FXUS66 KSEW 032224

Area Forecast Discussion
National Weather Service Seattle WA
324 PM PDT Sat Jun 3 2023

.SYNOPSIS...Upper level troughing will continue through the
weekend, with low level flow transitioning to northerly flow on
Sunday. Fire weather conditions will become more favorable Sunday
into early next week as breezy winds combine with drying fuels and
low relative humidity values. A ridge will slowly build into the
area by mid-week bringing a warming trend with continued dry
conditions. Afternoon and evening shower and isolated
thunderstorm potential will also increase through the latter half
of the week, with a return to a Pacific trough influence by next


.SHORT TERM /TONIGHT THROUGH TUESDAY/...The mid and upper levels
of the atmosphere this afternoon are characterized by an area of
closed low pressure along the BC Coast, while downstream from this
feature exists a fairly strong ridge of high pressure over the
northern tier of the US and into central and eastern Canada. Near
the surface, a weak frontal boundary was noted near the WA Coast,
while high pressure dominates much of the US West Coast and
troughing continues east of the Cascades across E WA and E OR.

Abundant sunshine this afternoon across most of W WA this
afternoon, especially east of the Olympics. Closer to the Coast,
more cloud cover is present, given the lifting of residual marine
stratus and cloud cover associated with the weak frontal
boundary. Through the evening and into the overnight period,
weather conditions will be generally benign with just some passing
thin cirrus across the area and continued lower level cloud cover
near the coast. A shortwave trough will round the periphery of
the aforementioned broader closed near the BC Coast. This system
will pass by hardly noticed, although it may be able to squeeze
out a shower or two near or just north of the US/Canada border.
Onshore flow will strengthen behind it, bringing mostly marine-
related impacts to the area and some stratus to the coast. Given
mostly clear skies, expect low temps to drop into the upper 40s
for most tonight (low 40s down near the Chehalis Valley and south
of Olympia.

Weak, loosely organized troughing will continue in the mid and
upper levels on Sunday, with very weak shortwave activity passing
through. Onshore flow will quickly transition to a more northerly
flow, becoming breezy by the afternoon, especially nearest water
and around the Puget Sound area. These conditions combined with
persistent dry conditions will elevate fire weather concerns,
particularly south of Puget Sound towards the OR border where
the breezy N/NE winds will coincide with relatively dry fuels and
min RH values below 30%. See the fire section below for more
details, but have issued an SPS to account for the elevated fire
threat in this area. Elsewhere, fire starts will still remain

Another swift-moving shortwave trough will skirt the area Sunday
night and Monday, bringing brief height falls to the northern
counties. This feature will also pass largely unnoticed with dry
conditions continuing and lows dropping back to similar values as
the previous night. Weak upper level troughing to continue on
Monday with low level winds remaining breezy out of the north.
Elevated fire weather concerns remain, especially south of Puget
Sound. Very little change then expected synoptically in the upper
levels for Tuesday, however, a warming trend is expected as
thermal troughing builds northward, making for high temps back
into the 70s (80s for areas south of Puget Sound). Elevated fire
concerns to continue.


build westward into the region on Wednesday, supported largely by
ensemble guidance, with center remaining over the central US. To
the west/southwest of this feature will exist a closed area of low
pressure, with southerly flow on its eastern periphery extending
into OR. Wednesday will be the warmest day of the week as
subsidence increases from ridging aloft. Moisture advection from
the south could help produce a few afternoon and evening showers
and thunderstorms across the southern Cascades, but current
forecast confidence in this is low as the deepest moisture looks
to remain displaced just to the south. Winds will be light before
transitioning back to onshore by the evening. Fire weather
conditions will likely still remain elevated given warm and dry

Upper level ridge influence will continue into Thursday, making
for continued warm and dry conditions. Daytime heating combined
with orographic support may help produce showers and isolated
thunderstorms over the Cascades, aided in part by moisture
advection from the weakening closed low to the south. Stronger
onshore flow in the low levels should knock temperatures down a
good 5 degrees or so from Wednesday. More of the same expected for
Friday, except with perhaps a slightly more favorable environment
for showers across the Cascades as well as slightly cooler
temperatures area-wide as low level onshore flow continues and
upper level trough influence over the Pacific increases and nudges
the ridge slightly eastward.

Upper level trough influence may increase a bit more into
Saturday, as hinted in the ensemble solutions. This may again help
aid in some precip over the higher terrain as well as increase
cloud cover along the coast.



.AVIATION...Southwest flow aloft continues as upper trough west of
Haida Gwaii gradually shifts eastward. The air mass is dry and
stable. Increasingly onshore today will likely bring some stratus a
little further inland tonight for areas of IFR or low MVFR west of
Puget Sound Sunday morning. Any stratus will quickly dissipate by
late Sunday morning for VFR regionwide in the afternoon.

KSEA...VFR. Surface winds W/NW 5 to 10 knots this afternoon turning
northerly tonight...then increasing to 10-15 knots Sunday afternoon.



.MARINE...Surface ridging parked over the coastal and
offshore waters combined with lower pressure east of the Cascades
will continue to produce onshore flow through the weekend. The
onshore gradient will increase today into Sunday in response
to a weak system moving onshore over British Columbia. This will
produce westerly gales over the central/east strait and
small advisory level winds over adjacent waters. In addition,
northwesterlies will increase over the coastal waters as surface
ridging strengthens offshore and thermally induced low pressure
expands over northern CA and OR.

The thermal trough to our south will expand northward early next
week turning winds more northerly across area waters.   27


.FIRE WEATHER...An area of low pressure along the BC Coast this
afternoon will move slowly eastward through tonight, while at the
surface onshore flow will strengthen. By Sunday, low level winds
will then begin to turn northerly and will then become breezy by
the afternoon. These winds combined with an overall already dry
atmosphere column will cause min RH values to drop to around 25%
across Puget Sound and lowland and foothill locations to the south
towards to OR/WA border. South of Puget Sound, ERC values
continue to creep towards 90th percentile values, raising the
threat for elevated fire weather concerns. An SPSSEW product has
been issued to highlight the potential for quick fire starts and
spreads, particularly across this area Sunday afternoon and
evening. Elsewhere, fire starts will still be possible.

No improvements to conditions are expected into Monday, with fire
starts possible area-wide, but with highest concern continuing to
be south of Puget Sound to the OR border. Will continue to
monitor conditions for possible need for a watch, but given some
of the fuel moisture profiles will hold off for now. Overall,
expecting breezy N/NE winds to continue with afternoon RH values
dropping to near 20%. Conditions will likely remain similar for
Tuesday and potentially on Wednesday, given continued warm and dry



.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed.


PZ...Small Craft Advisory until 2 AM PDT Sunday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 7 AM PDT Sunday for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 7 AM PDT Sunday for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 11 AM Sunday to 8 PM PDT Monday 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.



NWS SEW Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather