Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 241003
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
303 AM PDT Thu Aug 24 2017

.SYNOPSIS...A weak weather system moved by mostly to the north of
the area overnight. An upper level trough will move through the
area today bringing with it some cloud cover and cooler
temperatures but little if any precipitation. Friday will be a
transition day with a strong upper level ridge building over the
area Saturday. The ridge will remain over the area through the
early part of next week for another round of warm days in Western
Washington.

&&

.SHORT TERM...Satellite imagery shows weak system has moved by
mainly to the north of the area overnight with the southern
portion of the system in the Cascades early this morning. Skies
have cleared a bit along the North Coast in the wake of the system
with the remainder of the area reporting mostly cloudy skies at
09z/2 am. Temperatures because of the cloud cover were in the mid
50s to lower 60s except along the North Coast where temperatures
have dropped into the upper 40s.

Upper level trough moving through the area today behind the weak
system. Langley Hill radar showing nothing in the way of
precipitation echoes offshore. Model cross sections have very
little lift with the moisture field only extending up to around
7000 feet. Have taken the slight chance of showers out of the
lowlands this morning. The added lift of the Cascades could help
generate a light shower so have kept low pops just in the morning
hours in the mountains. For most of the area the main effect of
the trough will be a little more cloud cover and cooler
temperatures. Highs today will be below normal with 60s and lower
70s common.

Zonal flow aloft over the area tonight. Low level flow turning
northerly below about 700 mb. This is not a favorable pattern for
stratus especially over the interior. Still some cool air left
behind by the upper level trough with lows in the cooler locations
getting into the 40s. Most of the area will have lows in the
lower to mid 50s.

500 mb heights start to slowly rise on Friday. Low level flow
remaining light and northerly. With the temperatures warming aloft
and the lack of cloud cover high temperatures rebounding by a few
degrees on Friday with most places in the upper 60s and 70s.

Upper level ridge building quickly over the area from the
southeast Friday night into Saturday. 500 mb heights rise from the
upper 570 dms at 00z Saturday to the upper 580 dms by 00z Sunday.
Surface gradients remain northwesterly which will temper the
warming trend a little. Even with the northwesterly low level flow
highs on Saturday 5 degrees or so above normal with 70s to mid
80s in the forecast.

.LONG TERM...Some general agreement in the extended models for
Sunday and Monday with Western Washington on the back side of the
upper level ridge. The ECMWF is stronger with the ridge with 500
mb heights staying in the mid to upper 580 dms while the GFS moves
the ridge a little further to the east with 500 mb heights in the
lower 580 dms. Even with the difference aloft the high temperature
guidance on both models is similar with up to 5 degrees more of
warming on Sunday with highs a couple of degrees warmer on Monday
as the low level flow tries to turn offshore. Highs in the 80s
will be common from about Seattle south on Sunday with the warmer
locations in the upper 80s. Highs in the 70s for the remainder of
the area. On Monday 80s will be common over most of the interior
with some lower 90s in the warmer locations in the Southwest
Interior. GFS has a couple of shortwaves moving by to the north
and west Tuesday and Wednesday putting a dent in the upper level
ridge. The ECMWF is much weaker with these features. Both models
show an increase in low level onshore flow on Tuesday which will
knock a few degrees off the high temperatures. The low level
onshore flow pattern continues on Wednesday for a little more
cooling. Felton

&&

.AVIATION...An upper level trough will shift east of the region
today. Light southwest flow aloft becoming northwesterly this
afternoon and tonight. Air mass generally stable. Moist in the lower
levels and periods of scattered to broken high clouds through
tonight. Widespread MVFR stratus this morning. Isolated IFR
conditions could also develop along the coast and portions of the SW
interior and Kitsap peninsula. Improvement to VFR should occur by
18z. Partial clearing late in the day with only patchy low clouds
early Friday morning.

KSEA...Northeasterly wind 4-8 kt becoming variable to 5 kt 13-17z,
then northwesterly 4-8 kt after 17z. MVFR stratus should improve to
VFR by 18z.

&&

.MARINE...Onshore flow will gradually ease as a trough moves east of
the region later today and tonight. Westerly small craft winds in
the central/east strait may ease below 20 kt for a few hours later
this morning and early afternoon. Diurnal westerly winds should pick
up again late in the day and this evening. Decided to run the Small
craft through tonight but indicate a lull in winds around midday.
Small craft winds will also extend to the waters adjacent to the
eastern Strait with a lull in winds at times.

Light northerly flow will prevail Friday through this weekend, with
the flow trending weakly offshore on Monday. dtm

&&

.CLIMATE...Since records started at Seattle-Tacoma airport in 1945
the driest July and August is 0.03 inches in 1967. So far in 2017
the July and August total rainfall is 0.02 inches.

The streak of days with highs 70 degrees or more in Seattle is
now up to 56 in a row, six short of the record 62 days set in
2015. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Friday for Admiralty Inlet-
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

&&

$$

www.weather.gov/seattle

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


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