000 
fxus66 ksew 181240 cca 
afdsew 


Area forecast discussion...corrected 
National Weather Service Seattle Washington 
440 am PST Sat Nov 18 2017 


Corrected a couple of minor typos. 


Synopsis...weak high pressure aloft will prevail through 
tonight. Expect a Pacific storm to approach the area during the 
day Sunday. The cold front will sweep across the region during 
the late Sunday afternoon through Sunday evening time period. 
Expect locally strong winds and heavy precipitation ahead of and 
with the front. Weak high pressure aloft will return on Monday. 


&& 


Short term... 
there were scattered showers over the County Warning Area at this time. Expect the 
showers to decrease in coverage this morning, becoming confined to 
the far northwest part of the County Warning Area by this afternoon. 


Meanwhile, weak high pressure aloft will prevail over the region 
through tonight. The ridge aloft will weaken further early Sunday, 
allowing an upper level trof and associated cold front to approach 
from the northwest. The models were a little slower with this 
system than the previous runs; therefore, it appeared that the 
southeast part of the County Warning Area will probably not see any precipitation 
until Sunday afternoon. The front will sweep across the area 
during the late Sunday afternoon through Sunday evening time 
frame. Expect locally strong southerly winds over parts of the 
coast and northwest interior ahead of and with the front. A Wind 
Advisory will probably be needed for some of these zones for 
Sunday. 


Concerning snowfall: the north Cascades will remain in the colder 
air throughout the day sunday; therefore, the precipitation will 
fall as snow above the 3000 foot level. The Mount Baker ski area 
could pick up about a foot of snow during the day sunday; 
therefore, a Winter Storm Watch was issued for Whatcom and Skagit 
counties. The snow will taper off over this area behind the cold 
front Sunday evening. 


Locally heavy precipitation will continue on the Cascades of 
Pierce and Lewis counties as the snow levels fall Sunday night; 
therefore, the Crystal Mountain ski area and the southwest slopes 
of Mount Rainier could receive up to about a foot of snow Sunday 
night before the precip tapers off. Therefore, a Winter Storm 
Watch was issued for this area. 


Expect advisory snowfall amounts (5-10 inches) in the Cascades of 
Snohomish and King counties Sunday night. Stevens Pass may pick 
up about 10 inches of new snow. 


The GFS and European model (ecmwf) solutions were in better agreement as far as 
Monday was concerned. The Canadian model was the outlier. It looks 
as though a warm front will approach the region from the south 
during the day. Much of the County Warning Area will be dry on Monday. 


Long term... 
much of this period appeared mild, wet, and unsettled. Temps are 
anticipated to be above normal during this period. Expect locally 
windy periods, with the possibility of flooding on some area 
rivers. Confidence in the details was not high for this period. 


&& 


Aviation...a flat upper ridge centered over western Washington 
early this morning will shift east into the Idaho Panhandle late 
tonight. A strong and wet cold front will sag southeast through 
British Columbia today then will shift southward through western 
Washington on Sunday. Moderate westerly flow aloft. The air mass 
is moist and somewhat stable and will dry somewhat at lower levels 
today. Deep moisture will shift southward into the area late 
tonight into Sunday morning. 


Isolated convective showers over the coastal waters and extending 
into the interior from about kshn southward continue to weaken and 
diminish in coverage early this morning. Conditions are variable 
with pockets of MVFR or IFR ceilings and other areas seeing 
ceilings 035-045. Expect the lower level moisture to decrease this 
morning leaving just mid and high level clouds above about fl100. 
Cigs will fall to 008-015 after 10z tonight through Sunday 
morning as rain ahead of the incoming front shifts southeast into 
the area. Albrecht 


Ksea...south wind 8-11 knots. Ceilings 030-035 this morning then above 
fl100 this afternoon and evening. Ceilings falling to 010-015 after 
10z Sunday. Albrecht 


&& 


Marine...a strong cold front extending west-southwest from the Queen 
Charlotte Sound into the Pacific will slowly drag southeast down 
the British Columbia coastline today then will push southeastward 
through the coastal and inland waters on Sunday. Winds will 
increase gradually from north to south through tonight with gales 
expected over all of the waters except for Puget Sound and Hood 
Canal and the central Strait of Juan de Fuca late tonight into 
Sunday ahead of the front. Winds will rapidly ease behind the 
front later Sunday into Sunday night. 


