Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 211024

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT TUE JUN 21 2016

.SYNOPSIS....Weak high pressure will build over the region today
bringing a return to dry conditions through Wednesday. Another upper
level low will bring another round of showers Thursday and Friday as
it moves through the area Thursday night. An upper level ridge will
build over Western Washington for warmer and drier weather this
weekend into early next week.


.SHORT TERM...Showers associated with exiting upper level Low
continue to diminish this early morning as said Low continues to
move eastward. As the transition from exiting Low to entering weak
ridge commences...does not seem as though inland push of marine
stratus may be as strong as previous shifts had thought. Current obs
from HQM at the time of this writing suggest BKN to OVC bases are higher than expected. Taking this into
account...have opted to diminish coastal cloud cover from what was
inherited for this morning...with skies generally clearing in the
afternoon. Did leave in the prospect for some patchy drizzle along
the coast.

This weak ridge will dry things out for today and keep conditions
dry into Wednesday before transitioning eastward and replaced by
another upper level Low. Temperatures today and Wed will be fairly
similar...with temps right around 70 in the interior lowlands. The
incoming upper Low will bring cooler temps and yet another round of
showers for the second half of the week.

Aside from the previously mentioned tweaks for this morning...very
little in the way of changes made to the inherited forecast.  SMR

.LONG TERM...It is nice to see the extended models playing nice for
a they are in agreement for the weekend. Low pressure
moving through on Friday and taking associated showers with it by
late Friday night/Saturday morning. In its place...upper level
ridging once again sets up shop over the region. The amplitude and
intensity of this ridge as it builds over the weekend and into the
first half of next week varies from model to model with the ECMWF
favoring a weaker...yet still dry...solution while GFS and Canadian
opt for a higher amplitude. The main impact of course being a return
to warm summer temperatures...with interior lowland locations
hitting the mid to upper 70s.  SMR


.AVIATION...Light northwest flow aloft over Western Washington
early this morning, as an upper trough moved across the forecast
area and east of the Cascades overnight. A small upper ridge over
the offshore waters will move inland today, and the upper flow
will back to southwest tonight ahead of a weak front and upper
trough approaching from offshore.

The area of convection that formed over the Western Washington
interior Monday evening evolved into a Puget Sound convergence
zone after midnight, and at 2 am it was moving into the Cascades
of King and Snohomish Counties. The air mass will stabilize and
dry somewhat today and tonight, but there will still be
considerable moisture below about 5000 ft.

In addition, marine stratus has not pushed inland overnight as
expected. The details of the cloud cover will be difficult to pin
down, but in general the forecast -- which was calling for
widespread MVFR conditions with ceilings mainly 012-020 -- will
probably not be so pessimistic. The morning clouds should burn
off late morning through afternoon for VFR conditions with
ceilings mainly above 120 by late afternoon.

KSEA...South wind 6-12 kt, shifting to to northwest 5-10 kt late
this afternoon and becoming northerly around midnight. We are less
confident that marine stratus will be solid this morning, but
still forecasting BKN013 beginning around 12Z. McDonnal


.MARINE...Onshore flow developed overnight. Brief westerly gales
occurred in the central and eastern Strait of Juan de Fuca after
midnight. The wind there has eased a bit and the UIL-BLI pressure
difference -- which peaked at 1 am -- has started to decrease. The
gale warning will end at 3 am. Onshore flow will weaken slightly
today, but small craft advisory westerlies will continue in the
central and eastern Strait most of today and tonight.

Onshore flow will weaken a bit today, but continue through
tonight. A weak front will move through the coastal waters
Wednesday and then move slowly inland Wednesday night and
Thursday, but onshore flow will prevail through the end of the
week. McDonnal


.SEW Watches/Warnings/Advisories...

PZ...Small Craft Advisory until 6 AM PDT early this morning for
     Admiralty Inlet-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-Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until midnight PDT tonight for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.



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