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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 191056
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
356 AM PDT Fri Jul 19 2019

.SYNOPSIS...Upper level trough over the northern portion of the
area will shift east later today and tonight. High pressure will
build into the area over the weekend for dry and warmer days.
Another upper level trough will move through Western Washington
later Tuesday into Wednesday. High pressure will rebuild behind
the trough Thursday.

&&

.SHORT TERM /TODAY THROUGH SUNDAY/...Satellite imagery shows
mostly cloudy skies over most of the area early this morning. Weak
convergence hanging on over the central Puget Sound while the
upper level trough to the north is producing a few showers from 
Skagit county northward. Isolated showers are also present along 
the coast at 3 am/10z. Temperatures are in the 50s.

Weak convergence dissipating over the central Puget Sound this
morning but not fast enough to keep a mention of showers out of
the forecast. Upper level trough to the north also weakening and
drifting east today with a chance of showers in the lowlands just
this morning. With the trough still hanging around in the 
afternoon will have to keep a small chance of showers in the North
Cascades. For the remainder of the area varying amounts of cloud 
cover this morning. This afternoon all locations should see at 
least partly sunny skies. Highs will continue to be below normal, 
in the mid 60s to mid 70s.

Trough moving east of the area tonight with an upper level ridge
beginning to build over the area by early Saturday morning. 
Onshore surface gradients weaken and become northwesterly which 
will limit the amount of morning low clouds and fog. With the 
clearing skies some locations will see the coldest low 
temperatures of the month with upper 40s in the Lower Chehalis 
Valley and Southwest Interior. Lows in the 50s across the rest of 
Western Washington. 

Upper level ridge continuing to build over the area on Saturday
with northwesterly low level onshore flow. Temperatures aloft
warming with model 850 mb temperatures going from plus 7 to 10C
Saturday morning to plus 11 to 15C Saturday afternoon. High
temperatures will be 5 to 10 degrees warmer than today in the 
70s and lower 80s.

Little change in the pattern Saturday night into Sunday.
Temperatures aloft continue to warm with the southwesterly flow 
aloft over the area into the plus 14 to 18C range by 00z Monday. 
Surface gradients remaining northwesterly. After lows in the 50s 
Sunday morning, the warming trend will continue with highs in the
lower 70s to mid 80s. Sunday looks to be the warmest day of July 
so far. 

.LONG TERM /MONDAY THROUGH THURSDAY/...Extended models in good
agreement with the upper level trough well offshore starting to
move toward the Pacific Northwest coast Monday. Low level flow
turning southwesterly onshore late in the day. With the trough
approaching temperatures aloft begin to cool off of the peak on
Sunday. Still plenty of sunshine with highs just a couple of 
degrees cooler than Sunday, in the lower 70s to lower 80s. Cooling
trend continues on Tuesday with increasing onshore flow as the 
trough moves closer to the area. By Wednesday the trough moves 
into Western Washington. At this point the trough does not look 
strong enough to produce any shower activity. The trough will 
enhance the low level onshore flow resulting in more cloud cover 
especially in the morning with high temperatures colling back to 
to near normal, upper 60s to mid 70s. Upper level ridge trying to
rebuild behind the trough on Thursday. Felton

&&

.AVIATION...Northwesterly flow aloft will continue through the day 
today and into tonight as a weak upper level trough remains over the 
Pac NW. Onshore surface flow will gradually diminish as the day 
progresses. 

Mixed bag of cigs over W WA again this morning with generally VFR 
conditions in place although locations that were within the PSCZ 
overnight //northern King and southern Snohomish counties// and as 
it diminishes this morning will have ample lower level moisture in 
place for MVFR to IFR conditions. Cigs are expected to gradually 
lift for these spots and widespread VFR conditions are expected by 
this afternoon...with clouds diminishing throughout the day and the 
potential for clear skies by this evening. With continued onshore 
flow...some low level clouds may be present very late tonight/early 
Saturday morning.

KSEA...VFR cigs persist with mid level clouds lingering. A stray 
shower is still possible during the morning hours before PCSZ 
finally shuts down. Cloud bases expected to continue to lift 
throughout the day with some clearing. SKC possible by this 
evening...although some SCT low clouds may be possible overnight 
tonight and into Saturday morning. Southeasterly winds turning more 
south to southwest by mid-morning before becoming more northerly 
this afternoon with speeds generally 5 to 10 knots. SMR

&&

.MARINE...Will cancel all headlines given winds have fallen below 
any headline criteria. Onshore flow will gradually weaken...but 
model progs suggest still enough oomph to bring another round of SCA 
winds to the central and eastern strait again this afternoon and 
into tonight. Thus...will be issuing new headline for morning 
forecast package. As onshore flow diminishes further over the 
weekend...the trend for SCA winds in the strait might take a bit of 
a rest...however current model solutions suggest that wind speeds 
might get close. SMR

&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.

&&

.CLIMATE...With 0.20 inches at Sea-Tac yesterday the monthly rain
for July is 1.12 inches. This is the 2nd wettest July in the last
20 years surpassed only by the 1.44 inches in 2007. The 0.20 
inches at Sea-Tac was the 4th wettest July 18th on record ( and 
only 0.09 inches off the record 0.29 inches in 2007 ). This  
includes the Federal Building records that go back into the 
1890's. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 2 PM this afternoon to 5 AM PDT 
     Saturday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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