High pressure will bring light winds to the waters on Monday. A 
strong warm front will push northward through the waters on 
Tuesday. A strong low pressure center developing well offshore 
will push northeast into haida gwaii on Wednesday. Albrecht 


&& 


Hydrology... 
the models were not as wet with the Sunday system as the previous 
runs; however, the forecast rainfall amount for the Skokomish 
River basin was still enough to drive the uniquely flood-prone 
Skokomish River near or slightly above flood stage Sunday night. 


Beyond Sunday, the models did not exactly agree on the details. 
However, it still appears that much of this period will be wet. 
The snow levels will be atypically high (generally at or above 8k 
feet) for this time of the year Tuesday through early Thursday. 
This means that the precip will fall primarily as rain in the 
mountains. It looks like this firehose will be a bit different in 
that instead of staying on for x amount of time, it will be more 
of an on and off type of deal. Thus, expect occasional heavy rain 
in the mountains and on the coast. Although rainfall amounts may 
not be significant for any given time period, the cumulative 
rainfall for the Tuesday through Thursday period will probably be 
enough to drive some rivers above flood stage, not just the 
Skokomish River. 


&& 


Sew watches/warnings/advisories... 
Washington...Flood Watch in effect for Mason County from Sunday morning 
through Sunday evening. 


Winter Storm Watch from Sunday afternoon through late Sunday 
night for Cascades of Pierce and Lewis counties above 4500 
feet. 


Winter Storm Watch from late tonight through Sunday afternoon 
for Cascades of Whatcom and Skagit counties above 3500 feet. 


Pz...Small Craft Advisory until 10 PM PST this evening for coastal 
waters from Cape Flattery to James Island out 10 nm-coastal 
waters from Point Grenville to Cape Shoalwater 10 to 60 nm- 
northern inland waters including the San Juan Islands. 


Gale Warning from 10 PM this evening to 3 PM PST Sunday for 
coastal waters from Point Grenville to Cape Shoalwater 10 
to 60 nm-northern inland waters including the San Juan 
Islands. 


Small Craft Advisory until 6 PM PST this evening 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. 


Gale Warning from 6 PM this evening to noon PST Sunday 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. 


Gale Warning from 10 PM this evening to noon PST Sunday for 
coastal waters from Cape Flattery to James Island out 10 nm- 
West Entrance U.S. Waters Strait of Juan de Fuca. 


Small Craft Advisory until 3 am PST Sunday for coastal waters 
from James Island to Point Grenville out 10 nm-East 
Entrance U.S. Waters Strait of Juan de Fuca. 


Gale Warning from 3 am to noon PST Sunday for coastal waters 
from James Island to Point Grenville out 10 nm. 


Small Craft Advisory from 9 am this morning to 3 am PST Sunday 
for coastal waters from Point Grenville to Cape Shoalwater 
out 10 nm. 


Gale Warning from 3 am to 3 PM PST Sunday for Admiralty Inlet- 
coastal waters from Point Grenville to Cape Shoalwater out 
10 nm-East Entrance U.S. Waters Strait of Juan de Fuca. 


Small Craft Advisory until 10 PM PST this evening for West 
Entrance U.S. Waters Strait of Juan de Fuca. 


Small Craft Advisory until 3 am PST Sunday for Admiralty Inlet. 


Small Craft Advisory from 10 PM this evening to 9 PM PST Sunday 
for Puget Sound and Hood Canal. 


&& 


$$ 



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