604
FXUS66 KSEW 171003
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
303 AM PDT Wed Jul 17 2024

.SYNOPSIS...An upper level disturbance will pass over Wednesday
with a chance for scattered thunderstorms along the Cascades and
Olympics. The mostly dry nature of these storms pose a critical
fire weather risk. Surface high pressure will remain the
predominant feature through the weekend. A longwave trough slowly
begins to approach from offshore into early next week, potentially
bringing cooler temperatures.

&&

.SHORT TERM /TODAY THROUGH FRIDAY/...A bit busier of a satellite
picture this morning as mid-level clouds are making their way from
south to north over western Washington. In the low levels, marine
stratus is evident over the southern half of the Washington coast,
slowly moving northeastward. Water vapor imagery shows the
shortwave vorticity maximum centered just offshore of southern
Oregon. As clouds continue to move into the region this morning,
cooling will be inhibited and low temperatures will only bottom
out in the mid 60s for most, mid to low 50s along the coast.
Temperatures as of 3AM are in the upper 50s to mid 60s.

The negatively-tilted shortwave trough will move northeastward
over Oregon and Washington throughout the day today, bringing with
it chances for thunderstorm development over the Cascades, with
the best chances in the early afternoon to early evening. With the
trough directly overhead by this evening, the atmosphere will
contain sufficient instability as well as increasing mid-level
moisture. A well-mixed and somewhat deep boundary layer (LCL
heights reaching above 2000 ft) will limit how much rain reaches
the surface, and this will also pose a threat of gusty downdraft
winds. The greatest threat of these storms will be lightning, with
implications for fire weather as well as outdoor recreation.
There is a lesser but non-zero chance of thunderstorm development
over the Olympics, especially after the morning inversion has
burned off. However, the Olympics will be farther from the main
source of forcing and overall model ensemble guidance is much more
confident in the thunder potential over the Cascades.

Wednesday will be another warm day through the interior, with
highs in the mid to upper 80s, reaching the low 90s through the
Cascade valleys and foothills. How warm temperatures get will
depend on the duration of cloud cover Wednesday morning, as well
as the degree of returning onshore flow Wednesday afternoon. The
best probability for seeing a high temperature of 90 or greater
will be across the south and eastern sides of the Seattle metro
area and into the eastern King County lowlands (reaching 35-45%).

Thursday and Friday look to be cooler with continued onshore flow
behind the through, pushing the marine layer inland each morning.
High temperatures Thursday and Friday look to be in the mid 70s
to mid 80s, cooler along the coast. Lows will be in the mid 50s to
near 60 in the Seattle metro.

.LONG TERM /SATURDAY THROUGH TUESDAY/... The large upper level
ridge centered over the Four Corners will amplify and retrograde
slightly, bringing another warm day to the area with highs in the
80s to low 90s. Temperatures look to begin to cool off starting
Sunday as a large upper level low begins to creep towards the
area. Ensemble cluster analysis reveals differences in how quickly
this trough will make in into the region. Expect a slow cooling
trend, with highs potentially as cool as the upper 70s to near 80
into the beginning of next week. Lows may reach the upper 40s to
low 50s.

LH

&&

.AVIATION...Southerly flow aloft with weak upper level disturbance
moving through later this afternoon into the evening hours. Flow
aloft becoming southwesterly behind the disturbance. Onshore flow
in the lower levels. Air mass somewhat unstable over the higher
terrain.

Stratus along the central and southern coast this morning
spreading inland into the Lower Chehalis Valley with IFR ceilings.
Stratus staying west of the Puget Sound and retreating back to
the coastline by 17z. Weak upper level disturbance this afternoon
could trigger some thunderstorm activity over the Cascades and to
a lesser extent the Olympics after 21z. Expecting the storms to
stay over the higher terrain with light southerly flow aloft.
Thunderstorm chances decreasing rapidly after 04z.

Increasing low level onshore flow today with stratus moving inland
after 06z with IFR ceilings getting into the Puget Sound after 09z
Thursday.

KSEA...Middle level cloud deck this morning thinning out this
afternoon. Stratus returning to the terminal around 09z Thursday
with IFR ceilings. Light northwest wind becoming southerly 6 knots
or less around 15z. Winds trying to come around to northwesterly
this afternoon ending up in the 250-280 degree range 22z-04z.
Winds returning to southerly 4 to 8 knots after 04z. Thunderstorms
over the Cascades 21z-04z could be in the approach path for
planes arriving from the east. Felton

&&

.MARINE...Onshore gradients increasing later this morning
through tonight as high pressure builds over the waters. High
pressure centered over the offshore waters Thursday through the
weekend with varying degrees of onshore flow.

Small craft advisory westerlies in the Central and Eastern Strait
of Juan de Fuca tonight and again Thursday night. Small craft
advisory westerlies also possible in the Strait Sunday night.
Felton

&&

.FIRE WEATHER...The overall pattern continues to be driven by
very persistent high pressure over the Pacific Northwest with warm
and dry conditions. Low-level onshore flow across the region has
maintained the marine layer helping limiting minimum relative
humidities and supporting good overnight relative humidity
recoveries.

The primary issue of concern remains the potential for thunderstorm
development Wednesday as an upper-level disturbance moves across the
region. This will enhance instability and promote the potential
for thunderstorm development over the Cascades and Olympic
Mountains - resulting in critical fire weather conditions. Given the overall
atmospheric instability, fuel conditions, and lightning risk with
very low chances of wetting rains a Red Flag Warning has been
issued for zones 658, 659, 652 and 661.

&&

.CLIMATE...In 80 years of records at Seattle-Tacoma airport this
was the warmest first half of July. The average temperature of
72.0 degrees was fractionally warmer than the previous warmest in
2015. The average high temperature of 85.3 degrees was two degrees
warmer than the previous mark also set in 2015. The average low
temperature of 58.7 degrees was 4th warmest ( 60.6 2015, 58.9
2019 and 58.8 2007 ). Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning from 8 AM this morning to 11 PM PDT this
     evening for East Portion of the Olympic Mountains-West
     Portion of the Olympic Mountains-West Slopes of the Central
     Cascades Generally above 1500 Feet-West Slopes of the North
     Cascades Generally above 1500 Feet.

PZ...Small Craft Advisory from noon today to 5 AM PDT Thursday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

&&

$$