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000 
FXUS66 KSEW 220930
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
230 AM PDT Mon Jul 22 2019

.SYNOPSIS...Dry weather will continue into Monday with
temperatures slightly above normal. A cold front and upper level
trough will bring showers and a chance of thunderstorms in the
Cascades on Tuesday. Strong onshore flow will prevail on Wednesday
then weaken on Thursday. Mild weather will continue into the
weekend. 

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...Upper level ridging will 
continue to weaken today as an upper trough over the Northeast 
Pacific gradually shifts closer to the region. Low level onshore 
flow has increased over the past 24 hours. This has brought some 
shallow marine air inland. This combined with lowering heights will 
knock around 10 degrees off of high temperatures over the interior 
compared to yesterday. The main area of marine stratus is still 
offshore early this morning, but should spread onto the immediate 
coastline later this morning. For the remainder of the area, it will 
be another sunny day ahead. Onshore flow increases tonight and 
Tuesday as the upper trough offshore edges closer. This should 
initiate a solid marine push tonight with stratus reaching most of 
the lowlands by daybreak Tuesday. Temperatures will cool further on 
Tuesday with the stubborn marine layer likely only yielding some 
partial afternoon clearing over the interior lowlands. Increasingly 
moist and unstable southerly flow aloft ahead of trough will produce 
a slight chance of thunderstorms over the Cascades Tuesday afternoon 
and evening. The upper trough will quickly exit to the east on 
Wednesday with a flat upper ridge moving over the area. Any shower 
threat should end by Wednesday morning. Highs will be near normal on 
Wednesday.  

.LONG TERM /THURSDAY THROUGH SUNDAY/...Low amplitude ridging will 
produce dry conditions and slightly above average temperatures 
Thursday into Friday with persistent low level onshore flow. The 
models are advertising a weak trough passing mainly to our north on 
Saturday. Apart from a chance of showers in the North Cascades, it 
looks to do little more than deepen the marine layer and cool 
temperatures a bit. Ridging weakly rebuilds for the later half of 
the weekend for perhaps a slight warmup and continued dry weather. 

27

&&

.AVIATION...An upper trough over the Northeast Pacific will slowly
shift east today with southwest flow aloft for W WA. At the surface, 
onshore flow will continue to increase  through the day. The air 
mass remains dry and stable.

Current IR satellite shows some high clouds over portions of the 
area. With onshore flow expected to increase...low level clouds are 
expected to push inland at least as far as coastal obs sites this 
morning...perhaps a bit further...but given that these clouds have 
failed to push inland in as far as said coastal sites...suspect this 
push may not be as far inland as previous shift thought. VFR 
conditions in place for most of the area and that will likely 
persist with coastal locations such as HQM likely seeing MVFR to IFR 
conditions develop at some point with the aforementioned low clouds. 
Any cloud cover that sets up there will likely recede by noon. 
Stronger onshore flow tonight should bring about better penetration 
of marine low level clouds by Tuesday morning.

KSEA...Mostly clear today. Light and variable winds this morning 
will become more southwesterly this afternoon with speeds 5-10 
knots before becoming more northerly tonight.  SMR

&&

.MARINE...Onshore flow will increase today and then continue for the 
remainder of the week with high pressure offshore and lower pressure 
inland. Small craft advisory west winds are likely each day for the 
Central and Eastern Strait of Juan de Fuca with winds peaking each 
evening. In accordance with this expectation...have hoisted another 
SCA for this area...as well as the adjacent northern inland waters 
and Admiralty Inlet...starting this afternoon and extending into 
Tuesday. Models remain consistent that gales will be possible in the 
strait Tuesday..but timing falls just outside third forecast period. 
THus...will leave any needed headlines for that time frame to future 
shifts.  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.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT early this morning for 
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance 
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from noon today to 5 PM 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 5 PM this afternoon to 11 PM PDT this 
     evening for Admiralty Inlet-Northern Inland Waters 
     Including The San Juan Islands.

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