Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 050318

Area Forecast Discussion
National Weather Service Seattle WA
818 PM PDT Tue Aug 4 2020

.UPDATE...The forecast is in good shape this evening with no
changes appearing necessary. The main forecast concern overnight
will be the extent of the marine push inland.

Water vapor imagery this evening reveals cyclonic flow dominating
the Gulf of Alaska, with a notable area of closed low pressure
within the center of the flow. Moving south from this expansive
feature lies a split flow pattern across the West Coast of the US
into Canada as ridging prevails over BC and a baggy trough lies
along the CA Coast, with another notable shortwave along the
northern CA Coast. At the surface, high pressure remains anchored
offshore across the Pacific, with thermal trough extending from
interior CA into eastern WA.

Above setup with allow for benign conditions overnight amidst
onshore flow regime. Current vis imagery shows stratus along the
central and southern portions of the WA Coast. Stratus will expand
in coverage overnight and blanket coastal locations, down the
Strait of Juan de Fuca and into portions of the SW interior. Does
not appear it will make significant progress east of the Sound.
tho could spill into portions of this area. Aside from these
morning clouds, expect another mostly sunny day, with clouds
increasing late in the day as a frontal system approaches. Lows
tonight in the 50s with highs in the upper 70s most lowland
locations (upper 60s near waters) Wednesday. Previous discussion
below with updated marine and aviation sections.



.PREV DISCUSSION... /issued 300 PM PDT Tue Aug 4 2020/

.SYNOPSIS...A short return to upper level high pressure is in
store for today and Wednesday ahead of the next low pressure
system to arrive on Thursday. Dry and seasonably warm temperatures
expected today and Wednesday. Cooler temperatures are expected with
the Thursday system.


.SHORT TERM /TODAY THROUGH THURSDAY/...Mostly sunny skies over
the majority of W WA this afternoon with just some lingering
marine stratus mainly just off the coast but a little bit still
lingers on the coastline between Forks and Hoquiam. Temperatures
today right on track with where they were yesterday for most
locations although some spots are a degree or three warmer.

Models still in good agreement as upper level ridging will shift
eastward today amplifying as it does so. This combination will keep
temps today and tomorrow pretty similar. An upper level trough will
make its way toward W WA early Thursday morning. While models still
leaning more for a Thursday morning entrance...decided to split the
difference with inherited forecast and compromise on late Wednesday
night. Still pretty lackluster instability with this feature so will
maintain forecast that best chances for thunder will be east of the
Cascade crest. This system will eject quickly with generally dry
conditions expected Thursday night. High temps thursday will be a
welcome break from summer...with interior highs ranging from the mid
60s to lower 70s.  18

.LONG TERM /FRIDAY THROUGH MONDAY/...Models...both deterministic
and ensemble...leaning dry for Friday and into the weekend as a
pretty weak upper level ridge moves in as the aforementioned
trough exits. This feature exits Saturday and while there is a
general troughy pattern in place over the region...neither model
really wants to bring anything organized into W WA..each showing
maybe a patch of PW here and there but nothing to hang a forecast
on. Something better organized looks to be in the works for
Monday...but...again...models are keeping the bulk of this system
to the north in BC. Would not take much variance in model
solutions however to bring this down into the merits
watching in future model runs. 18


.AVIATION...Evening satellite shows clear skies at all TAF sites,
with a thin area of stratus along the Coast just west of HQM.
Stratus will expand over the next few hours and through the
overnight hours, bringing ceilings down anywhere from LIFR to
MVFR into portions of the interior. Lowest ceilings likely
confined to coastal locations and SW interior, with HQM and OLM
having greatest chance for low ceilings. Elsewhere SCT-OVC sub-VFR
deck is expected. Stratus should retreat back to the Coast into
the afternoon, leaving all sites generally clear except HQM. A
frontal system will approach late Wednesday, bringing widespread
clouds and some rain just outside of this TAF cycle. North winds
tonight will become light overnight then shift to the S/SW
Wednesday 8-12kts (more W at HQM and CLM 10-15kt).

KSEA...Clear skies into a good portion of the overnight. Marine
stratus may reach the airfield and bring MVFR ceilings with it.
Confidence is rather low on this potential and should it
materialize, will likely only last a few hours early morning.
Generally clear skies by the afternoon with higher cloud over
increasing late. North wind this evening becoming light overnight
and picking up from the S/SW Wednesday morning 10kts or less.



.MARINE...No changes were needed to the marine forecast this
evening. The pressure gradient down the Strait of Juan de Fuca is
beginning to increase this evening, with several observations
indicating winds starting to pick up. Given this, the Small Craft
Advisory for the Central/Eastern Strait looks good overnight and
through much of Wednesday. Guidance continues to pick up
increasing south winds across the Northern Inland Waters Wednesday
morning so will leave that Small Craft Advisory as is, too. Winds
and waves elsewhere appear insignificant tonight and tomorrow.
Previous discussion included below:

Onshore flow will strengthen overnight. Small Craft Advisory
winds will continue tonight and into tomorrow for the Strait of
Juan de Fuca. Winds in the Northern Inland Waters are expected to
increase Wednesday morning with a Small Craft Advisory in effect
until Wednesday evening. Onshore flow within the Strait is
expected to strengthen Thursday afternoon into evening, there is a
possibility for gale force wind gusts in along the Eastern
entrance to the Strait. Conditions on Friday and into the weekend
are expected to improve with weaker onshore winds in the Strait as
well as along the coastal waters.



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


PZ...Small Craft Advisory until 11 PM PDT Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory from 5 AM to 11 PM PDT Wednesday for
     Northern Inland Waters Including The San Juan Islands.


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