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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 161726
AFDSEW

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

.SYNOPSIS...An upper level trough over the area today will move 
south tonight and Sunday. Northerly flow aloft Sunday night and 
Monday will give dry but cold conditions to the area. The ridge 
will flatten Monday night with a system arriving from the 
northwest on Tuesday. Unsettled weather is expected later Tuesday
through the end of the week, perhaps with a break between systems
on Thursday. 

&&

.SHORT TERM /TODAY THROUGH MONDAY/...An upper level low over
Washington is giving scattered light showers to the area today and
considerable cloudiness. Snow levels are generally around 1000
feet. Short term models continue to show the upper level low
dropping off to the south later today through tonight, so showers
will decrease from north to south. 

Arctic high pressure over the Canadian Northwest Territories and
Yukon is forecast to move southward into the central interior of
British Columbia later tonight into Sunday. As the upper low to
the south moves away, expect north to northeast pressure gradients
to increase allowing cold Fraser outflow to resume. Both the NAM12
and the GFS give -15mb Bellingham to Williams Lake BC pressure
gradients Sunday morning for breezy to windy northeasterlies in
Western Whatcom County and the San Juan Islands. At this time,
indications are that winds will stay below advisory strength in
those areas, but we will take another look as the full suite of
12Z models and guidance comes in.

The north to northeasterly flow Sunday into Sunday night will
result in drying and clearing across the area. Highs will
generally be in the 30s to lower 40s and lows will fall mainly
into the 20s Sunday night. 

Some cloud cover and precipitation amount forecasts were refined
this morning, but the going forecast looks good and no major
changes are needed. Albrecht

.LONG TERM /TUESDAY THROUGH FRIDAY/...From the 404 AM PST Long
term discussion: Models very 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...Southerly flow aloft with spotty showers through 
the evening. Ceilings will range from IFR in the South to MVFR in 
the north with mainly MVFR cigs by afternoon. Surface winds will 
mainly be light to 10 knots through today.

KSEA...IFR ceilings showers this morning with light rain possible 
during the afternoon. Ceilings will improve to MVFR and VFR this 
afternoon and evening with MVFR into Sunday morning. 
Southwesterly winds around 10 kts. 

&&

.MARINE...A surface low off will drift southward through the 
Washington coastal waters with high pressure 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. Fraser outflow will increase late tonight and Sunday 
across the northern inland waters peaking Sunday morning. This 
will support low- end gales over the northern inland waters and 
small craft conditions elsewhere.

&&

.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. 

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 4 AM PST Sunday for 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.

     Small Craft Advisory until 4 PM 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.

     Small Craft Advisory from 6 AM Sunday to 4 AM PST Monday for 
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 10 PM this evening to midnight PST Sunday 
     night for Northern Inland Waters Including The San Juan 
     Islands.

     Small Craft Advisory from midnight tonight to 4 AM PST Monday 
     for West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from midnight tonight to 4 PM PST Sunday 
     for Central U.S. Waters Strait Of Juan De Fuca.

&&

$$

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