Scientific Forecaster Discussion
fxus66 ksew 211612
Area forecast discussion
National Weather Service Seattle Washington
913 am PDT Thu Jun 21 2018
Synopsis...moist marine air and clouds should give way to a partly
to mostly sunny skies and a cooler afternoon across western
Washington. Onshore flow will continue Friday, and an upper trough
will bring a chance of showers. An upper ridge will cross the area
this weekend for sunny warmer weather, followed by another upper
trough early next week.
Short term...current satellite shows West Washington pretty socked in with
marine stratus...and while they may thin over some locations late
this morning and this afternoon...current forecast looks to be on
track in thinking that most areas will likely see more clouds than
sun today. For interior lowland locations...this is expected to
result in much cooler high temperatures for this afternoon...with
many locations seeing temps on the order of 10 degrees cooler than
highs observed yesterday.
Looking at the models...a weak shortwave ridge overhead today will
keep the weather fairly inactive...aside from the aforementioned
marine influence. This feature will move out of the area tonight
allowing for an upper level trough to approach the area late tonight
and passing through Friday. Precip signatures not looking terribly
convincing for this system...with generally just slight chance pops
for much of the area. The exception being that there are hints for
the development of a weak pscz during the evening hours...with
chance pops in place over those portions of King and Snohomish
counties. This will fizzle out as the trough exits the area Friday
Models remain persistent bringing an upper level ridge into the area
Saturday bringing a return to mostly sunny skies. Temps will warm
somewhat although this warming will be limited by continued onshore
flow...with highs in the interior lowlands getting back into the mid
Inherited forecast handles all this well...and as such no morning
updates are needed. Smr
Long term...from previous discussion...the upper ridge will shift
east of the Cascades Saturday night or Sunday for a sunny and warmer
day, with highs in the 70s to lower 80s over much of the interior
lowlands; an exception to this is the north coast, where the leading
edge of the next system might arrive in the afternoon. A cold front
will move across western Washington Sunday night, followed by an
upper trough on Monday. The upper trough will probably keep a chance
of showers over parts of western Washington into Tuesday, with
another small upper ridge arriving on Wednesday. Mcdonnal
Aviation...weak westerly flow aloft will continue today and
tonight. Strong onshore flow will continue into Friday. The air mass
is stable and dry except for low level clouds.
Marine stratus moved inland to the Cascade crest in most areas
overnight. Ceilings this morning are generally 1-2k ft with a few
IFR spots. Ceilings will rise a bit later this morning, then some
breakout is likely over the interior for at least a few hours this
afternoon and evening. The stratus will spread back into the
interior late this evening and early Friday morning with ceilings in
the 1-2k ft range by 09z-12z Friday.
Ksea...ceilings around 1k ft rising to near 2k ft around 18z,
scattering out 20z-22z. Ceilings developing around 2k feet 07z.
Southwest wind 5-10 kt. Chb
Marine...onshore flow will continue through the weekend. Gale force
westerlies are forecast tonight in the Strait, and a Gale Warning is
in effect there, plus a Small Craft Advisory for the northern inland
waters. Small Craft Advisory strength westerlies are likely each
afternoon and evening in the Strait, with gale force westerlies
possible at times. For now have kept winds at 15-30 kt each night
after tonight. Chb
Pz...Small Craft Advisory until 2 PM PDT this afternoon for central
U.S. Waters Strait of Juan de Fuca-East Entrance U.S.
Waters Strait of Juan de Fuca.
Gale Warning from 2 PM this afternoon to 5 am PDT Friday 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 PM this afternoon to 5 am PDT Friday
for northern inland waters including the San Juan Islands.