Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 151539
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
839 AM PDT Sun Oct 15 2017

.SYNOPSIS...A weak upper level ridge will remain over Western
Washington through tonight with dry weather and warmer afternoon
temperatures. The ridge will start to retreat to the southeast on
Monday, allowing a wet cold front to drop southward through the
area on Tuesday. A vigorous warm front will lift back northward
on Tuesday night and Wednesday, giving stronger wind and heavier
rain. An upper trough will reach the offshore waters on Thursday,
preceded by another wet cold front over Western Washington. Cooler
and showery weather will follow on Friday. A wet and windy
atmospheric river pattern is possible next weekend.


&&

.SHORT TERM...In typical mid October fashion, the jet stream
across the North Pacific is strengthening and moisture from the
western Tropical Pacific is streaming eastward. After a couple of
quite weather days, the weather pattern across Western Washington
will abruptly transition to a stormy one Monday night, and the
stormy weather will likely persist into next weekend or beyond.

For today, fog is limited to river valleys and portions of the
south interior. Some stratus is seen around Port Angeles and over
the coastal waters. The clouds and fog in the interior will
rapidly dissipate midday. 500 mb heights are high and
temperatures aloft are warming, so a mild mid October day is
expected today with highs in many places getting into the lower to
mid 60s. Clouds in the interior will remain rather scarce this
evening.

Monday will be a transition day as a frontal system from Haida
Gwaii and the Queen Charlotte Sound southwestward into the Pacific
approaches the area from the northwest. Models differ on the
amount of cloud cover moving into the area late tonight into
Monday morning with the incoming 12Z NAM12 showing some low level
moisture pushing inland in a weak prefrontal push of onshore flow.
This impacts high temperatures in the interior on Monday with the
NAM showing temperatures mainly in the 50s and the less aggressive
GFS and ECMWF solutions about 5 degrees warmer.

A morning update will feature a decrease in the coverage of fog
this morning and an increase in the coverage of stratus for Monday
morning along with slightly cooler highs. Further adjustments to
Monday`s forecast may be required later this afternoon after we
get a chance to look at all of the incoming information.

Today really is a good day to make preparation for the upcoming
stormy pattern...like making sure gutters are clean, drains are
cleared of any early fall leaves, lawn furniture is brought in,
etc. The weather will go down hill on Monday night as the front to
the northwest approaches the area.

Tuesday will be wet and breezy as the front moves southeast
across the area during the day. Rainfall amounts in the mountains
with this front will be typically heavy, but not sufficient to
cause any flooding due to the progressive nature of the front. In
the interior lowlands, rainfall amounts will be cut back to around
a half inch as we see shadowing from the Olympics, mountains of
Vancouver Island, and Willapa Hills. Albrecht

.LONG TERM...From the previous long term discussion: Looks like a
stormy fall day on tap for Wednesday, probably requiring the
eventual issuance of wind- related and possibly flood-related
watches, warnings, or advisories. A jet streak will develop in the
base of an upper trough over the Gulf of Alaska. As the jet noses
into B.C., it will push a strong warm front inland on Tuesday
night. This will bring a new round of heavy rain, though the ECMWF
hold heavy rain off until Wed morning. Southeast gradients will
become tight on Wednesday, supporting windy conditions. Windiest
weather will be in southeast-wind prone areas, such as the coast
and areas from Everett north through the islands and Bellingham.
Statistical guidance from the GFS and ECMWF show sustained winds
at Whidbey Island reaching 34 knots and 31 knots respectively. The
first such wind event of the autumn is commonly more impactful
than a similar event later in the season, after nature`s tree
trimmer has already been through a few times. As for rain, models
are developing a consensus in keeping heaviest rain over the
windward Olympic Mtns and North Cascades. The 06z GFS shows
24-hour rain totals as high as 8 inches over the northwest Olympic
Mtns as of 11 pm Wed. The official NWS forecast has 6.5" at this
wettest location during the same 24 hours. Currently looking for
3-4 inches over the North Cascades. The interior lowlands
currently forecast to range from 0.75" south of Olympia to near 2"
around Bellingham. See the hydrology section for more impacts.

An upper trough will make headway off the eastern Pacific, pushing
a cold front through Western Washington on Thursday. Once it
passes, the atmospheric river type weather should ease. Cool and
showery post-frontal conditions will prevail on Friday.

Yet another strong, consolidated jet will nose into the Pac NW and
B.C. next Saturday. Looks like this will be another atmospheric
river setup for heavy rain. With 850 mb southwest winds of 40-60
kt, should be windy too.  Haner

&&

.AVIATION...Flat upper ridge over Washington this morning to
shift slowly east through Monday morning. Westerly flow aloft. A
Pacific frontal system is forecast to approach Western Washington
from the NW during the day Monday with increasing moisture aloft. At
the surface, light offshore flow to become onshore Monday as
pressures begin to fall to the north with the approaching front. Air
mass stable. Local valley fog and stratus to dissipate before 19z.

KSEA...VFR today with some higher clouds well above 10K ft passing
over the terminal at times through tonight. Northerly surface
gradients through Puget Sound for a north wind around 5 kts today,
then becoming light and variable after 02z. Buehner

&&

.MARINE...Higher pressure east of the Cascades today with light
offshore flow. An approaching Pacific frontal system moving into
British Columbia tonight and Monday will drop pressure to the north
and turn the flow onshore across Western Washington by Monday. The
front is expected to sag southeast across the area Monday night and
Tuesday. Small craft advisory winds likely to develop over the
coastal waters Monday afternoon and could spread into parts of the
inland waters Monday night, especially north of Puget Sound.

The next stronger Pacific frontal system is set to arrive Wednesday
and anticipate gale force winds over the coastal waters. Solid small
craft advisory winds are likely over the inland waters. Forecasts
should be monitored closely as the precise track of the front could
bring stronger or weaker winds than forecast, especially over the
inland waters.

These Pacific frontal system will also help generate rising ocean
swells, building well above 10 feet, possibly close to 20 ft, for
the coastal waters by later Wednesday into Thursday.  Buehner

&&

.HYDROLOGY...From the previous hydrology discussion: A few hours
of heavy rain on Tuesday is not expected to produce river flooding.

An atmospheric river type event is expected Wednesday, with the
focus of heaviest precip aiming at the North Cascades and the
Olympic Mtns. All area rivers will see rises. Will need to monitor
the potential for minor flooding on a couple of rivers,
especially rivers flowing off the Olympics and North Cascades.
This includes the uniquely flood-prone Skokomish River. In the
lowlands, 1-2 inches of rain and autumn leaf litter clogging
drains could lead to localized urban flooding.

Another period of heavy rainfall is possible next weekend,
renewing concerns for flooding. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$

www.weather.gov/seattle

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



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