Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS66 KSEW 251054

Area Forecast Discussion
National Weather Service Seattle WA
354 AM PDT MON JUL 25 2016

.SYNOPSIS...High pressure will bring dry and mild weather today. A
weak system will move through tonight into Tuesday morning bringing
some marine air into the interior. The ridge will strengthen
Wednesday into the end of the work week with the warmest
temperatures of the month expected Thursday and Friday. Another
weak system will cool temperatures back down to near normal for
the weekend.


.SHORT TERM...Satellite imagery shows low clouds along the coast
spreading down the Strait and moving into the lower Chehalis
valley at 3 am/10z. For the remainder of the area skies are
clear. Temperatures at 3 am were in the mid 50s to mid 60s.

Low clouds along the Coast this morning are indication of some
small changes in the weather pattern taking place today. KOTH-KSEA
gradient, which was near -3 mb early Sunday morning, now at plus
1.5 mb. Gradient through the Strait has also increased slightly
peaking out near plus 1 mb around midnight. While the flat upper
level ridge will remain over Western Washington today the low
level onshore gradients will slowly increase as the day wears on.
This will help keep the low clouds intact this morning along the
Coast with cooler daytime highs but have little effect on the
interior. Persistence will be a good forecast for the interior
today with another day of highs in the 70s to mid 80s. Highs on
the Coast today should remain in the 60s.

00z model runs a little slower with the weak shortwave moving
through the area with the trough axis not moving through until
Tuesday morning. At the surface onshore flow strengthening enough
to push the marine air into the interior late tonight into Tuesday
morning. Temperature guidance only showing a couple of degrees of
cooling but with the surface ridge axis to the south during the
day on Tuesday will go for a little more cooling and undercut the
guidance by a couple of degrees. In general highs in the 60s again
on the coast Tuesday with mid 60s to mid 70s inland with morning
clouds giving way to afternoon sunshine for the interior.

Flat upper level ridge rebounds slightly on Wednesday with 500 mb
heights back into the mid 580 dms range. Surface onshore gradients
weaken and model 850 mb warm a touch...from around plus 12c on
Tuesday to plus 14-15c. With the weaker onshore flow and warming
temperatures aloft daytime highs will rebound on Wednesday back to
near todays readings for the interior...mid 70s to mid 80s. Highs
on the Coast Wednesday a little warmer as the lower 70s.

.LONG TERM...Small small differences in the extended models this
morning. No changes for Thursday with the 500 mb heights near 590
dms with northwesterly surface gradients. Look for highs in the
mid 70s on the Coast and mid 70s to near 90 for the interior. The
00z run of the gfs is a little faster with the approaching weak
trough on Friday. 500 mb heights falling a bit and the model
temperatures aloft cooler than previous runs...850 mb temps plus
17-18c versus plus 20c. The ECMWF is staying with the idea of a
slower approach of the trough leaving Friday as the warmest day of
the week. Since the gfs solution is a new one will stay with the
slower trough idea and keep Friday as the warmest day with warmer
locations in the lower 90s. Models are slowly trending stronger
with the trough over the weekend. Not strong enough to produce
any precipitation but if the trends continue the stronger onshore
flow will result in more morning cloudiness and cooler forecast
highs. Current forecast has temperatures near normal over the
weekend with highs in the mid 60s on the coast and 70s for the
interior. Will stay with this idea for now. Felton


.AVIATION...Light westerly flow aloft for the next 24 hours with
slowly falling upper heights. An upper level shortwave will
approach the coast late Monday, strengthening low-level onshore
flow. For this morning, expecting similar coverage of marine
stratus as Sunday morning, meaning IFR marine stratus will reach
inland to near KSHN and through the Strait to Whidbey Island.
Definitely have fog moving down the Strait now, with Ediz Hook
reporting 1/2SM FG at 3 AM. Marine clouds will burn back to the
coast this afternoon, then penetrate even deeper into the interior
lowlands tonight due to increased onshore flow. Above the low-
level marine layer, the air mass above will be dry and stable
through Tuesday morning.

KSEA...Clear skies and a northerly breeze will continue through
this evening. North wind will shift to a light SW wind around 06z
this evening. Marine stratus will spread over the terminal during
the wee hours of Tuesday morning, with IFR ceilings in the 010-018
range. Haner


.MARINE...High pressure offshore with lower pressure east of the
Cascades will result in onshore flow all week long. The flow will
become strong later today, with westerly gales expected in the
central and eastern Strait tonight. Moderate onshore flow will
resume from Tuesday through Thursday, becoming strong again on
Friday. Otherwise, Small-Craft northwesterlies expected over the
coastal waters for most of the week ahead.Haner


.SEW Watches/Warnings/Advisories...
PZ...Small Craft Advisory from 2 PM this afternoon to 2 AM PDT
     Tuesday for Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm.

     Small Craft Advisory until 9 AM PDT Tuesday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm.

     Gale Warning from 6 PM this evening 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.

     Small Craft Advisory from 6 PM this evening to 3 AM PDT Tuesday
     for Admiralty Inlet-Northern Inland Waters Including The
     San Juan Islands.



You can see an illustrated version of this discussion at is the U.S. government's official web portal to all federal, state and local government web resources and services.