Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
415 am PDT Sat Sep 20 2014
Synopsis...upper level ridge over western Washington today shifting
east on Sunday. Low level offshore flow today will push highs well
above normal. The low level flow will turn onshore on Sunday with
cooler temperatures along the coast and another warm day for the
interior. A weakening front will arrive on Monday. A stronger system
will reach the area on Wednesday.
Short term...satellite imagery shows low clouds and fog have been
on the decrease overnight. At 10z/3am most of the stratus and fog
was confined to the coast...Strait and northwest interior with
temperatures in the 50s and lower 60s.
Offshore surface gradients strengthening today as a thermally
induced trough expands northward up the coastline. VAD wind profile
from the katx radar shows north winds to 20 knots at 1000 feet. This
matches up well with the model cross sections over the area this
morning. The combination of increasing offshore flow and good
northerly winds right off the ground will make it tough for more fog
to form this morning over the area. Will cut back on the areal
coverage of the fog. With the low level flow expect the most
persistent fog and low clouds to be in the Strait with the northerly
flow upsloping against the Olympics. Temperatures aloft warming
today with the 850 mb temperatures as warm as 20c by 00z Sunday. 850
mb winds becoming more easterly during the day with increasing
offshore flow at the surface. End result of all these variables is a
warmer day for western Washington today with highs well above
normal...in the 70s and lower 80s in most places. The coolest
location will be the Strait of Juan Delaware fuca with the coast having
the potential to be the warm spot.
Upper level ridge beginning to shift east tonight with the low level
flow remaining offshore. This will help keep low temperatures up
despite the clear skies with 50s common and only the colder
locations dropping down into the upper 40s.
Bit of a tricky forecast on Sunday as far as the highs go for the
interior. Upper level ridge continuing to move to the east on
Sunday. Low level flow starts off the day offshore but by 00z Monday
the thermally induced trough has moved east of the Cascades and the
low level flow has turned onshore. Transition to onshore flow will
result in a stratus surge up the coast on Sunday cooling highs back
down to near 70 degrees. Highs for the interior on Sunday will be
dependent on when the transition to onshore flow takes places. With
the current forecasted transition in the afternoon will result in
highs a couple of degrees warmer on Sunday versus saturdays readings
..70s to middle 80s. A little quicker transition to onshore flow than
forecasted will keep highs near saturdays maximum temperatures.
Low level onshore flow continuing into Sunday night with a marine
push bringing stratus into the interior overnight into early Monday
morning. Lows will be mostly in the 50s.
Mostly cloudy day in store for Monday with the stratus in the
morning followed by middle and high level clouds in the afternoon as a
weakening front approaches the area. Highs much cooler over the
interior with 10-15 degrees of cooling putting highs in the middle 60s
to lower 70s.
Weakening front moving through the area Monday night for a chance of
showers with lows in the 50s.
Long term...extended models in good agreement on the 00z run. Both
the GFS and European model (ecmwf) staying with the idea of a break on Tuesday with
the next system approaching the area late in the day. This will
induce a little low level offshore flow pushing highs into the lower
to middle 70s over the interior with a little cooler temperatures along
the coast. Large upper level trough setting up off the coast Tuesday
night and remaining off the coast for most of the remainder of the
extended period. Much stronger and more organized system spinning
out of the trough moving through western Washington on Wednesday
with the precipitation out ahead of the system moving into the area
Tuesday night. With the trough remaining offshore chance of rain
will stay in the forecast on Thursday. Both models have the trough
moving inland on Friday which would lower the probability of precipitation. Will wait for a
couple of more similar solutions before taking the chance of rain
out of the Friday forecast. Felton
Aviation...an upper ridge over the area will shift inland tonight
with increasing southwest flow aloft. At the surface...offshore flow
will prevail today and tonight with a trough along the coast. Air
mass stable with areas of low level moisture this morning.
Skies have been clearing overnight as offshore flow strengthens.
Clearing has allowed fog to form in some locations but it has been
somewhat patchy so far. The fog will probably increase a bit through
the early morning hours...then burn off by middle to late morning.
Ksea...skies have cleared as offshore flow increases. There is a
chance some fog could affect the terminal for a few hours this
morning which will be covered with a tempo group in the taf.
Northerly wind 4-8 knots...increasing to 8-12 knots this afternoon.
Marine...low level offshore flow will prevail today and tonight
with a surface trough along the coast. Small Craft Advisory east
winds are likely for the western straight and adjacent coastal
waters zone tonight and Sunday morning. There will also be some 10
foot west swell over the coastal waters this morning.
Onshore flow will return Sunday as the trough along the coast shifts
inland. There could be a period of Small Craft Advisory southerlies
for the coastal waters Sunday as onshore flow develops.
A weak front will cross the waters Monday. A stronger frontal system
will move through the area Tuesday and Wednesday. This system could
bring gale force southerly winds to the coastal waters and Small
Craft Advisory winds to parts of the inland waters. Schneider
Pz...Small Craft Advisory for hazardous seas coastal waters.
You can see an illustrated version of this forecast discussion at