Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 191705 CCA
AFDSEW

Area Forecast Discussion...CORRECTED
National Weather Service Seattle WA
1005 AM PDT Mon Jun 19 2017

.SYNOPSIS...High pressure aloft will prevail over the area today
for warmer weather. A weak cold front will bring a threat of
showers to the region mainly later tonight and Tuesday. Expect
onshore flow to keep temperatures near normal Tuesday and
Wednesday. Anticipate a warming trend the latter part of this
week.

&&

.SHORT TERM...
The stratus and/or fog over the lowlands from the Puget Sound
region southward was slowly lifting this morning. Expect skies to
become partly sunny over this area by noon today. Look for clouds,
associated with a low pressure system centered over the Gulf of
AK, to continue streaming across the northwest half of the CWA
today. There may be some light showers at times from these clouds,
so will update the forecast for this possibility. Elsewhere,
expect mostly sunny to partly cloudy skies.

The air mass is warmer with an upper level ridge over the region.
Afternoon temps will reach the 70s over the interior lowlands,
with a few lower 80s away from the Sound. Expect highs in the mid
60s along the coast with onshore flow.

The ridge over the area will flatten on Tuesday as a cold front
moves inland. This front will bring isolated to scattered showers,
mainly during the morning. Onshore flow will increase behind the
front Tuesday afternoon and may see a Puget Sound Convergence
Zone form over the central Sound. The stronger push will bring
marine clouds farther inland on Wednesday for near normal temps.
33/05

.LONG TERM FROM THE PREV DISCUSSION...
The weather remains dry on Thursday as an upper level ridge
builds offshore. Pressure gradients are weaker with less stratus
coverage during the morning. Temperatures will remain seasonal.

An upper level ridge over the Pac NW will bring dry and stable
weather over the weekend with a slight warming trend expected.
Guidance shows highs reaching the lower to mid 80s Friday through
Sunday, with mid 60s to 70s along the coast. Models have been
waffling on the placement of a closed upper level low late in the
period. Current solutions show the low parked over B.C. early next
week which drop down into WA by midweek. The result is cooler
conditions as heights drop into the 570s. 33

&&

.AVIATION...An upper ridge over the area today will weaken as it
moves eastward tonight and Tuesday. Southwest flow aloft. A
weakening cold front now nearing the Washington offshore waters
will push east across the area on Tuesday. The air mass is dry
with widespread low level moisture this morning. The lower levels
will dry this afternoon and tonight as mid and high level moisture
increases from the west ahead of the incoming frontal system.
The air mass is stable.

Morning visible satellite imagery shows widespread low stratus and
fog in the Chehalis gap and in the interior from south of KNUW
southward along the I-5 corridor, especially from the Puget sound
eastward. Observations and pilot reports show bases around FL004
and tops 015-017. With a strong June sun, local applications show
the stratus rapidly burning off and scattering out between 18Z and
19Z with conditions across the region becoming VFR. Increasing
mid and high level clouds now seen from just west of KHQM to KBLI
this morning will gradually spread inland this afternoon and
tonight.

There is uncertainty about how much low stratus and fog will form
into the area early Tuesday morning. Onshore flow that develops
late this afternoon and tonight will advect more marine moisture
from the coastal waters inland overnight. This supports the LAMP
guidance that shows rather widespread IFR to LIFR conditions
developing late tonight and continuing into Tuesday morning ahead
of the incoming frontal system. However, the increased mixing
ahead of the front combined with abundant mid and high level
moisture favors MVFR stratus as shown in the current terminal
forecasts. Will stay with the current thoughts in the terminal
forecasts for now and reevaluate conditions for the 00Z package
this afternoon. Albrecht

KSEA...Ceilings near the deck and tops around 1700 feet this
morning favor a quick burn off and scatter out of stratus and fog
shortly after 18Z. Mid and high level clouds will gradually
increase late this afternoon and tonight. Will maintain the idea
of MVFR stratus developing at the terminal late tonight through
Tuesday morning. Northerly winds 5 to 9 knots through this evening
will become southerly between 06Z and 09Z tonight. Albrecht

&&

.MARINE...High pressure well offshore combined with lower pressure
east of the Cascades will give increasing onshore flow this
afternoon and tonight. A small craft advisory remains in effect
for the central and eastern portions of the Strait of Juan de
Fuca for tonight.

A weak front will move through the waters on Tuesday. Onshore flow
is expected to increase behind the front Tuesday afternoon and
evening. A gale watch remains in effect for the central and
eastern portions of the Strait of Juan de Fuca from Tuesday
afternoon into Tuesday evening. Small craft advisory westerly
winds are expected over most of the remainder of the waters late
Tuesday into Tuesday night.

Moderate onshore flow will prevail on Wednesday with the onshore
flow weakening on Thursday. The low level flow will turn
northerly on Friday with high pressure over southern British
Columbia. Albrecht/Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to noon PDT Tuesday
  for the central and eastern Strait of Juan de Fuca.

 Gale Watch from Tuesday afternoon through Tuesday evening for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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