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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 191231
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
431 AM PST Tue Feb 19 2019

.SYNOPSIS....A frontal system will bring rain and mountain snow 
later today and tonight. A cool upper level trough will keep
showers in the forecast on Wednesday. Drier northerly flow aloft 
will develop Thursday with an upper ridge offshore. A second 
frontal system will reach the area on Friday followed by an upper 
low later on Saturday. Cool upper level trough remaining over the
area Sunday and Monday.

&&

.SHORT TERM /TODAY THROUGH THURSDAY/...Satellite imagery shows
cloudy skies over Western Washington early this morning. Doppler
radar has a few light rain showers along the Central Coast and
over the Puget Sound area. Temperatures at 3 am/11z were in the 
30s. 

Leading edge of the precipitation out ahead of the next frontal
system to reach the area over Central Vancouver Island at 3 am.
Precipitation spreading over the area late morning into the
afternoon hours. Increasing onshore flow ahead of the front will
push the snow levels up to at least 500-1000 feet. The exception
to this will be in Whatcom county where the increasing onshore 
flow will not arrive until late in the day keeping the snow levels
near the surface for the onset of the precipitation especially 
over the interior of Whatcom county. Have issued a winter weather
advisory for up to 3 inches of snow for the county with the 
higher accumulations away from the water and toward the Canadian 
border. Snow increasing in the mountains this afternoon with 
increasing snow amounts later this afternoon into the evening as 
the orographic flow picks up. Models have 850 mb winds rising to 
west northwesterly 20-30 knots by late in the day. The air mass 
is already somewhat cold so expect this system to be a good snow 
producer with higher than normal snow to water ratios. Heaviest 
precipitation over the Central and Southern Cascades. Have issued 
a winter storm warning for the Cascades from Snohomish to Lewis 
counties for 8 to 18 inches of new snow this afternoon and 
tonight and a winter weather advisory for the Cascades of Whatcom
and Skagit counties. Highs today in the lowlands will range from 
the mid 30s in Whatcom county to the mid 40s over the southern 
interior. 

Front moving through the area tonight with a quick convergence
zone developing over Central Puget Sound area late tonight into
the early morning hours. Enhanced precipitation rates possible
with the convergence zone which could lower the snow level down to
300-500 feet. For now will leave the mention of snow out of the
forecast for the central Sound. With the low level onshore into
Wednesday morning and plenty of cloud cover over the area. Lows 
will remain in the mid 30s for the most part.

Cool upper level trough behind the front moving over the area on
Wednesday. Snow levels will remain low, around 500 feet or so, but
showers will be widely scattered and precipitation amounts will be
light. Highs on Wednesday mostly in the lower 40s. 

Upper level trough moving south of the area Wednesday night with
showers confined to the extreme southern portion of the area in
the evening. Drier northerly flow aloft will help reduce the cloud
cover a little which in turn will allow temperatures to drop into
the mid 20s to lower 30s by Thursday morning. 

Drying trend continuing on Thursday with north northeasterly flow
aloft over the area. Some weak Fraser river outflow winds possible
in Whatcom county with the Bellingham to Williams Lake gradient
in the -10 to -12 mb range. Highs on Thursday in the lower to mid
40s. 

.LONG TERM /FRIDAY THROUGH MONDAY/...Some minor differences in the
extended models early on with the 00z GFS speeding up the timing
of the next system moving down out of the northwest starting the
precipitation over western Washington in the morning on Friday. 
The ECMWF is about 6 hours slower with the onset of the 
precipitation with most of it beginning in the afternoon. The 
earlier precipitation start on the GFS brings up the possibility 
of a rain snow mix with snow levels low, 500 feet or less, in the 
morning. Same scenario as the system for today with snow levels 
rising in the afternoon with increasing low level onshore flow. 
Have used a model blend on Friday and gone for chance pops in the 
morning. The precipitation currently looks pretty light so even if
it does start out as snow no significant accumulations expected. 
Front moving through the area later Friday night into Saturday. 

Cool upper level low settling over the area for Saturday night for
another showers and low snow levels forecast. Models try to bring
in some drier northerly flow aloft on Sunday but then develop
another upper level trough over the area Sunday night into Monday.
With the trofiness will have to keep a chance of showers in the
forecast as well as the continuation of the low, 1000 feet or 
less, snow levels. Highs through the period will remain well below
normal, mostly in the lower 40s. Felton

&&

.AVIATION...Northwest flow aloft becoming westerly today as an upper 
level trough shifts southward along the British Columbia coast. The 
air mass is moist and stable. Widespread low MVFR and areas 
of IFR are expected today. Scattered showers this morning will 
become more numerous this afternoon and evening. A Puget Sound 
Convergence Zone is expected to form over the northern part of Puget 
Sound this evening and shift southward overnight. 

KSEA...Low MVFR and periods of IFR expected through the forecast 
period. Showers expected through the day though the flow 
aloft favors some rain shadowing by the Olympics. Surface winds S-SW 
7 to 15 knots.   27

&&

.MARINE...A frontal system will bring westerly small craft
advisory strength winds to all waters later today into Wednesday 
morning. The flow will turn more northerly on Wednesday and then 
northeasterly Wednesday night and Thursday as high pressure moves 
over southern British Columbia. Another frontal system will drop 
down from the northwest on Friday with a trailing weak low moving 
over the area on Saturday. Schneider

&&

.HYDROLOGY...River flooding is not expected for the next 7 days.


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning from noon today to 6 AM PST Wednesday for 
     Cascades of Pierce and Lewis Counties-Cascades of Snohomish 
     and King Counties.

     Winter Weather Advisory from noon today to 6 AM PST Wednesday 
     for Cascades of Whatcom and Skagit Counties.

     Winter Weather Advisory from noon today to 6 PM PST this evening 
     for Western Whatcom County.

PZ...Small Craft Advisory from 1 PM this afternoon to 10 AM PST 
     Wednesday for Coastal Waters From Cape Flattery To James 
     Island Out 10 Nm-Coastal Waters From James Island To Point 
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To 
     Cape Shoalwater Out 10 Nm-Northern Inland Waters Including 
     The San Juan Islands.

     Small Craft Advisory from 1 PM this afternoon to 10 PM PST 
     Wednesday for Coastal Waters From Cape Flattery To James 
     Island 10 To 60 Nm-Coastal Waters From James Island To 
     Point Grenville 10 To 60 Nm-Coastal Waters From Point 
     Grenville To Cape Shoalwater 10 To 60 Nm.

     Small Craft Advisory from 1 PM this afternoon to 7 AM PST 
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Puget 
     Sound and Hood Canal.

     Small Craft Advisory from 4 PM this afternoon to 4 AM PST 
     Wednesday for West Entrance U.S. Waters Strait Of Juan De 
     Fuca.

     Small Craft Advisory from 7 PM this evening to 10 AM PST 
     Wednesday for Admiralty Inlet.

&&

$$

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