Wildfire and Strong Wind in Washington, September 7 to 19, 2020

Area Forecast Discussion
National Weather Service Seattle WA
831 PM PDT Sun Sep 6 2020

.SYNOPSIS...An extended period of warm and dry weather is expected
across W Washington through the upcoming week. Strong ridging in
the upper levels combined with the development of lower level
offshore flow will make for critical fire weather concerns and
enhanced heat risk across portions of the area. A return to
onshore flow is possible by the weekend.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Current forecast on
track. No updates planned this evening. Previous discussion from
340 PM PDT Sun follows.

Water vapor imagery this afternoon reveals a strong, elongated
area of high pressure in the upper levels stretching from the
desert SW northwestward into the far NE Pacific. Downstream from
this trough lies a strong polar jet that separates the ridge from
more substantial troughing towards Hudson Bay. At the surface,
weak onshore flow continues over portions of the Pacific Northwest
as high pressure lies offshore and a thermal trough continues
over E WA.

Marine stratus have largely eroded across W Washington this
afternoon, per vis satellite loop, with the most extensive
area remaining along and just off the Coast. This has allowed for
a sunny day across the majority of the area, with high temps
topping off a few degrees higher than yesterday.

Changes are in store as we head into this upcoming week. Within
the stronger belt of flow between the high amplitude Western US
ridge and Hudson Bay trough will exist a rapidly digging shortwave
trough (seen in water vapor currently across far NE BC). As this
trough digs from Canada into the Northern and Central Rockies, its
attendant sfc cold front will also sweep across the large majority
of the northwestern US (a backdoor cold frontal passage for W
Washington). In the front's wake, an unseasonably strong area of
high pressure will begin to settle into portions of the
intermountain west. This will set the stage for the development of
a noticeably strong pressure gradient along the Cascades. With
high pressure to the east of the Cascade Crest, the pressure
gradient will be oriented E-W, or essentially become offshore/
downslope for W Washington. Initially, as the high drops through
Alberta/vicinity on Monday, breezy north to northeast winds
increase across the region through the day, with gusts to 30 mph
becoming common across the lowlands from north to south. The winds
will become increasingly from the east later Monday as the
offshore surface pressure gradient continues to increase. This
will lead to stronger winds Monday night and early Tuesday across
the Cascades and down into the foothills. Latest ensemble guidance
suggest gusts around 45 mph could mix down to the surface in
locations downwind of the Cascade gaps such as North Bend and
Enumclaw. Expect the development of critical fire conditions
through the day on Monday and into Monday night.

By Tuesday morning, the digging shortwave trough is expected to
develop into a closed low in the 4 Corners vicinity, with somewhat
enhanced ridge influence across the PNW. Winds will slowly relax
a bit across W Washington through the day on Tuesday, however,
downslope-oriented winds will begin to noticeably dry out the
local area. Critical fire conditions will remain in place.

By Wednesday, the closed 4 Corners low becomes cutoff from the
main belt of stronger westerly flow over Canada and the northern
tier of the US. This essentially creates an omega block over the
Western US/adjacent Pacific as the ridge remains along the West
Coast/PNW and a cutoff low continues on it upstream side in the
Pacific. This will essentially lock the upper pattern in, locally
making for synoptic scale subsidence over W Washington and rising
temps. Warming temps will allow for the development of thermal
trough along the West Coast which will allow easterly/downslope
winds to continue. As the core of the strong sfc high slides
towards the Central Plains, gradients will have relaxed further,
so while wind will not make for as big an issue as Monday
night/Tues, RH will bottom out in the 10-20 percent range Wed
afternoon. Critical fire conditions will continue. You can refer
to the Fire Weather portion of this discussion towards the bottom
of the page for more details on this event. Also reference the
RFWSEW for latest Red Flag Warnings.

