Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 131544

Area Forecast Discussion
National Weather Service Seattle WA
844 AM PDT Mon Jul 13 2020

.UPDATE...No major changes were needed to the forecast this
morning. Synoptically, western Washington is caught in dry NW flow
aloft as a large area of low pressure remains centered roughly
along the northern Alberta/Saskatchewan border and high pressure
remains centered over the far N Pacific. Some weak disruption can
be noted within the NW flow regime over the Pacific Northwest
using water vapor imagery, with IR satellite capturing some of the
passing mid and high clouds associated with this. Aside from some
of these passing higher level clouds today, expect a mostly sunny
day with highs in the low to mid 70s across the lowlands (mid to
upper 60s near water). Previous discussion available for reference
below with an update to the marine and aviation section.



.PREV DISCUSSION... /issued 316 AM PDT Mon Jul 13 2020/

.SYNOPSIS...High pressure aloft over the Pacific will allow for
dry conditions to continue into at least mid-week with slightly
warmer temperatures. Some weak systems passing to the north of the
area may allow for scattered showers at times for the second half
of the week.


.SHORT TERM /TODAY THROUGH WEDNESDAY/...Current satellite shows
mostly clear skies over W WA this early morning  with some passing
scattered high clouds south of the Seattle metro area.

High pressure aloft over the Pacific will strengthen today and into
Tuesday...amplifying the resulting ridge and expanding its influence
over W WA for the entire short term period. This will leave partly
to mostly sunny skies over the CWA and steadily increasing daytime
highs...with Wednesday still forecast to be the warmest day.
Interior lowland highs will start off in the lower to mid 70s
today...then into the mid to upper 70s Tuesday before peaking
Wednesday as highs approach 80. High temps for coastal locations
will remain mostly steady in the lower to mid 60s.

.LONG TERM /THURSDAY THROUGH SUNDAY/...The upper ridge flattens
and makes its way eastward...exiting the area late Wed/early
Thursday. Deterministic models disagree on what...if any...impact
the approaching upper level trough and associated front will have
on the area Thursday as the ECMWF holds the feature together
better allowing for some rain to enter the area while the GFS
dissolves the feature...keeping the CWA dry. Ensembles lean toward
the ECWMF solution...although they are about 6 hours slower with
the feature bringing the front into the area Friday evening and
overnight. This feature exits Friday morning and while there may
be a slight chance/chance of some lingering showers...generally
dry conditions look to resume for most the area for the remainder
of the long term as another Pacific ridge sets up shop just off
the coast.

The aforementioned system will cool temps somewhat...with interior
lowland highs for most of the long term sitting generally in the
lower to mid 70s.  18


.AVIATION...Continued northwesterly flow aloft continues through
Tuesday as high pressure remains centered offshore. Meanwhile,
onshore low-level flow remains in place. This will maintain the
clear skies that are generally across the region today. A few areas
of shallow stratus remain along the Strait of Juan de Fuca and in
portions of the Southwest interior/lower Chehalis Valley through 17-
18z, but this too should clear quickly. Otherwise, a return of
shallow marine stratus to the immediate coastline and as far as
around KHQM, but not much of an onshore push into the interior

KSEA...Clear skies continue today with northerly winds.  Cullen


.MARINE...Varying degrees of onshore flow this week with high
pressure over the coastal waters and lower pressure inland. Expect
diurnally driven increases in west winds through the Strait of Juan
de Fuca each evening, leading to advisory strength winds through the
central and eastern portions of the Strait. Over the coastal waters,
northerly winds persist through the next several days, with winds
largely remaining below advisory thresholds. Expect seas to remain
generally 4 to 6 feet over the coastal waters, largely dominated by
a short period swell.    Cullen


.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 from 8 PM this evening to 5 AM PDT Tuesday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.


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