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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 161204

Area Forecast Discussion
National Weather Service Seattle WA
404 AM PST Sat Feb 16 2019

.SYNOPSIS...An upper level trough over the area today will move 
south tonight and Sunday. Dry northerly flow aloft Sunday night 
and Monday with an upper level ridge nosing into British Columbia.
The ridge will flatten Monday night with a system arriving from 
the northwest on Tuesday. Unsettled weather will continue through 
the end of the week with a break between systems on Thursday.


.SHORT TERM /TODAY THROUGH MONDAY/...Satellite imagery shows upper
level trough over Western Washington early this morning. Doppler
radar has a few showers over the area with the heaviest
concentration over the central Puget Sound. Temperatures at 3
am/11z were in the 30s.

Upper level trough lingering over the area today keeping showers
in the forecast. The more organized bands of showers will remain
offshore and to the south with the trough slowly drifting south
during the day. Snow levels will be low this morning, 500 to 1000
feet for the most part from the Puget sound eastward, so some of 
the higher hills could see some flakes in the air. No snow
accumulations in the lowlands expected. As the trough digs south
this afternoon the snow levels will rise up into the 1000-2000
foot range. Highs today will be in the upper 30s to mid 40s. 

Chance of showers lingering into the evening hours with the 
trough still in the vicinity. Shower chances decreasing rapidly 
after 06z as the trough southerly movement increases. Fraser river
outflow creeping back into the picture with the Bellingham to 
Williams Lake gradient down to -13 to -16 mb by 12z Sunday. Lows
will be in the upper 20s and 30s. 

Sunday looking like a dry day at this point. Fraser river outflow
pretty much confined to Whatcom Skagit and San Juan county. For
the remainder of the area surface gradients fairly light. Although
the upper levels of the air mass will be drying out with the
northerly flow aloft, light gradients and plenty of low level
moisture stratus a good possibility at least through Sunday 
morning. Will word the forecast partly sunny. Highs will mostly be
a couple of degrees either side of 40.

Dry weather continuing Sunday night into Monday with an upper
level ridge nosing into British Columbia keeping the flow aloft 
northerly over Western Washington. Northerly surface gradients 
increasing over the entire area which will help dry out the lower 
layers of the air mass resulting in sunny skies on Monday. Windy 
conditions in the north but the models have the Bellingham to 
Williams Lake gradient peaking around -16 mb. Will keep the winds 
just below advisory, in the 20 to 30 mph range with gusts to 40 
mph. Highs once again will be a couple of degrees either side of 

inconsistent early on in the extended with the GFS bringing a cool
upper level low down over the area Monday night while the ECMWF
does not have this feature at all. Snow levels will be low with
model 1000-850 mb thickness values on the GFS below 1300 meters,
cold enough for snow. With the ECMWf showing nothing Monday night
will go with just a chance pops forecast with low, 500 feet or 
less snow levels. Better consistency with a shortwave moving down 
from the northwest later Tuesday into Tuesday night. This system 
does not have a lot of moisture with it but once again snow levels
will be low, 500 feet or less, so there is a chance of some light
accumulations Tuesday night. Upper level trough behind the 
shortwave moving through Wednesday into Wednesday night keeping 
low snow levels in the forecast. Drying trend on Thursday with the
next shortwave embedded in the northwesterly flow aloft a little 
slower on the 00z models, arriving Friday night into Saturday. 
Will lower the pops out of the likely category and go for just a 
chance of rain or snow on Friday. Snow levels will still be low on
Friday and Friday night so there is another chance for some light
accumulations especially above 500 feet. Highs through the period
well below normal, upper 30s and lower 40s with lows below 
freezing most of the mornings. Felton 


.AVIATION...South to southwesterly flow aloft will continue through 
today as an upper level trough lingers over the area. Residual 
moisture will keep spotty showers through today, though not 
everywhere will see precipitation, best chance of light rain will be 
during the afternoon and evening hours. Ceilings will continue to 
fluctuate between VFR and MVFR through the morning with mainly MVFR 
cigs by afternoon. Surface winds will mainly be light to 10 knots 
through today.

KSEA...VFR cigs currently are expected to drop to MVFR by late 
morning. A few spotty showers around this morning with light rain 
possible during the afternoon. Fluctuation between VFR and MVFR cigs 
expected Saturday night into Sunday morning. Winds generally under 
10 kts. JD


.MARINE...A 1010 mb surface low will consolidate off the north 
Washington coast later today, then drift southward. High pressure 
will build to nearly 1040 mb over southern British Columbia late 
tonight and Sunday. Small craft winds over the coastal waters in 
addition to westerly swell of 12 to 14 feet at 14 seconds will 
support keeping the small craft in effect there through tonight. 
Elsewhere, winds have eased between systems and should remain 20 kt 
or less. 

Fraser outflow will increase late tonight and Sunday across the 
northern inland waters with BLI-YWL gradient peaking near -15 to 
-17 mb Sunday morning. This will support low-end gales over the 
northern inland waters, with a Gale Watch now in effect there late 
tonight through Sunday evening.


.HYDROLOGY...Precipitation that falls through Saturday night will 
mainly be in the form of rain, or a rain snow mix. The 
precipitation, combined with all the snow on the ground that will be 
melting over the same period, will result in some ponding water on 
roadways and yards, especially with snow and ice plugging storm 
drains. In addition, melting snow is efficient at saturating the 
ground and is increasing the threat of landslides more than the 
rainfall-based indices would suggest. The SPS for the landslide 
threat will be continued through Saturday. 

As far as river flooding, none is expected for the next 7 days.  


PZ...Small Craft Advisory until 4 AM PST Sunday for 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.

     Small Craft Advisory for rough bar until 4 AM PST Sunday for 
     Grays Harbor Bar.

     Gale Watch from late tonight through Sunday evening for Northern 
     Inland Waters Including The San Juan Islands.