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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 172202
AFDSEW

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

.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/...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/...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 with VFR conditions over most of
the area. The exception remains IFR conditions in the South Puget
Sound associated with log fog. Ceilings will improve to VFR this 
evening then lower to MVFR levels with pockets of IFR again late 
tonight into Monday morning. Surface winds will generally be light
under 10 knots.

KSEA...VFR cigs will linger through the evening then lower to MVFR 
level after midnight into Monday morning. Light northerlies 6-8 kts 
will ease and become southeasterly 2-4 kt after midnight.

&&

.MARINE...A weak surface low in the Oregon coastal waters coupled 
with weak high pressure over Southern British Columbia will result 
in weaker Fraser outflow. Have downgraded  the low-end gale over the 
northern inland waters to a small craft advisory and dropped the 
small craft advisory for all other waters. The 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.

&&

.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.

     Small Craft Advisory for rough bar until 4 PM PST this afternoon 
     for Grays Harbor Bar.

&&

$$

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