Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 211636
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
936 AM PDT THU JUL 21 2016

.SYNOPSIS...An upper trough will move into the coastal waters
today and inland tonight, bringing partly to mostly cloudy skies
and some showers beginning this afternoon. Thunderstorms are
possible over the Olympics, Cascades, and south interior. A few
showers could linger into Friday. A weak upper ridge should bring
sunny weather Saturday through Wednesday.

&&

.SHORT TERM...Dry southwest flow aloft continues over Western
Washington this morning ahead of an offshore upper trough. There
is marine stratus and patchy fog along the coast, in the western
Strait of Juan de Fuca and in the lower Chehalis Valley but
otherwise skies are clear at 9 am.

The upper trough will approach the Pacific Northwest coast today,
and recent model solutions show the closed upper low currently
near 42N 130W tracking northeast into the northern Oregon coastal
waters late today. Previous runs showed a more open and positively-
tilted trough, but this scenario looks more favorable for
scattered showers and possibly thunderstorms with almost due
southerly flow aloft, upper level diffluence and instability, and
placing us in the left front region of a 90 kt jet streak along
the southeast side of the upper low.

So we are making some morning updates to the forecast to increase
the chance of showers this afternoon and evening, and to increase
the coverage of possible thunderstorms to the Olympics and the
lowlands from around Tacoma southward. This is similar to recent
changes made by WFO Portland to the adjacent area to our south,
and reflects the latest Day 1 Convective Outlook from SPC.

The upper low will continue northeast across Western Washington
late tonight and east of the Cascades midday Friday. This timing
is a bit slower than previous runs, which has been a trend over
the past several runs. Scattered showers should continue tonight
and linger into Friday morning, then probably become limited to
the mountains Friday afternoon. The upper trough should also
result in stronger onshore low level flow tonight, with marine
stratus pushing inland overnight. So Friday should start generally
cloudy, with the marine layer clouds probably giving way to
partly sunny skies over the interior lowlands in the afternoon.

Drier northwest flow aloft will develop Friday night and Saturday,
ahead of a weak upper ridge over the offshore waters. However low
level onshore flow will maintain a moist marine layer below about
5000 ft. So we can expect some marine stratus again Friday night
and Saturday morning, and then a mostly sunny afternoon. McDonnal

.LONG TERM...Here is the long term section from the early morning
discussion -- Models try to prog a shortwave passing through the area
Saturday night and into Sunday. Previous runs have hinted at this
feature...but current runs have it a bit more pronounced. Nothing in
the way of moisture associated with it...and given how it passes
through during the overnight hours and is quickly replaced by
another ridge...its impact over Western Washington will be minimal
at best. Otherwise...the remainder of the long term continues to
look unimpressive...a succession of weak ridges and troughs that
will keep interior lowland highs in the mid to upper 70s. Large
scale ridging is hinted at out in the Day 8 time frame which may
kick temps into the mid 80s for the interior lowlands. Consider that
to be fair warning for local fans and air conditioners. SMR

&&

.AVIATION...An upper level low along 130w off the south Oregon
coast this morning will track northeast across Western Washington
tonight.  Moderate south to southwest flow aloft becoming light
northwest flow aloft by Friday morning. Stable and dry except for
low level moisture near the coast. Moisture will increase in the mid
levels across the entire area this afternoon...and become moist at
all levels and unstable by this evening. This will lead to a chance
of thunderstorms over the Cascades late this afternoon and evening,
with mostly high-based showers elsewhere. However, there is around a
20 percent chance of isolated thunderstorms spilling into the
interior lowlands mainly south and west of Puget Sound and the
Olympics. Low level onshore flow will strengthen later today and
tonight, causing marine stratus to fill in across the interior
lowlands later tonight and Friday morning.

KSEA...Mostly clear skies and light NE wind 4-8 KT...becoming NW 8-
12 KT by midday...then switching to light south wind during the
evening. Mid-level Accas clouds spreading across the sky this
afternoon and evening, with a chance of showers by this evening and
overnight. The odds of an isolated thunderstorm reaching the
terminal is only around 10-15 percent and will not include in TAF. A
quick shift from northerly winds to S/SW wind is expected around 04z-
06z this evening. Marine stratus with IFR cigs will then invade the
terminal during the pre-dawn hours on Friday and persist into Friday
morning. DTM

&&

.MARINE...Onshore flow will become strong during the course of
this afternoon, leading to westerly gales in the central and eastern
Strait by this evening. Gale Warning remains in effect with small
craft winds in areas near the East Entrance to the Strait.
Otherwise, moderate onshore flow will then continue for the week.
DTM/Haner

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Gale Warning from 5 PM this afternoon to 5 AM PDT Friday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 PM this afternoon to 9 AM PDT Friday
     for Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 5 PM this afternoon to 1 AM PDT Friday
     for Admiralty Inlet.

&&

$$

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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