conditions during the afternoon hours from
about Tacoma northward to Everett. At this time it appears that
winds will top out locally at 30 mph with a few gusts to 40 mph.
While windy, this is below the threshold that causes any problems
in the area. Highs on Wednesday will again be in the lower to mid
50s.

The incoming 12Z NAM and GFS continue to hint at a break in the
precipitation on Thursday. The NAM is slower than the GFS in that
it keeps precipitation on the coast and westward at 00Z while the
GFS is a couple of hours faster. At this time, we will keep the
forecast for Thursday as is and wait for the 12Z ECMWF and GFS
ensembles before making timing adjustments.

Gridded and zone forecasts were updated for very minor changes to
cloud cover and POPS. Otherwise the forecasts are in good shape
today. Albrecht

.LONG TERM...From the previous long term discussion: Extended
models in good agreement with the system late Thursday night into
Friday being the wettest one of this current series. With the
upper level trough behind the front continuing to dig the front
will move slowly through the area on Friday. Westerly flow aloft
with just a chance of showers on Saturday. Model solutions become
inconsistent on Sunday with the GFS taking the next system south
into Oregon and Northern California while the ECMWF brings the
system right through western Washington. Inconsistencies in the
solutions continue on Monday with the GFS building a temporary
upper level ridge over western Washington while the ECMWF brings
yet another system into the area. Given the lack of consistency in
the model solutions will trend the forecast toward climatology
and have chance pops for Sunday and Monday with temperatures near
normal. Felton

&&

.AVIATION...An upper level trough will dominate the Pacific
Northwest through Wednesday. Moderate south-southwest flow aloft.
The air mass weakly unstable and moist. There will be slight risk of
isolated thunderstorms south of KOLM from 20z-01z. Generally VFR
conditions with showers through the period. A surface trough will
move through early Wednesday bringing low level onshore flow.

KSEA...Mostly VFR cigs with scattered showers. Southeasterly wind 5-
9 kt becoming more sly this afternoon. dtm

&&

.MARINE...Low pressure at the surface will remain offshore through
tonight with southeasterly small craft winds over most waters except
Puget Sound. Winds in the West/Central Strait may stay more easterly
before easting late this afternoon or evening. A surface trough will
reach the coastal waters late tonight and lift across the inland
waters Wednesday morning. Expect a switch to onshore flow Wednesday
afternoon and westerlies picking up in the Strait, possibly near
gale force. dtm

&&

.HYDROLOGY...The White River at R Street near Auburn will remain
near flood stage into Wednesday morning. Elsewhere, river
flooding is not expected for the next 7 days. Felton

&&

.CLIMATE...All climate stations for western Washington have
exceeded their normal monthly precipitation for March...and there
is more rain on the way for much of this week. Stations...their
normal monthly amounts for March and the precip they have received
so far for March 2017 through March 20th are as follows:

Sea-Tac -- Normal: 3.72 inches -- March 2017 so far: 5.39 inches
Olympia -- Normal: 5.29 inches -- March 2017 so far: 8.27 inches
Hoquiam -- Normal: 6.99 inches -- March 2017 so far: 11.32 inches
Quillayute -- Normal: 10.83 inches -- March 2017 so far: 12.95"
Bellingham -- Normal: 3.22 inches -- March 2017 so far: 3.90"

The combined rainfall for Sea-Tac for February and March through
the 20th is 14.24 inches. This is the 3rd wettest on record
including the Federal Building records which go back to the
1890`s. The only two years there has been more precipitation in
February and March, 2014 15.55 inches and 1972 with 14.85 inches.
There is a chance Seattle will break this record late this week.
Felton/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 PM PDT Wednesday 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 until 5 AM PDT Wednesday for Admiralty
     Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 3 PM PDT this afternoon for Central
     U.S. Waters Strait Of Juan De Fuca-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

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.