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Area forecast discussion 
National Weather Service Seattle Washington 
1113 am PST Sat Nov 21 2009 


Synopsis...a weak upper level ridge will move over western 
Washington through midday today...followed quickly by a vigorous 
frontal system this afternoon and evening bringing rain and windy 
conditions to the area into Sunday. Heavy snow will fall in the 
mountains with the snow level remaining at or below 3000 feet. A 
weaker front will move through the area Monday and Monday 
night...and another weak front will arrive later in the week. 


&& 


Short term...northwest flow aloft over western Washington and 
lingering scattered showers across the area this morning. The 
freezing level has fallen to around 3000 feet. The weak offshore ridge 
will continue to move quickly across western Washington this 
morning...followed by the south end of an occluded front swinging 
into British Columbia bringing significant precipitation to the the 
coast and Olympics this afternoon. A frontal wave and strong jet 
will 45n/145w will deepen and move through the forecast area 
tonight. 


Heavy snow for the mountains remains on track with impressive 
precipitation amounts over the Olympics in the 3 to 4 inch range and 
the Cascades in the 1 to 3 inch neighborhood through Sunday morning. 
Snow levels rise to around 3000 feet as this feature moves through... 
with roughly to 30 to 40 inches of snow expected for the Olympics 
and 10 to 30 inches for the Cascades. Even the relatively drier 
Hurricane Ridge area should receive Winter Storm Warning 
amounts...ie 12 inches in 12 hours or 18 inches in 24 hours. 


The track of the surface low remains one of the forecast chalanges 
for the day. The most likely scenario remains that it will reach the 
central Washington coast around midnight tonight...move northeast 
across the olympic peninsula and into the north part of the Puget Sound 
sound region to around Montana Baker at 4 am and then head into British 
Columbia. It will produce breezy to windy conditions across the 
area. Stongest winds will likely be with the southerlies ahead of 
the low tonight especially along the coast and northwest 
interior...and again with the westerlies Sunday morning through the 
east Strait into Whidbey Island. For now have issued a high wind 
watch for the central Washington coast through the Chehalis gap and 
into the southwest interior for the strong westerlies with the low 
Sunday morning. In addition have issued a Wind Advisory for the 
north interior...Admiralty Inlet...San Juans and east Strait for the 
southerlies this afternoon and tongith followed by the westerlies 
with and behind the low. Will be watching evolution and track of the 
low as it approaches the coast this afternoon. Winds should begin to 
come down by late Sunday morning all areas. 


Post-frontal showers will taper off Sunday afternoon evening as an 
upper ridge will move overhead Sunday evening. The ridge kicks east 
quickly Monday ahead of another frontal system. This system could 
still bring decent precipitation amounts to the coast and olympic 
peninsula Monday afternoon...but weakens as it moves inland Monday 
night. Compared to last several systems this will be an uneventful 
system...marking the beginning of a period of quieter weather. Cook 


Long term...from previous discussion...a weak upper ridge will 
follow the front that moves through Monday and Monday night. There 
is just a chance of rain in the forecast for Tuesday and 
Wednesday...with a fairly good chance for some dry weather over 
those two days. A weak front will slowly move across western 
Washington Wednesday night and Thursday. Another frontal wave will 
probably keep the area wet Thursday night and Friday...though 
disagreement between models drops off considerably by late in the 
week. Mcdonnal 


&& 


Hydrology...todays system will bring impressive amounts of 
precipitation to the Cascades and Olympics...but relatively low snow 
levels will mitigate the impact on rivers except the Two Rivers that 
are already close to or above flood stage -- the skokomish and the 
Lower Reach of the Chehalis in Grays Harbor County. The snow level 
will only rising to around 3000 feet...making this mostly a mountain 
snow event and not a rain event. 


The Skokomish River has fallen to near or just below flood stage at 
this time...new forecast guidance shows the river rising again to 
just below 17 feet by early Sunday morning. In addition...there will 
be a planned release of 3000 cfs from Cushman dam #2 into the North 
Fork Skokomish River this afternoon. Accordingly will extend the 
warning for the skokomish into Monday. 


A Flood Watch remains in effect for the Lower Reach of the Chehalis 
river in Grays Harbor County...which has been within a foot of flood 
stage for the past two days. It remains conceivable that the 
precipitation from the incoming system could help finally nudge the 
lower Chehalis above flood stage. If flooding does occur it will be 
minor. 


The next system on Monday looks to be progressive enough and weak 
enough to be of little concern with respect to flooding. The air 
mass over the northwest for the remainder of the week will be warmer 
with higher snow levels...however the systems that are expected to 
affect the area will be fairly weak and are not expected to cause 
any additional flooding concerns. 


No flooding is expected at any time on the Green River. Cook 


&& 


Aviation...a warm front will spread rain into western Washington this afternoon 
through tonight. SW flow aloft will increase through the day and 
become strong southerly flow by this evening. The atmosphere will remain 
moist and stable. 


A strong surface low will develop offshore today and 
tonight...reaching the S or central Washington coast by 12z Sun morning as a 
potential 985 mb low. This will bring strong southerly flow aloft and at 
the surface...shifting westerly after 12z as the low tracks inland near 
khqm...probably tracking just south of kolm. However...the track is 
uncertain at this time with some models showing a more northern 
solution. 


Ksea...southerly winds 8-15 knots today...becoming more Ely tonight. There 
will be a potential for low level wind shear between 03z and 12z sun due to Ely 
winds at the surface and strong southerly winds near 2k feet. Confidence is 
still low but may will monitor and if needed update the taf prior to 
the 00z issuance. Latest 12z model solutions continue to vary 
considerable regarding the track and depth of a surface low now 
expected to affect the area. Winds will increase dramatically from 
the SW sometime between 12z and 15z. Concerning ceilings...anticipate 
ceilings to gradually lower during the day...although there will be 
occasional ceilings near 2500 feet this morning. By late this 
evening...ceilings should be near 2k feet...with rain and fog reducing 
visibilities into the 3-5sm range. Dtm 


&& 


Marine...winds will pick up along the coast by midday and across 
the inland waters during the afternoon as a warm front reaches 
western Washington. Latest models show gales to develop with this 
initial front from around the East Entrance/Admiralty Inlet northward and 
also the coastal waters. Wind Advisory for the sound still looks 
good. 


A potentially strong 985 mb surface low will enter the coastal 
waters after 09z tonight and track into the central/S Washington coast 
around 12z. Winds behind the front will rise drastically so I upped 
the winds to high end gale and extended the warning through Sun 
morning. Depending on the exact track...storm force winds could be 
possible across the central/S Washington coastal waters. Will evaluate this 
possibility before the afternoon coastal forecast issuance. Dtm 


&& 


Sew watches/warnings/advisories... 
Washington...flood warnings and/or flood watches remain in effect for 
a portion of the County Warning Area. 


A Winter Storm Warning is in effect for the Olympics and 
Cascades from this afternoon through Sunday morning. 


Wind Advisory in effect for the north interior...east 
Strait...and Admiralty Inlet through Sunday morning. 


High wind watch in effect for the central coast...lower 
Chehalis...and southwest interior from late tonight through 
Sunday morning... 


Pz...a Gale Warning is in effect for the coastal waters...and 
from Admiralty Inlet northward through early Sun morning. 
.A gale watch is in effect for the central and 
east Strait...and Admiralty Inlet from early 
Sun morning through Sunday afternoon. 
.A Small Craft Advisory is in effect for Puget Sound sound from 
noon through Sunday morning. 




$$ 


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www.Weather.Gov/Seattle/gafd/latest_webafd.Html. 

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