Heat in Washington, September 5, 2024

Area Forecast Discussion
National Weather Service Seattle WA
934 AM PDT Thu Sep 5 2024

.SYNOPSIS...A strong upper level ridge combined with low level
offshore flow will lead to unseasonably warm temperatures through
Friday. The ridge will slowly weaken and slide eastward over the
weekend for increasing onshore flow and a cooling trend. Troughing
will develop offshore early next week.

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.UPDATE...No major changes to the forecast discussion below.
Temperatures have already reached the mid and upper 60s for most
locations (a few sites in mountain valley areas have temperatures
in the 70s to near 80 already, particularly in the South Olympics
and West Slopes of the Central Cascades). The heat advisory will
continue for today, as temperatures are expected to soar well into
the 80s and 90s for locations in Western Washington. Please see
the discussion below for more details.

HPR

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.SHORT TERM /TODAY THROUGH SATURDAY/...What a difference 24 hours
makes... A strong upper ridge axis is overhead and the switch to
low level offshore flow is well underway. A cursory look at some
of the current observations in the thermal belt of the Olympic
Mountains are quite impressive with widespread temperatures in the
70s at 2000 to 3000 feet and relative humidity values a paltry 15
to 25 percent. This is the kind of air mass that lower elevations
will be tapping into after the sun comes up and mixing begins.
Numerous locations are nearly assured to surpass daily record high
temperatures and it wouldn't be at all surprising if several
record high minimums occur as well. Thermally induced low pressure
at the surface will shift ever so slightly inland on Friday
allowing immediate coastal areas a cooling afternoon sea breeze,
but much of interior lowlands will see a near repeat of today's
temperatures. As is often the case with late season warm spells,
the duration will be relatively short as upper ridging weakens and
low level onshore increases. Models continue to hint at a weak
upper level disturbance lifting northward across the region late
Saturday. This could become the mechanism for some thunderstorm
activity in the central Cascades, but confidence isn't high enough
to include in the forecast at this time.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...A cooling trend continues
into Sunday. Upper level ridging centered well to our east will
maintain enough influence to keep the region dry, but temperatures
will be back closer to seasonal values. Ensembles continue with
the idea of anomalous upper troughing developing well offshore
early next week. Initially, it likely won't produce much more than
night and morning clouds and temperatures around normal. However,
by the middle of the coming week, we could be talking about a
frontal system with enough stamina to actually make it onshore and
produce some light precipitation. Stay tuned. 27

&&

.AVIATION...An upper ridge will continue to be located over the area
today with offshore flow in the lower levels. Lingering LIFR
stratus/fog in the Snohomish River Valley this morning with
generally SKC elsewhere. Clear skies will continue through this
evening. Elevated haze will increase across Western Washington into
this afternoon due to regional fires in OR and mainly S/SE flow
aloft. Otherwise, N/NE surface winds this morning will increase from
N/NW this afternoon.

KSEA...VFR conditions with clear skies. Elevated haze will increase
this afternoon into this evening due to ongoing wildfires in OR.
Surface winds will range 6 to 9 kts from the N/NW. JD

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.MARINE...Strong upper level ridging remains over W WA again
today as thermal pressure trough makes its way eastward,
continuing offshore/easterly flow over interior waters and
northerlies over the coastal waters. Some patches of early morning
fog possible...mainly within the Strait...but neither as
widespread nor as thick as yesterday.

Onshore flow returns Friday with surface high pressure building over
the coastal waters and the thermal pressure trough exiting the area.
The resulting northwesterlies over the coastal waters will allow for
wind speeds to increase up to around 20 kts...perhaps with isolated
gusts nudging into SCA criteria...but headline conditions not
expected to be widespread and thus no need for such at this time.
Surface flow will become southerly early next week as the next
disturbance brings a chance of rain showers to the region.

Seas generally 5 to 7 feet and remaining that way for much of the
remainder of the week before decreasing to 3 to 5 feet Sunday.
Expect shorter periods and more wind-driven seas today and Friday
once winds transition to northwesterlies.

18

&&

.FIRE WEATHER...Upper level ridging and an associated surface
thermal trough along the coast will allow conditions to become
increasingly warm, dry, and unstable. As fuels rapidly dry, these
conditions will combine to create critical fire weather conditions
over the mountains today. A return to onshore flow on Friday over
the Olympic Peninsula will begin to bring cooler, moist air to the
Olympics while the Cascades remain warm, dry and unstable.

Winds will also turn easterly over the Cascades. While they will not
be overly strong, but a period of easterly winds are possible this
morning as air funnels through the gaps over the Cascade crest.
Another period of offshore flow is expected later tonight into
Friday morning, but it looks to be less strong.

Red flag warnings remain in place through tomorrow due to continued
hot, dry and unstable conditions.

Cooler and more moist air will return area-wide through the weekend
with the potential of some wetting rains by the middle of next week.

LH/27/McMillian

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.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Heat Advisory until 11 PM PDT this evening for Central Coast-
     Eastern Strait of Juan de Fuca-North Coast-Western Skagit
     County-Western Strait of Juan De Fuca-Western Whatcom
     County.

     Heat Advisory until 11 PM PDT Friday for Bellevue and Vicinity-
     Bremerton and Vicinity-East Puget Sound Lowlands-Everett
     and Vicinity-Hood Canal Area-Lower Chehalis Valley Area-
     Olympics-Seattle and Vicinity-Southwest Interior-Tacoma
     Area-West Slopes North Cascades and Passes-West Slopes
     North Central Cascades and Passes-West Slopes South Central
     Cascades and Passes.

     Red Flag Warning until 8 PM PDT this evening for East Portion of
     the Olympic Mountains-West Portion of the Olympic Mountains.

     Red Flag Warning until 11 PM PDT Friday for West Slopes of the
     Central Cascades Generally above 1500 Feet.

     Red Flag Warning until 11 PM PDT Friday for West Slopes of the
     North Cascades Generally above 1500 Feet.

PZ...None.
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