Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 241028
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
328 AM PDT SAT SEP 24 2016

.SYNOPSIS...A strong ridge will bring dry weather today through
Monday. temperatures will warm each day with the warmest weather
in September possible on Monday for the interior. A marine push
will cool things down on Tuesday. An upper level trough moving out
of the Gulf of Alaska in the middle of the week will bring a
threat of more rain to the area late in the week.

&&

.SHORT TERM...Satellite imagery shows upper level ridge building
offshore with cloudy skies over most of Western Washington. There
is some clearing over the Northwest Interior and San Juans.
Temperatures at 3 am/10z were in the upper 40s to mid 50s.

Upper level ridge will continue to build today and move east to
the near shore coastal waters by 00z Sunday. Warm front off the
coast will weaken as it tries to move through the ridge with just
some mid level cloud cover for later this morning into the
afternoon hours. In the meantime the low cloud cover that is
already in place over the area will be tough to burn off this
morning. Model cross sections and the Sandpoint profiler show
light winds from the surface up to 850 mb. Surface gradients are
flat. The combination of these two factors will result in little
mixing in the lower levels today. Throw the mid level deck from
the weakening warm front over the top later this morning and the
end result will be a fairly cloudy day with just some filtered
sunshine in the afternoon. This will keep high temperatures near
or just a little below the normal for this time of year...in the
60s and lower 70s.

Ridge continue to build tonight into Sunday with 500 mb heights
near 590 dms by 00z Monday. Surface gradients remain light
overnight into Sunday morning. Mid level cloud deck from the dying
warm front hanging over the area overnight before dissipating
Sunday morning. With the clearing and the light surface gradients
expect fog to form especially in the fog prone places like the
Southwest Interior and the Snohomish River Valley Sunday morning.
Lows on Sunday morning will be in the mid 40s to lower 50s. What
fog that does form will not last through the morning hours.
Temperatures aloft continue to warm with model 850 mb temperatures
ranging from plus 12 to 16c by 00z Monday but surface gradients
remain northwesterly. With more sunshine and warmer temperatures
aloft highs on Sunday will be 5 to 10 degrees warmer than on
Saturday but without the low level flow turning offshore highs
will remain in the upper 60s and 70s.

Low level flow turning offshore briefly Sunday night into Monday
morning before turning back to onshore Monday afternoon. The
transition to onshore flow will cool high temperatures on the
coast down a few degrees. For the interior temperatures aloft
continue to warm with model 850 mb temperatures as high as plus
18c Monday afternoon. Transition to onshore flow too late in the
day to prevent Monday`s high temperatures from being the warmest
in the short term with the warmer locations getting into the lower
80s and the remainder of the interior in the 70s. For every other
day in September the highs on Monday would not be near records but
many locations have their coolest records of the month on the 26th
so it is possible some records could be broken.

.LONG TERM...Upper level ridge well to the east on Tuesday.
Marine push Monday night will bring low clouds back to the
interior Tuesday morning. With the low level onshore flow highs
Tuesday will be 10-15 degrees cooler than on Monday for the
interior and a few degrees cooler on the Coast...mostly in the
60s.

Upper level trough digging well offshore on Wednesday with the
flow aloft becoming southwesterly. Low level onshore flow weakens
resulting in less morning cloud cover on Wednesday. Temperatures
aloft cooling with 500 mb heights near 570 dms by 00z Thursday.
Wednesday looks to be a pretty normal late September day with
highs in the 60s.

Some changes to the forecast on Thursday with all of the models digging
the upper level trough further south keeping Western Washington
dry. Onshore surface gradients increasing keeping highs in the
60s.

Model solutions differ on Friday with the Canadian model bringing
the trough inland...the GFS moving the trough to just offshore and
the ECMWf digging the trough further south. Will stay with the
chance pops for now on Friday. If the model trends continue look
for the precipitation to get taken out of the forecast on Friday.
When the trough finally moves inland it has the potential to be
the coolest air mass to move into the area for awhile with snow
levels possibly as low as 5000 feet. Felton

&&

.AVIATION...Low level moisture and low clouds will give way to
drying air and just mid/high clouds for the afternoon. A weak
warm front will brush the area but high pressure will continue to
build over the region.

KSEA...Morning low clouds and then cirrus, light winds.

&&

.MARINE...High pressure has built over the area with light pressure
gradients. A warm front brushing by will have little effect on the
area. High pressure will strengthen into early next week.

&&

.CLIMATE...Thursday the 22nd was the latest date Seattle has ever
had a 90 degree plus day. Even 80 degree plus days are hard to
come by this time of year. Since records started at Sea-Tac in
1945 there has only been 21 total days with highs 80 degree plus
the last week of September and October. The last time Seattle had
a 80 degree plus day in the last week of September or October was
10 years ago...September 27th, 2006. So far this year Seattle has
had 38 days 80 degree plus which ties for the 8th most on record
at Sea-Tac. This is the fourth year in a row with a top 10 finish
for the number of 80 degree plus days in Seattle...2015 51 days
(1st)...2014 45 days (4th) and 2013 40 days (6th).

September 26th is the only day in September to not have an 80
degree plus day. The record for September 26th is 78 degrees which
has occurred 4 times...the last time in 2006. Felton

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory for hazardous seas until 11 AM PDT this
     morning for Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From Cape Flattery To
     James Island Out 10 Nm-Coastal Waters From James Island To
     Point Grenville 10 To 60 Nm-Coastal Waters From James
     Island To Point Grenville Out 10 Nm-Coastal Waters From
     Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory for rough bar until 11 AM PDT this morning
     for Grays Harbor Bar.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html


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