Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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437
FXUS66 KSEW 212215
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
315 PM PDT Fri Apr 21 2017

.SYNOPSIS...An upper level ridge over Western Washington will
shift east of the Cascades tonight. The combination of the upper
ridge and high pressure to the east will give dry weather tonight.
Wet and cool weather will return to the region on Saturday and
continue through next week as a steady stream of weather systems
takes aim at the Pacific Northwest.

&&

.SHORT TERM...The 500-mb ridge that gave a very nice day to
Western Washington today is now over the area and is headed to the
east. Dry weather will continue through tonight as higher pressure
at lower levels east of the Cascades and lower pressure with a
negatively tilted front near 135W, give offshore flow to the
area. High clouds spreading east of the front will invade from the
southwest later this evening then will thicken late tonight into
early Saturday morning. With the warmer start today, low level
offshore flow, and increasing upper level moisture, expect
temperatures to be warmer tonight, especially in the normally
colder pockets that fell into the mid 30s last night.

Rain will rapidly spread northeast across the area Saturday during
the morning through midday hours as upper level diffluence
increases. The precipitation will then become showery behind the
front that will lift NE across the area during the mid afternoon
hours. With and behind the front, lifted indices fall to near
zero and the mid levels destabilize. Forecast soundings generally
show a skinny CAPE profile and marginal instability, but it does
not take much instability or lift to produce an isolated TSTM this
time of year in the environment shown by the short term models.
We included a slight chance of TSTMS during the afternoon and
evening hours for the area from 18Z-06Z, excluding the Olympic
Peninsula westward, and this generally lines up with the thinking
of the Storm Prediction Center in Norman OK. High temperatures on
Saturday will generally range from the mid 50s to lower 60s across
the area. It will become breezy behind the front Saturday into
Saturday evening as a weak low curls northward through the
offshore waters and flow aloft to the east of this low interacts
with the terrain to form a mesolow to the northeast of the Olympic
Mountains.

Showers diminish Saturday night then increase again Sunday
afternoon and evening as the base of the negatively tilted upper
trough to the west of the incoming front lifts northeast across
the area. With the cooler air aloft and some diffluence to the
northeast of this disturbance, there could be another risk of
isolated TSTMS Sunday afternoon and evening. Will allow later
forecast shifts to include these if timing of that feature
remains consistent.

Models show a more organized system moving into the area on
Monday. Rain chances with this system will be highest over the
southern half of the forecast area. The heaviest precipitation
amounts appear to be south of where a low moves into the area
(most likely the southern Washington Coast). The GFS remains
rather dry over the northern interior zones Monday but is trending
in the direction of the favored ECMWF that spreads moisture north
through the area -again heaviest south. Highs on Monday will be
in the 50s. Albrecht

.LONG TERM...Tuesday through Friday will see a series of systems
moving in on rather zonal flow aloft. The ECMWF appears to be
wetter than the 12Z GFS with the jet axis closer to or over the
forecast area while the GFS is a bit farther south. POPS were
initialized with a model mean and temperatures assume clouds and
occasional precipitation (2-4 degrees below normal for late
April). Any migration of the jet to the north of the current model
position will result in windy conditions at times. Albrecht

&&

.AVIATION...An upper level ridge over W WA this afternoon will
move over E WA by 06Z. A cold front and associated upper level
shortwave trough about 400 NM offshore will slowly approach the
region through tonight. The ridge will keep the air mass dry and
stable through tonight except for increasing high clouds late
tonight. Low level E to SE flow will slowly increase tonight ahead
of the approaching front.

The cold front will bring light rain to the coast around 15Z
Saturday then spread NE across the interior 18Z-21Z Saturday. The
air mass will become unstable immediately behind the front, with the
upper level trough following close behind. There is a slight chance
of afternoon and early evening thunderstorms as this feature moves
NNE across W WA.

KSEA...An upper level ridge will produce excellent VFR conditions
through 12Z with only increasing high clouds later tonight. Surface
winds will be NE 7-14 KT. Kam

&&

.MARINE...A cold front about 400 NM offshore will approach
the region tonight. E-SE offshore flow will gradually increase
tonight as the front approaches with SCA winds developing over the
coastal waters and the west entrance to the Strait of Juan de Fuca.
The front will become more negatively tilted with time and will
curve NE across the coastal waters and W WA Saturday morning and
early afternoon. The air mass will become unstable Saturday
afternoon immediately behind the front, for a slight chance of
thunderstorms.

A much weaker surface trough will move across the area Sunday
afternoon. The GFS and ECMWF are in good agreement that another
frontal system will affect the area on Monday. The models have about
a 1001 MB eastward moving surface low approaching the coast Sunday
night. The low will weaken slightly before moving inland across the
N WA coast Monday afternoon. At this point the models have the low
generating SCA winds. Another frontal system will affect the area
Tuesday and Wednesday. Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 11 PM this evening to 8 PM PDT
     Saturday 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 from 8 PM this evening to 8 AM PDT Saturday
     for West Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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