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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 171657
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
857 AM PST Sun Feb 17 2019

.UPDATE /TODAY/...Weak low pressure aloft combined with some
convergence along a weak boundary that is moving south across
Western Washington is producing some isolated light showers from 
Seattle northwestward to the Hood Canal Bridge and Port Townsend. 
Weak Fraser outflow is also producing some isolated rain showers 
over the San Juan Islands and along the north slopes of the 
Olympic Mountains from Sequim to west of Port Angeles. The air 
mass is quite cool, but snow levels appear to be 500-700 ft MSL, 
so precipitation is in the form of rain most areas. Models appear
to have overforecast the degree of outflow from the Fraser as 
Bellingham to Williams Lake BC pressure gradients are 3-5 mb 
weaker than indicated by the models. There is also some fog that 
has formed in the south interior, at Hoquiam, and in some of the 
protected river valleys. 

The today period of the forecast was updated to include the patchy
fog where it is being observed and to increase POPS and the
coverage of shower activity for today. The remainder of the
forecast looks good for now. Albrecht

&&

.SYNOPSIS...An upper level trough over Oregon this morning will
continue to move southeast today with northerly flow aloft
developing over Western Washington. Northerly flow aloft
continuing tonight into Monday with a weak upper level ridge over
British Columbia. A weak system will develop off the Oregon coast
Monday afternoon giving the southern portion of the area a slight
chance of showers. A stronger system will move down over the area
from the northwest on Tuesday. Unsettled weather will continue
into next weekend with a break between systems Thursday. 

&&

.AVIATION...Northerly flow aloft with a few lingering showers over 
the interior this morning. Ceilings generally MVFR to VFR with a few 
lingering pockets of IFR ceilings/visibilities this morning. 
Conditions will improve to MVFR/VFR after 18Z through the evening. 
Conditions will lower to MVFR levels with pockets of IFR late 
tongight into Monday. Surface winds will generally be light under 10 
knots.

KSEA...MVFR cigs currently will likely drop to IFR between 11-12z. 
LIFR also possible during the morning hours (14z-18z). Ceilings will 
improve around midday Sunday into the afternoon to MVFR and even VFR 
levels late. Light southerly surface winds will become 
northerly 6-8 kts this afternoon.

&&

.MARINE...A weak surface low tracking south along the Washington and 
Oregon coast with surface high pressure over Southern British 
Columbia will support low-end gales over the northern inland waters 
and small craft conditions over most other waters. Westerly swell 10 
to 11 feet at 12 seconds today will subside to 8 or 9 feet by this 
evening. Another weak front will reach the waters Tuesday. 

&&

.PREV DISCUSSION... /issued 408 AM PST Sun Feb 17 2019/ 

SYNOPSIS...An upper level trough over Oregon this morning will
continue to move southeast today with northerly flow aloft
developing over Western Washington. Northerly flow aloft
continuing tonight into Monday with a weak upper level ridge over
British Columbia. A weak system will develop off the Oregon coast
Monday afternoon giving the southern portion of the area a slight
chance of showers. A stronger system will move down over the area
from the northwest on Tuesday. Unsettled weather will continue
into next weekend with a break between systems Thursday. 

SHORT TERM /TODAY THROUGH TUESDAY/...Satellite imagery shows
cloudy skies over most of the area early this morning with a few
breaks in the overcast. Still a few showers left on the doppler
radar mainly along the northern slopes of the Olympics. Wide
variety of temperatures with a varying cloud cover with Tacoma at
27 degrees at 3 am/1z while Whidbey Island was at 41. Most 
locations were in the lower to mid 30s. 

Upper level trough continuing to sink south today with drying
northerly flow aloft developing over Western Washington. With the
flow aloft becoming more northerly upslope flow into the northern
slopes of the Olympics will weaken bringing about an end to the 
shower activity ( northeasterly flow aloft is better ). For the 
remainder of the area mostly cloudy skies this morning with some 
patchy fog where there is bigger breaks in the cloud cover. 
Decreasing clouds later this afternoon into the evening hours as 
the air mass dries out a bit with increasing northerly flow aloft.
Fraser river outflow winds over the North Interior are a little 
behind schedule Bellingham to Williams Lake gradient -10 mb 
instead of the predicted -15 mb. The gradients is becoming more 
negative with time so have just slowed down the onset of the 
outflow winds today. With the gradient peaking around
-15 mb and most of the cold air bottled up over British Columbia 
will keep the winds below advisory levels. It will be another cool
day with highs int he upper 30s and lower 40s. Through the 16th 
Seattle is 8 degrees below normal for the month. This is the 4th 
coldest first 16 days of February in the 75 years of records at 
Sea-Tac and the coldest in the last 30 years. The 3 years that 
were colder 1989, 1949 and 1956.

