Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 210954
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
254 AM PDT Wed Aug 21 2019

.SYNOPSIS...A cold front will bring rain to the region today.
Expect dry weather by Thursday afternoon with a weak high pressure
over the area. A series of systems will pass generally to our
north Friday into the weekend keeping temperatures near normal and
bringing a chance of showers mainly to the coast and northern
areas. Warmer and drier conditions are expected to develop early
next week.

&&

.SHORT TERM /TODAY THROUGH FRIDAY/...Light rain ahead of an
approaching frontal system has spread onto the coast early this
morning. There is a distinct lack of cloud cover from the central
Puget Sound southward as of 2 AM PDT, but that is expected to
abruptly change as we approach sunrise this morning. The models
have backed off with QPF with this system as it loses steam
bumping up against a broad ridge over the western US. Nonetheless,
some light rain will spread into Puget Sound by midday with the
front as it moves through. For north King and Snohomish counties,
the best shot of precipitation may actually be with a brief
convergence zone that is expected to develop behind the front this
evening. High temperatures will be considerably cooler across the
interior today with 60s to near 70 common.

The front will shift east of the area tonight with shower activity
lingering...especially around the convergence zone. That activity
will dissipate by daybreak Thursday as post-frontal onshore flow
relaxes and upper level ridging begins to build back into the
area. We should see a return of some sunshine by mid or late
afternoon Thursday with high temperatures returning to near
normal. Upper level ridging will flatten by Friday as the first of
a series of systems pass by mainly to the north. It may produce an
increase in high and mid clouds later in the day Friday, but most
areas should see a partly sunny day with temperatures similar to
those of Thursday.

.LONG TERM /SATURDAY THROUGH TUESDAY/...Another system moves to
the north on Saturday. While still rather weak, heights will fall
further and there will be a chance of showers for the coast and
the interior from Seattle northward. Partly to mostly cloudy
conditions are expected with temperatures a little below normal.
Sunday looks much the same as the last in the series of upper
level disturbances passes through British Columbia.

And this is where things begin to change. For much of the past
few weeks, the longwave pattern has favored upper level troughing
to our west and a broad ridge over the interior west. As the
trough over British Columbia pushes eastward early next week, it
is expected to carve out a decent trough over the Northern Plains
states. In response to this, a fairly high amplitude ridge is
expected to develop over the Pacific Northwest with 500mb heights
well into the 580s Tuesday and beyond. In addition, a thermal
trough may develop along the coast with low level flow turning
offshore. The models have been hinting at this for a couple days
now. It is still several days out, but we may be looking at at
least a few very warm days to close out the month of August.
Summer isn`t over yet...meteorological or astronomical. 27


&&

.AVIATION...An upper trough and an associated weakening frontal
system will move through the area today. Weak high pressure aloft
will build over the area tonight. The flow aloft will be westerly.
Low level southerly flow ahead of the front will turn westerly
behind the front. The air mass is moist and stable.

Clouds will increase across the area today with lowering ceilings
as a front moves onshore. Low clouds will persist at the coast.
The interior should see increasing mid clouds at first this
morning then low clouds will become widespread this afternoon and
evening.

A weak Puget Sound Convergence Zone is possible tonight.

KSEA...High clouds early this morning. Mid clouds will develop
later this morning and then low clouds this afternoon. Low
clouds should persist tonight. Southerly wind 4-8 knots. Schneider

&&

.MARINE...A weakening front will move onshore today with small
craft advisory strength southerly winds. Onshore flow behind the
front will bring small craft advisory strength westerly winds to
the outer Coastal Waters and for the central and eastern Strait of
Juan De Fuca tonight. Winds in the strait will ease late tonight.
Winds over the outer Coastal Waters will ease Thursday but there
will probably be another round of small craft advisory strength
west to northwest winds for the Coastal Waters Thursday evening.
There will also be some 10 to 11 foot seas over the outer Coastal
Waters on Thursday.

An onshore flow pattern will continue Friday and into the weekend
with the usual diurnal small craft advisory strength west winds in
the Strait each evening. A weak frontal system will approach the
area on Friday and then dissipate over the area on Saturday. The
main impact will be to turn the winds a bit more southerly but
speeds should remain sub-advisory strength. Schneider

&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 11 AM PDT Thursday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm-Northern Inland Waters Including The San Juan
     Islands.

     Small Craft Advisory until 11 AM PDT this morning for Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville Out 10 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater Out
     10 Nm.

     Small Craft Advisory until 5 AM PDT Thursday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
     Thursday for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until noon PDT today for Admiralty Inlet.

&&

$$

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