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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 181152

Area Forecast Discussion
National Weather Service Seattle WA
352 AM PST Mon Feb 18 2019

.SYNOPSIS...Upper level ridge nosing into British Columbia with
northerly flow aloft over Western Washington today. The ridge will
dissipate tonight with a system embedded in the northwesterly flow
aloft moving over the area Tuesday into Wednesday. Snow levels 
will be low with this system but low level onshore flow ahead and 
with the system will keep snow levels at least around 500 feet. 
After a break Thursday another system following a similar track 
will move into the area for Friday and Saturday. Snow levels will 
remain low with this system as well.


.SHORT TERM /TODAY THROUGH WEDNESDAY/...Satellite imagery cloudy
skies over the central and southern Puget Sound area extending
into the Cascades and also along the Strait of Juan De Fuca and
the North Coast. For the remainder of the area just partly cloudy
skies. Temperatures at 3 am/11z in locations with less cloud 
cover have dropped into the 20s with lower to mid 30s in the more 
cloudy locations.

Doppler radar continues to show a small area of showers between
Port Angeles to just south of Port Townsend as a result of the
weak upslope flow into the Olympics. With the flow weakening and
becoming more northwest expect these showers to come to an end
this morning. For the remainder of the area cloud cover is going
to be tricky today with light surface gradients and little flow in
the lower levels. Upper level ridge to the north is weak and
weakening which will not provide much subsidence to get rid of 
the cloud cover. The February sun is not going to be much help 
either. With a lack of mixing in the lower levels will make a 
persistence forecast as far as the cloud cover goes this morning.
This afternoon the models continue with the idea of a weak system
developing over Northwest Oregon spreading some clouds and a 
slight chance of showers over the southern portion of the area 
later this afternoon. A variety of cloud cover for the remainder 
of Western Washington this afternoon with highs in the 40s.

Weak system to the south fizzling out this evening with clouds out
ahead of the next system approaching from the northwest arriving 
overnight. Lows will be in the upper 20s and 30s. 

Forecast trend of keeping the snow levels at least 500 feet with
this next system will continue with this forecast package.
Bellingham to Williams Lake gradient has been overdone by the
models the last couple of days keeping the cooler air north of
Western Washington. As the system approaches on Tuesday the low
level flow will go onshore lifting the snow levels up into the 500
to 1000 foot range at least. Chance the precipitation starts in 
the morning but most of the precipitation will hold off until the
afternoon hours. Northwesterly flow aloft keeping the central
Puget Sound area rain shadowed for a bit before the precipitation
fills in later in the afternoon. Highs on Tuesday will mostly be
in the lower to mid 40s.

System slowly moving through the area Tuesday night night with
some cooler air aloft beginning to filter into the area. Low level
flow remaining onshore with the Quillayute to Bellingham and
Olympia to Bellingham gradient peaking above plus 2 mb Tuesday
night. Snow levels in general remaining around 1000 feet. The 
best chance for snow in lower elevations will be with the 
developing convergence zone over Snohomish and Northern King 
county. This feature could enhance the precipitation rates enough
to lower the snow level a few hundred feet within the zone. Lows 
will be in the 30s.

Cooler air continuing to move into the area Wednesday morning but
as what is usually the case as the colder air arrives the
moisture is decreasing. Snow levels getting down to around 500 
feet during the day Wednesday but shower activity by afternoon 
will be spotty and light. Highs on Wednesday mostly in the lower 

.LONG TERM /THURSDAY THROUGH SUNDAY/...Extended models showing
good consistency with a break in the weather on Thursday and the
next system arriving later Friday into Saturday following a 
similar track to the one for Tuesday. Just like the system for 
Tuesday low level onshore flow increasing ahead of the front
pushing snow levels up to at least 500 feet and more likely near 
1000 feet. Cool upper level trough over the area on Sunday for a 
low snow levels with showers forecast. Highs will remain well
below normal through the period. Felton


.AVIATION...Northerly winds aloft will continue through today. 
VFR/MVFR cigs currently will become mainly MVFR into the early 
morning hours as cloud levels lower. Areas of IFR conditions are 
possible with pockets of LIFR mainly confined to any development of 
fog in prone-areas. Improvement to cigs by early afternoon with 
mainly VFR conditions late afternoon into the evening hours. Surface 
winds will generally be light under 10 knots.

KSEA...VFR conditions will likely become MVFR cigs between 10-12z. 
Expect MVFR levels to continue through early afternoon before 
transitioning to VFR through the evening. MVFR cigs expected Monday 
night into Tuesday ahead of the next system. Light southerly surface 
winds under 10 knots will continue through the TAF period. JD


.MARINE...Fraser outflow winds have eased across the north interior 
and should remain below 20 kt. Low pressure will move into the 
northern waters tonight and Tuesday with increasing southerly flow. 
The 1005 mb surface low will drift south of the waters Wednesday. 
Brief Fraser outflow winds may develop but forecast gradients do not 
support strong winds. Will keep winds below 20 kt but small craft 
Northeasterly winds are possible. Lighter northerly or offshore 
winds elsewhere. A stronger front moves inland Friday with stronger 
southerly flow.  Mercer


.HYDROLOGY...No river flooding is expected for the next seven days.