Kovacik/Cullen

.LONG TERM /THURSDAY THROUGH SUNDAY/...Previous discussion...The
omega blocking pattern will continue into Thursday, though will
begin to dampen as a shortwave traverses across the Gulf of
Alaska. Another hot and dry day is expected with temperatures
expected to peak with highs in the upper 80s to mid 90s for most
lowland locations. Fire weather concerns are likely to continue.
By Friday, guidance is suggestive of a return to onshore flow,
allowing for slightly cooler temperatures, most noticeably along
the Coast. The ridge will be weaker but still appears to linger
over the region per ensemble guidance. This pattern will likely
carry us through the remainder of the extended forecast period. If
onshore flow does materialize, highs look to fall back towards 80
(around 70 close to water). This would help bring an end to an
extended period of critical fire weather concerns and late summer
heat risk.

Kovacik

&&

.AVIATION...An upper ridge will continue to build offshore
tonight and Monday. At the same time, an upper trough will deepen
over the northern Rockies. Northwest flow aloft for Western
Washington will turn more northerly tonight then north to
northeast on Monday. At the surface, weak onshore flow this
evening will switch to offshore later tonight and increase on
Monday as strong high pressure slides south out of Alberta into
western Montana and a thermal trough moves over the Coastal
Waters. A backdoor cold front associated with the inland upper
trough will push into Eastern Washington with Fraser outflow
developing for Western Washington on Monday.

The air mass will be stable and generally dry. There is currently
stratus along the immediate coast which might sneak into HQM this
evening but the developing offshore flow later tonight should put
an end to any low clouds Monday morning.

KSEA...Clear skies. Northerly wind 5-10 knots will rise to 12-22
knots with gusts to 30 knots Monday afternoon. Schneider

&&

.MARINE...Weak onshore flow this evening will switch to offshore
later tonight and Monday as a thermal trough sets up over the
Coastal Waters. Offshore flow will be northeasterly on Monday with
some Fraser outflow. It will then turn more easterly Monday night
and Tuesday as strong high pressure shifts south along and east
of the Rockies. Offshore flow will continue Wednesday and Thursday
but gradually weaken with time. Weak onshore flow could return
later Friday.

There will be north to northeast small craft advisory strength
winds for the inland waters and East Entrance Strait of Juan de
Fuca Monday and Monday night. There will then be a period of small
craft advisory strength easterly winds for the western half of
the Strait and parts of the inner Coastal Waters near gaps in the
terrain Monday night and Tuesday.

The outer Coastal Waters will also have northerly small craft
advisory strength winds through Tuesday evening or so with short
period seas possibly building to around 10 feet later tonight and
Monday. Schneider

&&

.FIRE WEATHER...Previous discussion...Breezy northerly winds will
develop Monday and then turn easterly Monday night as surface
high pressure over B.C. and Alberta drops south across the inland
portion of the Pacific Northwest. Thermally induced low pressure
will develop along the Oregon coast and Willamette through
Tuesday. Easterly winds through the Washington Cascades will peak
between midnight and mid morning Tuesday and then pressure
gradients will ease later Tuesday. Record high temps are possible
Wednesday and Thursday. Red flag warnings are in effect for
portions of Western Washington for Monday afternoon through early
Wednesday evening and the air will be rather dry with poor
nighttime recovery Monday night through Wednesday night. A switch
to light onshore flow with higher humidity in the lowlands could
occur Thursday night.

&&

.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...Red Flag Warning from 11 AM Monday to 8 PM PDT Wednesday for
     Black Hills and Southwest Interior Lowlands-Central Coastal
     Lowlands-East Portion of the Olympic Mountains-Northeast
     Puget Sound Lowlands Generally Below 1500 Feet-Southeast
     Puget Sound Lowlands Generally Below 1500 Feet-West Slopes
     of the Central Cascades Generally above 1500 Feet-West
     Slopes of the North Cascades Generally above 1500 Feet.

PZ...Small Craft Advisory until 11 AM PDT Monday 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.

     Small Craft Advisory from 7 AM to 8 PM PDT Monday for East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory from 8 AM to 8 PM PDT Monday for Admiralty
     Inlet-Puget Sound and Hood Canal.

&&

www.weather.gov/seattle