Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 251627
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
927 AM PDT SUN SEP 25 2016

.SYNOPSIS...A weak warm front offshore will lift northeast into
British Columbia today. Clouds will erode from the south midday
with rain chances over far northern portions of Western Washington
coming to an end. A strong upper ridge will give warm weather to
the area through Monday. A cold front will move through the area Monday
night. Generally dry weather is expected Tuesday through Thursday.
An upper trough will bring a chance of showers Friday and Saturday.

&&

.SHORT TERM...A weakening warm front extends east from a low near
49N 140W into the offshore waters this morning. A somewhat thin
higher stratus layer around 4000-5000 feet continues to stream
into Western Washington this morning but light rain has all but
disappeared from regional observations and radar images. Short
term models quickly erode the cloud deck during the midday to
early afternoon hours as the warm front lifts northeast and
weakens further and as strong 500 mb ridging builds northward into
Western Washington.

The air mass across Western Washington will be rather warm
through Monday with most areas seeing highs well into the 70s and
lows tonight holding in the 50s. Some locations in the urban
Seattle area may remain around 60 tonight. Nocturnal cooling in
this warm and somewhat muggy air mass will allow areas of fog and
low stratus to form tonight, especially in the more fog-prone
river valleys and south interior zones. Fog and stratus will
quickly burn off Monday. Temperatures will rise well into the 70s
to around 80 Monday, the warmest day of this warm spell, as
temperatures at 850 mb continue to appear to top out around 16.5C
Monday afternoon. The latest 12Z NAM12 temperature guidance for
SeaTac airport is 80 degrees -which would break the old record of
78 degrees.

The cold front now seen dropping southwest from the low near 49N
140W into the east-central Pacific will move through western
Washington Monday night. A rather strong prefrontal marine push
can be expected to develop around sunset Monday and be followed by
the front itself late Monday night. The front will be weakening as
it moves through, but there will be a small threat of showers with
the front and with a weak convergence zone Tuesday morning.

Current forecasts are in good shape. No morning update is required
today. Albrecht

.LONG TERM...From the previous long term forecast: Extended
models in good agreement during the first part of the extended
with an upper level trough digging offshore on Wednesday and
Thursday. Run to run consistency not very good with the models yo-
yoing around on when the trough will be close enough to introduce
some precipitation into the forecast. 00z runs...like they did
yesterday...have backed off on the timing pushing the initial
precipitation back into Friday. Current forecast has chance pops
in the forecast for Thursday night. With the lack of consistency
will keep the chance pops in the forecast for Thursday night at
this point. Models in pretty good agreement that when the trough
moves into the area later Friday into Saturday the air mass will
be one of the cooler ones to move into the area in awhile with the
snow levels lowering down to near 5000 feet. Paradise on Mount
Rainier...and maybe the higher mountain passes like the North
Cascades could see a little snow next weekend. Felton

&&

.AVIATION...An upper level ridge of high pres will prevail over
the region today for WNW flow aloft. The low level flow will be
nearly flat. MVFR CIGs over the coast should scatter out by 2100
UTC.

KSEA...VFR. Winds will become northerly 7-10 knots this afternoon.

&&

.MARINE...A surface high over western WA will keep the flow
nearly flat today. Expect light northerly flow to develop tonight
before turning onshore or westerly on Monday. The onshore flow
will strengthen Monday night in response to the passage of a cold
front. This will lead to the potential for gale force westerlies
over parts of the Strait of Juan de Fuca. The onshore pressure
gradient will weaken some on Tuesday.

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...None.
&&

$$

www.weather.gov/seattle

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



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