Northerly flow aloft continuing tonight with the slow drying of
the air mass continuing. With the decreasing cloud cover it is
going to be another chilly morning on Monday, most places will be
in the 20s. 

Another dry day for most of the area on Monday. Northerly flow
aloft continuing but the ridge over British Columbia will start to
weaken. A weak system develops off the Oregon coast in the
afternoon which could give the extreme southern portion of the
area some light precipitation Monday afternoon. Snow levels will
be low, around 500 feet, but if any precipitation does develop it
will be light so no snow accumulations in the forecast. Highs on
Monday around 40.

Upper level ridge over British Columbia dissipating Monday night
with the weak system to the south floating over Western
Washington. Can't totally rule out a light shower of a flurry 
with this feature Monday night into early Tuesday morning. With 
the increasing cloud cover lows will be a little warmer Tuesday 
morning, in the mid 20s to lower 30s. 

Models showing good consistency in bringing the next system down
from the northwest into the area on Tuesday. If the precipitation
starts early enough in the day we could see a little bit of snow
but right now the model trends are slowing down the onset of the
precipitation into the late morning and afternoon hours. As the
system moves into the area the low level flow ( surface to around
2000 feet ) will turn onshore across most of the area. This will 
lift the snow level off the surface into the 500 to 1000 foot 
range. The exception will be in the north with the potential for 
the cooler air in place due to the outflow winds Sunday into 
Monday keeping the snow levels near the surface. Precipitation 
amounts at this point do not look very impressive with most places
getting 0.10 or less in the afternoon. The low level onshore flow
will add a couple of degrees to the highs on Tuesday with lower 
40s forecast. Highs over Whatcom county will remain in the 30s. 

LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...Tuesday night into
Wednesday a tricky part of the forecast this morning with the 
track of the surface low with the system the key to who if 
anybody gets snow Tuesday night into Wednesday morning. Models 
are in pretty good agreement this far out with the ECMWF bringing 
the low in along the Canadian border while the GFS has the low 
near the East Entrance to the Strait. The GFS track is a little 
more conducive to lower snow levels than the ECMWF track but both 
models do not have much in the way of really cold air moving into 
the area. Without any cold air already in place it is going to be 
hard to get snow below 500 feet with low level onshore flow and 
the models continue to show low level onshore flow Tuesday night. 
Will have 500 to 1000 foot snow levels in the forecast. The air 
mass cools down a touch on Wednesday but we are back to the usual 
Western Washington snow scenario at that point with the moisture 
decreasing as the cooler air arrives. A few lingering showers 
Wednesday night with the short dry spell still on track for 
Thursday. Another low snow level event later Friday into Saturday 
but unless the models get rid of the low level onshore flow the 
snow level will remain above the surface. Highs will remain well 
below normal through the period. Felton

AVIATION...Easterly flow aloft will transition more northerly 
through today as the upper-level low slides southward. Lingering 
showers likely confined to KCLM through 14z before precipitation 
tapers. Ceilings generally IFR through the morning hours with LIFR 
levels possible at times. Fog development mainly confined to KOLM. 
Cigs should begin to lift to MVFR/VFR after 18Z for most. MVFR 
levels with pockets of IFR expected again Sunday night into Monday. 
Surface winds will generally be light under 10 knots.

KSEA...MVFR cigs currently will likely drop to IFR between 11-12z. 
LIFR also possible during the morning hours (14z-18z). Ceilings will 
improve around midday Sunday into the afternoon to MVFR and even VFR 
levels late. Surface winds light under 10 knots. JD

MARINE...A weak 1013 mb surface low driving south off the 
Washington coast will reach the Oregon coast this afternoon. Surface 
high pressure will build over Southern British Columbia. This will 
support low-end gales over the northern inland waters and small 
craft conditions over most other waters. Westerly swell 10 to 11 
feet at 12 seconds today will subside to 8 or 9 feet by this 
evening. Another weak front will reach the waters Tuesday. Mercer 

HYDROLOGY...No river flooding is expected for the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 4 PM PST this afternoon for Central 
     U.S. Waters Strait Of Juan De Fuca-Coastal Waters From Cape 
     Flattery To James Island 10 To 60 Nm-Coastal Waters From 
     Cape Flattery To James Island Out 10 Nm-Coastal Waters From 
     James Island To Point Grenville 10 To 60 Nm-Coastal Waters 
     From James Island To Point Grenville Out 10 Nm-Coastal 
     Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater Out 
     10 Nm-West Entrance U.S. Waters Strait Of Juan De Fuca.

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

     Small Craft Advisory until 6 AM PST Monday for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until midnight PST tonight for Northern Inland 
     Waters Including The San Juan Islands.

     Small Craft Advisory from 4 PM this afternoon to 4 AM PST Monday 
     for Puget Sound and Hood Canal.

&&

$$

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