Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 201139

Area Forecast Discussion
National Weather Service Seattle WA
339 AM PST Tue Feb 20 2018

.SYNOPSIS...A weak disturbance dropping in from the north will
bring generally light snow to mainly the southwest part of the
state through this evening. Expect a slightly stronger system to
impact the region during the Wednesday afternoon through Thursday
morning time frame. This system will exit the area Thursday
afternoon for clearing and drying from the north. Temperatures
will remain below normal.


Temperatures over the interior were generally 2-5 degrees colder
than what they were at the same time yesterday. On the coast,
temperatures were about the same or slightly warmer due to the
presence of clouds. KLGX radar showed an area of precipitation
just off the coast at this time. This precip was associated with a
weak upper level disturbance moving south across the offshore WA
waters. It will be off the Oregon coast by midday.

Meanwhile, a weak upper level trof over British Columbia will move
south over the CWA this afternoon and will be out of the area by
Wednesday. Expect precipitation to develop over mainly the
southern part of the CWA in response to the development of weak
diffluence aloft between the offshore disturbance and the one
dropping in from the north. Of course, the snow levels will remain
quite low; therefore, expect the precip to fall in the form of
snow or a rain/snow mix on the lowlands. The heavier precip is
expected over the southwest interior, where a Winter Weather
Advisory is in effect for up to 3 inches of snow. On the central
coast, snowfall amounts are forecast to be generally less than an
inch. Elsewhere across the lowlands, expect a dusting or nothing
at all.

Alas, another (but slightly stronger) system will head south
towards the area during the day Wednesday. The ECMWF solution was
the preferred model because it has been the most consistent with
this system and, also, the other models were starting to lean in
its direction. Expect light snow to develop over the northwest
part of the CWA during the day Wednesday and then quickly
overspread the remainder of the area Wednesday evening. The
lowlands are anticipated to pick up anywhere from a dusting to up
to 3 inches of snow Wednesday night from this system. At this
time, it looks like the heaviest amounts are expected over the
western part of the CWA.

Look for the upper level low to move south across the area on
Thursday, reaching Oregon by late Thursday afternoon. Thus, look
for clearing and drying from the north during the day Thursday.

The pattern looks quite active during this period due to a series
of storms moving across the region from the northwest. The
screaming message is that it will be chilly and wet. Expect a
strong system to impact the area the end of this week for the
possibility of significant lowland snow in some areas, especially
within any convergence zones. Another, possibly potent, lowland
snow-maker might impact the CWA early next week. For now, look
for below normal temps and above normal precip during this period.


.AVIATION...North to northwest flow aloft continues over
western Washington this morning. The air mass is stable with areas
of high and mid level moisture. A weak surface low passing southward
over the offshore waters today will spread some light precipitation
into the southern third of the forecast area this afternoon. Areas
of MVFR ceilings with light snow are expected after 18Z south of a
line KHQM-KTIW with VFR ceilings continuing northward of that line.
Ceilings will improve from the north after 03Z-06Z with VFR expected
across the region tonight. Near the surface, light east-southeast
gradients with turn northeasterly tonight as the offshore low moves
into the Oregon waters.

KSEA...VFR with increasing high and mid level moisture this morning.
Ceilings may lower to BKN-OVC040-050 in the 21Z-00Z time frame with
the odd light flurry in the vicinity of the terminal. No
accumulation is expected. Ceilings with lift this evening with
just scattered or broken high clouds into Wednesday morning. Light
southeast wind this morning will shift to north-northeast 7 to 11
kt this evening. 27


.MARINE...A 1018 mb low near 49N 130W will drop SE through the
Washington offshore waters today and move onto the central Oregon
coast tonight. Southeast winds up to 25 knots are expected at
times over the coastal waters today as the low passes by to the
west. Winds will back to northeasterly behind the low and will
increase to small craft advisory strength in the Northern Inland
Waters and through the western two-thirds of the Strait of Juan de
Fuca tonight. Small craft advisories have been extended through
tonight for the coastal waters and have been issued for tonight in
the Northern Inland Waters and in the western and central strait.

A weaker low will move rapidly southward through the waters
Wednesday night. At this time, winds throughout the waters are
expected to remain at or below 20 knots.

A stronger system will arrive Friday and will remain around the
waters through the upcoming weekend. This system is likely to
bring at least small craft advisory strength winds to all waters,
and could bring gale southerlies or westerlies to some waters.


River flooding is not expected the next 7 days.


WA...Winter Weather Advisory from 10 AM this morning to 10 PM PST
     this evening for Lower Chehalis Valley Area-Southwest

PZ...Small Craft Advisory until 3 AM PST Wednesday 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 from 4 PM this afternoon to 3 AM PST
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands-West
     Entrance U.S. Waters Strait Of Juan De Fuca.


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