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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 191013

Area Forecast Discussion
National Weather Service Seattle WA
312 AM PDT Thu Sep 19 2019

.SYNOPSIS...A weak upper ridge will build inland today. A weak 
system will slide down the back of the ridge tonight and Friday for 
a slight chance of showers. Better chances for rain are expected 
Saturday night and Sunday as frontal system arrives.


.SHORT TERM /TODAY THROUGH SATURDAY/...Pretty quiet conditions out 
there this early morning with current IR satellite imagery showing a 
distinct lack of mid to high level clouds. However...current obs 
show temp/dewpt spreads to be pretty minimal for many locations and 
as such already starting to see reports of reductions to visibility 
due to fog. Thus...that portion of the inherited forecast is panning 
out well and some caution will be needed for the morning 
commute...especially in fog prone areas.

A weak upper ridge will be the primary weather feature of the day as 
it builds over W WA...keeping conditions dry and partly to mostly 
sunny. Models remain persistent in showing an extremely weak feature 
hot on its heels though...dipping down into the area tonight and 
into Friday...but impressive is not one of the first thousand words 
to describe this feature. The ECMWF plays things a little wetter 
with it...but ultimately the best moisture remains just along the 
border of the PQR/SEW CWAs. Ultimately...POPs with this feature will 
remain pretty low with the most widespread impact simply being 
increased cloud cover. Another round of ridging builds in for Friday 
night and into Saturday but both flattens and moves eastward rather 
quickly with the ridge axis east of the Cascades by Saturday 
afternoon. Models remain in good agreement regarding next frontal 
system expected to reach the coast Saturday night.

Afternoon high temperatures will remain fairly static throughout the 
short term...ranging in the mid to upper 60s for lowland locations.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/... The aforementioned frontal 
system will bring widespread rain to the CWA for the bulk of Sunday 
before it moves east of the Cascades Sunday evening and showers 
taper off Sunday night. An upper level ridge does follow closely 
behind but like the ridges mentioned in the short term...does not 
hang around for any prolonged amount of time...making a hasty exit 
Monday evening. The system behind that ridge is a point of 
contention for the models...with the GFS fizzling the system out 
pretty quickly while the ECMWF also weakens the system as it passes 
through but slowly enough so that precip with the system would still 
be a very real possibility. Opted to take an average of these two 
solutions...leaning maybe a bit more toward the drier GFS. THe gulf 
in model solutions widens for Wednesday as the GFS has a so-weak-as- 
to-nearly-be-zonal-flow ridge over W WA while the ECMWF keeps a 
troughier pattern overhead. Having leaned toward the GFS for 
Tuesday...opted to follow suit for Wednesday as well.  SMR


.AVIATION...A upper level ridge is centered well offshore with an 
upper trough over the Great Basin producing northerly flow aloft 
over Western Washington. The low level flow is weakly onshore. The 
air mass is generally dry and stable except for some areas of low 
level moisture along the west slopes of the Cascades and areas of 
fog over the lowlands. Areas of IFR/LIFR in low clouds and patchy 
valley fog should burn off by late this morning. High clouds will be 
increasing this afternoon ahead of a weather system moving into 
British Columbia. 

KSEA...Areas of stratocumulus drifting southward along the west 
slopes of the Cascades may slow or prevent fog/stratus formation at 
the terminal this morning. 09Z dew point depression is still 4C as 
well. Considering these factors, will likely trim back IFR ceilings 
in the upcoming 12Z TAF to just a tempo group this morning. 
Otherwise, VFR expected through this evening. Surface winds light 
and variable becoming S-SW 7 knots or less from mid morning through 
the afternoon.  27


.MARINE...Relatively light onshore flow will prevail through
Friday with high pressure offshore and lower pressure inland. 
Onshore flow will be enough for marginal small craft advisory 
westerlies in central/east strait this evening. A frontal system 
will approach from the west on Saturday and then move through the 
area Saturday night and Sunday.  27


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


PZ...Small Craft Advisory from 5 PM this afternoon to 2 AM PDT Friday 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca.