Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 221614
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
914 AM PDT Sat Apr 22 2017

.SYNOPSIS...There will be showers and a chance of thunderstorms
today as a frontal band lifts northeast across the area. Showers
will taper off this evening. Showers will increase again Sunday
and Sunday afternoon as an upper level disturbance moves through
the area from the southtwest. A series of low pressure systems and
fronts will give wet and slightly cooler than normal weather to
the area Monday through the end of the upcoming week.

&&

.SHORT TERM...Radar and satellite imagery show an incoming
negatively tilted frontal system sitting from the Central
Washington coastal waters southeast to around Portland Oregon at 8
AM. This front will lift northward into the Central Puget Sound
area between 11 AM and Noon today then northeastward to the
Canadian border during the early afternoon hours. It is dry and
mild ahead of the front with downslope easterly winds giving
temperatures that are probably the highs for the day in many
locations early this morning. Conditions will very rapidly
deteriorate from the south as the front lifts through the area
midday. This frontal system is accompanied by good 500 mb height
falls and cooling aloft. Lifted indices drop to -1 to -3 by early
afternoon while CAPE rises to 300-500 J/kg in forecast NAM BUFR
soundings this afternoon. With the rather deep moisture profile,
the nature of any thunderstorms that form will be short-lived.
The current forecast of a slight chance of tstms for this
afternoon will be maintained, and Nowcasts will be used to refine
details if any development occurs. It will become breezy from the
south behind the front as southwest winds of 30-35 kt aloft mix
downward in the unstable air mass.

Showers will taper off rapidly early this evening as the frontal
system and upper level disturbance passes off to the north and as
we lose insolation. With some breaks in the clouds and the cooler
air mass in place, temperatures will fall mainly into the 40s
tonight.

Another disturbance, this one now nearing 45N 140W, will lift
northeastward through the area on Sunday afternoon. At this point,
upper level diffluence with the approaching disturbance and
cooling aloft looks marginal for any thunderstorm activity Sunday,
but we will be taking a closer look at the 12Z data to determine
if they are needed in the next forecast package.

After the passage of Sunday`s disturbance, models appear consistent
in taking a 1000 mb low onshore over the far southern Washington
coastline Monday afternoon. Rain increases ahead of the low from
about Everett or Seattle and Forks southward during the early
morning hours and continues all day. Areas over the north interior
will only see some showers with the low moving in to the south.

As far as temperatures are concerned: warmth of this morning will
be fleeting as we fall into a cool and wet pattern this afternoon
through Monday -highs in the 50s and lows in the 40s.

Current forecasts appear to be on track. No morning updates are
needed. Albrecht

.LONG TERM...From the previous long term discussion: As has been
the story for what seems to be an eternity...when one low leaves
another is close behind. Such is the case as a front associated
with another upper level low...this time to the northwest off the
Canadian coast...will bring another slug of rain to the area
Tuesday afternoon and evening. Thankfully neither model projects
that this feature will linger like the previous low and has it
swooping down into the area...although there is about a 12 hr
difference between models as to when this occurs...the ECMWF being
much faster. Further disagreement arises when considering the
strength of the upper level ridge that moves in afterward...the
ECMWF solution favoring a dirty ridge that still allows for a
chance of showers Wed night while the GFS solution shows a weak
ridge akin to what has generally been part of the pattern over the
area for a while now...strong enough to allow for dry conditions
Wed night and into Thursday afternoon. This disagreement does not
allow for much confidence...but as this year has gone so far...if
there is a way to squeeze rain into the forecast...likely to be
more right than wrong if it is in there. As such...although they
fall at the lower end of chance range...pops remain in the
forecast for the second half of next week. SMR

&&

.AVIATION...A cold front extending from the N WA coast across
Olympia at 1530Z will move NE across the rest of W WA through early
this afternoon. The air mass will become unstable this afternoon
immediately behind the front and just ahead of the trailing upper
level shortwave trough. There will be showers and a chance of
thunderstorms as the upper level trough axis moves NE across the
area. The trough axis should be over B.C. by 03Z.  Gusty S-SW winds
19015G25KT are expected to develop over W WA today. MVFR to low VFR
conditions are expected with and behind the front with CIGS from
around BKN025 up to BKN-OVC050 in -RA and SHRA.

KSEA...VFR conditions up through 19Z ahead of the cold front, with
light rain beginning around 18Z. Showers are expected after 21Z with
a slight chance of a thunderstorm. CIGS will lower to BKN025-035
around 19Z then continue through this evening. S-SW winds will rise
to 15G25KT around 19Z then diminish after 03Z. Kam

&&

.MARINE...A cold front extending from the N WA coast to around
Olympia at 830 AM will move NE across the rest of W WA through early
this afternoon. SCA winds will develop across most of the coastal
and inland waters as the front moves through and then continue
behind the front through this evening. The air mass will become
unstable immediately behind the front this afternoon for a slight
chance of thunderstorms. Winds will eventually diminish later
tonight.

A weak surface trough will move NE across the area on Sunday.

Models still show another frontal system approaching from the W
Sunday night, with the surface low moving due E and reaching the
coastal waters 18Z/11 AM Monday morning then weakening rapidly as it
pushes inland Monday afternoon. The models have become more
inconsistent with some of the smaller details regarding the surface
low, notably the strength of the low and the track. At this point it
still could generate SCA winds over the coastal waters, but SCA
winds are looking less likely over the inland waters.

Another frontal system will affect the region Tuesday and possibly
Wednesday, but the models are becoming more flaky with this one as
well. Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
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 until 9 PM PDT this evening for Admiralty
     Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands-Puget
     Sound and Hood Canal.

     Small Craft Advisory until 11 AM PDT this morning for Central
     U.S. Waters Strait Of Juan De Fuca-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

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You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html


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