Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
830 am PDT Friday Jul 29 2016
Synopsis...today will be sunny and warm. Low level onshore flow
will become strong this evening and bring marine stratus further
into The Lowlands both Saturday and Sunday. An upper level low will
move across western Washington on Monday and Monday night, bringing
showers and possibly a few thunderstorms. Sunny skies and higher
temperatures will return by the middle of next week.
Short term...stratus this morning is confined to the southwest
interior and the coast, and will be gone from even those places in a
few hours. With 500mb heights only a little lower than yesterday,
high temperatures over the interior this afternoon will be close to
those on Thursday--mostly 80s, perhaps touching 90 in the south.
Onshore flow will strengthen this afternoon which should keep highs
on the coast around 70.
Stronger onshore surface flow will bring marine stratus further
inland by Saturday morning. Heights on Saturday will be in the 570s
and highs will be at least ten degrees lower than Friday. Strong
onshore flow continues Saturday night into Sunday, which should give
pretty widespread marine stratus Sunday morning. Heights will be
about the same on Sunday and so will temperatures. There will be
afternoon clearing on both Saturday and Sunday with perhaps a bit
less on Sunday. Burke
Long term...previous discussion...most significant weather feature
for the next 7 days will be an upper low that models have
consistently shown moving across northwest Washington on Monday
afternoon and Monday night. The upper low will be preceded by strong
height falls and large-scale lift supported in part by upper level
divergence on the east side of the low. Marine moisture will deepen
significantly with marine clouds surging into the interior early
Monday. Large-scale lift and available moisture will promote quite a
bit of shower development on Monday afternoon and evening. Models
show enough instability near the track of the low center for a few
thunderstorms. Right now, the northern half of the forecast area
looks most favorable for thunder in the afternoon and evening.
Models show quite a bit of mid-level speed shear, which could
support some loose convective organization in the form of short
lines or multi- cellular clusters.
The upper low will exit into southern Alberta on Tuesday, followed
by a healthy dose of height rises over the pac northwest. Should still
be a few lingering weak showers during the day, but showers will
end quickly by Tuesday evening. Northwest flow aloft and a deep marine
layer will lead to below normal temperatures on Tuesday. For
Wednesday and Thursday, moderate onshore flow will prevail with
seasonable upper heights. This will ultimately lead to a rebound
to near normal temperatures and seasonably rain-free weather.
Aviation...dry stable air except for patches of coastal marine
stratus and fog--mainly from near Grays Harbor and south early this
morning. These should thin out through the morning hours and reveal
clear skies by early afternoon. The remainder of western Washington
will be under clear skies for much of the day today.
Marine layer cloudiness should develop along the coast this evening
and tonight...then push inland somewhat by daybreak Saturday. Model
guidance is in some disagreement on this and have opted for a more
pessimistic solution that would bring some stratus as far east as
sea-tac...thus giving rise to possibility of MVFR conditions in some
Ksea...clear skies with northerly winds increasing to 5-10 kts by
19z. Winds expected to become more southerly overnight tonight. Some
marine stratus possible by 12z Saturday...dropping cigs down to
Marine...high pressure is offshore with lower pressure east of the
Cascades. Just this set up alone will make conditions prime for
Small Craft Advisory winds over coastal waters. As onshore flow
increases tonight with an upper level trough moving into the
area...expecting gales over the northern and central outer coastal
waters so Gale Warning for later today will remain in place.
Inherited forecast suggests that those gales may not die down all
that much by the time inherited warning is set to expire Saturday
afternoon...suggesting that an extension may be necessary. Will
leave that for the afternoon package at the earliest. There is also
a Gale Warning for the Strait of Juan de Fuca for a strong westerly
push tonight. Current solutions show that another round of gales may
be possible for Saturday night in the Strait as well. Smr
Pz...Small Craft Advisory in effect through late tonight for inner
coastal waters and southern outer coastal waters.
Small Craft Advisory in effect through this afternoon for
central and northern outer coastal waters.
Gale Warning from 5 PM PDT this afternoon through Saturday afternoon
for central and northern outer coastal waters.
Gale Warning from 5 PM PDT this afternoon through late tonight for
central and eastern Strait of Juan de Fuca.
You can see an illustrated version of this discussion at