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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 180541
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
941 PM PST Sun Feb 17 2019

.UPDATE 930 PM PST...Latest satellite and radar show clouds
clearing most of the lowlands late in the day. Temperature and 
dew point spreads are close, so fog or areas of low clouds will 
likely form overnight. The air mass is somewhat moist in the low 
levels as Fraser outflow is not as strong as expected. Low level 
moisture will linger with clouds or fog Monday morning which
should lift in the afternoon but probably not clearing with a lack
of flow over Western Washington. The next system is still poised
to arrive from the northwest on Tuesday. It will likely be cold
enough for light snow, but the low track is far enough north that
some areas could get rain or a mix by evening with snow levels
in the 500 to 1000 foot range. This flow pattern also tends to
shadow much of central Puget Sound, so precipitation will be
rather light in the greater Seattle metro area. Lowland snow
accumulation looks to be minor at this point, probably less than 1
inch with hilltops and north interior getting a couple inches 
with a transition to rain less likely. Mercer

&&

.SYNOPSIS...Northerly flow aloft and high pressure building into
the region from the northeast will give dry and continued cool
conditions to the area tonight through Monday. A weak system 
dropping southeast across the region Tuesday into Wednesday will 
give increasing chances of precipitation; snow levels will remain 
low and some accumulations will be possible, mainly on higher 
hills and near the Cascades. Another period of dry cool weather is
expected on Thursday. Another system will give wet conditions 
with low snow levels Friday into early next weekend.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Previous 
discussion...Satellite imagery shows drier air moving into the 
area from the north and northwest with some sunshine along the 
coast and in the interior north of Everett. Isolated very light 
showers, mainly over the Cascades, are coming to an end as they 
move south. Expect the gradual drying trend to continue this 
evening. 

One change in the short term is northeasterly flow out the 
Fraser...it hasn't developed as expected as the Bellingham to 
Williams Lake pressure gradients peaked lower and sooner than 
expected. The cold air northeast of the Canadian coastal ranges is
not as deep or cold as what has been seen recently nor it is as 
cold as modeled. So, winds have been significantly reduced over 
the north interior for this afternoon and tonight. It will still
be cold tonight with the decrease in cloud cover, drier air mass,
and light winds, but temperatures were raised upward a degree or
so toward a new blend of short range models. Monday will see
mostly to partly sunny skies, with isolated very light showers
confined to the Cascade crest and temperatures rising generally
into the lower 40s in the lowlands.

Moisture will spread northeast into the area Monday night into
Tuesday as it wraps around a system that is dropping 
southeastward from the Gulf of Alaska. Snow levels will initially
be low, generally below 500 feet, as the precipitation starts on
Tuesday, but precipitation is expected to be light during the
coolest part of the system. Snow levels will gradually rise to
500-1500 feet later Tuesday or Tuesday night. While 1000-850 mb 
thicknesses are generally below 1300 meters for the duration of
this system, the low level flow will generally be onshore Tuesday
afternoon and Tuesday night. The best chance for light 
accumulations would be in the northern interior, especially around
and northeast of Bellingham. 

Snow levels will fall back closer to around 500 feet Wednesday 
morning as the system passes, flow becomes convergent and 
northwesterly, and cooler air aloft moves into the area. A 
convergence zone on Wednesday morning could produce some snow 
accumulations, but the location and amounts cannot be nailed down 
at this time. Albrecht

.LONG TERM /THURSDAY THROUGH SUNDAY/...Previous discussion...Extended
models are generally consistent in showing a cool and drier 
period late Wednesday night through Thursday evening. Temperatures
will likely be around where they have been with lows in the 20s 
to lower 30s and highs generally in the upper 30s to mid 40s.

Yet another system moves through the area Friday into Saturday.
This system also starts with low snow levels Friday morning, but
they rise as moisture really gets into the area, and despite the
rather low snow levels, most lowland locations will see rain due
to time of day and onshore flow issues.

While forecasts have generally moved a bit warmer through the
upcoming week from previous forecasts, long wave troughing over
the Pacific Northwest means temperatures will still be 5-10 
degrees below normal for late February and puts us on track to 
have one of the coldest February's is several decades. Albrecht

&&

.AVIATION...Northerly flow aloft is becoming fairly light except
in the north interior. Clouds cleared most the area with VFR this
evening, then low clouds or fog will likely give MVFR late tonight
and Monday morning. There will be pockets of IFR except north of
KPAE. Surface winds will generally be light under 10 knots.

KSEA...VFR cigs will linger through the evening then lower to MVFR 
late tonight into Monday morning. Light or variable wind to 5 kt 
will become southeasterly 2-4 kt after midnight.  Mercer

&&

.MARINE...The BLI-YWL gradient peaked around -11 mb earlier today
and is gradually easing allowing winds to remain mainly below
small craft strength. The exception is the northern inland waters
where weak outflow will still produce northeasterly small craft
winds through tonight. The threat of gales has passed as gradients
peaked well below the forecast. Westerly swell over the coastal 
waters and west entrance has dropped to 7 to 8 feet and will 
remain at those heights through Monday. A frontal system will 
bring increased winds and seas to area waters Tuesday and 
Wednesday with gale conditions possible over the coastal waters. 

Mercer

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 10 AM PST Monday for Northern Inland 
     Waters Including The San Juan Islands.

&&

$$

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