Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 211040
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
340 AM PDT Mon May 21 2018

.SYNOPSIS...An upper level ridge will build over the area
through Tuesday for sunny and warmer weather. Southerly flow
aloft will develop midweek as an upper level low forms off the
California coast bringing high level moisture and instability
northward for a chance of mountain showers. Mild and generally
dry weather will continue for the lowlands. The shower chances
will increase next weekend as the low moves closer to Western
Washington.

&&

.SHORT TERM...Satellite imagery shows an upper level ridge
offshore beginning to build into the area early this morning. No
precipitation echoes on the doppler radar at 3 am/10z. Skies are
beginning to clear from the north with a few breaks in the cloud
cover over the southern portion of the area as well. Wide variety
of temperatures at 3 am with the different cloud cloud. Locations
already clearing out have dropped in the mid 40s while locations
still with cloud cover are in the lower to mid 50s.

Surface gradients in the process of becoming northwesterly this
morning. The marine layer is much more shallow than the last
couple of mornings. Throw in some subsidence with the upper level
ridge building into the area and the ingredients are there for a
morning break out into the sunshine across Western Washington.
With more sunshine this afternoon high temperatures will be around
5 degrees warmer than yesterday with mid 60s to lower 70s over
the interior and lower to mid 60s along the coast.

Upper level ridge continuing to build over the area tonight and
Tuesday. Surface gradients go onshore slightly overnight but with
the marine layer over the interior eliminated today only expect
stratus to develop along the coast Tuesday morning. Even there the
stratus will be shallow and burn off in the morning. Mostly clear
skies tonight will allow temperatures to once again fall into the
mid 40s in the cooler locations with the remainder of the area in
the upper 40s to mid 50s. Temperatures aloft warming with the
strengthening ridge. Model 850 mb temperatures go from around
plus 10 to 12C tonight to plus 14 to 16C by 00z Wednesday. Surface
gradients remaining northwesterly during the afternoon on Tuesday
which will put a cap on the high temperatures. Expect the warmer
locations to approach 80 with upper 60s to mid 70s common for the
remainder of the area.

Some small changes to the pattern on Wednesday with an upper level
low developing off the California coast increasing the southerly
flow aloft over the area. With the daytime heating the air mass
will become slightly unstable over the higher terrain. This plus
the increase in mid level moisture over the area could produce a
shower over the mountains late in the day. For the lowlands the
surface gradients turn onshore Wednesday morning and increase
during the day. The increase in onshore flow will cool down the
high temperatures along the coast on Wednesday by a couple of
degrees while highs in the interior will be similar to Tuesday.

.LONG TERM...Extended models in good agreement on Thursday with
the upper level low moving east and the flow aloft over Western
Washington becoming more southwesterly. This will cut off the
shower chances for the mountains. After increasing throughout the
day on Wednesday the low level onshore gradients weaken on
Thursday. With the weak marine push Wednesday night into Thursday
morning will go for slightly cooler, 5 degrees or less, high
temperatures on Thursday, mid 60s to mid 70s. Little change in the
pattern on Friday as the upper level low moves closer to the
Northern California coast. Some differences in the models start
showing up for the weekend with the GFS tracking the low inland
into Nevada while the ECMWF has a more northerly track. The ECMWF
solution will result in some showers for the weekend across the
area. With the inconsistency in the models will just go with
chance pops for now. With either solution the low level onshore
flow increases, lowering high temperatures back down into the 60s
for the most part Saturday and Sunday. Felton

&&

.AVIATION...Upper level ridging will build into Western
Washington today with light northwest flow aloft. The air mass is
stable and drying in the mid and upper levels. Areas of low level
moisture will dissipate by midday. Areas of MVFR ceilings this
morning should lift and scatter over the interior by 18Z and by
around 21Z near the coast. Weak low level onshore flow will bring
some low MVFR or IFR in stratus to the coast and locally inland
Tuesday morning, but the bulk of the region should remain VFR.

KSEA...A deck of stratocumulus between 040 and 050 is gradually
eroding from the north this morning as subsidence associated with
upper level ridging takes over. VFR will be the rule today with the
stratocu deck mostly gone by 18Z. Surface winds light and variable
this morning will make a gradual shift to northwest 4 to 8 knots
this afternoon then north-northeast this evening.   27

.MARINE...High pressure offshore and lower pressure east of the
Cascades will result in weak onshore flow through Tuesday night.
Small craft advisory westerlies are likely in the central and
eastern Strait of Juan de Fuca during the diurnal peak late
afternoon through early morning hours.

Onshore flow will probably be somewhat stronger Wednesday through
Friday. Gale westerlies are possible in the central and eastern
Strait of Juan de Fuca those nights, especially Wednesday night
and Thursday night. McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 3 AM PDT
     Tuesday for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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