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Area forecast discussion...updated 
National Weather Service Seattle Washington 
433 am PST Sat Nov 21 2009 


Updated the aviation and marine segments. 


Synopsis...a weak ridge will move through western Washington this 
morning...followed quickly by a vigorous frontal system. This system 
will bring rain and windy conditions tonight and 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 late in the week. 


&& 


Short term...we have northwest flow aloft over western Washington 
between an upper trough that moved through the area overnight and a 
weak upper ridge offshore along 130w. There are still scattered 
showers across the forecast area this morning...and a Puget Sound sound 
convergence zone extending across island and Snohomish counties. It 
is still breezy in some areas...but the wind should ease over the 
next few hours as the surface high pressure that has been building 
into the area shifts east of the Cascades ahead of the next front. 
The freezing level has fallen to around 3000 feet and temperatures 
across the western Washington lowlands were in the upper 30s to middle 
40s at 3 am. 


The weak ridge offshore will move quickly across western Washington 
this morning...and the convergence zone will likely weaken and 
dissipate. The south end of an occluded front swinging into British 
Columbia will quickly follow the ridge...with significant 
precipitation reaching the coast and Olympics this afternoon. A 
frontal wave beginning to develop near 45n/145w will deepen as it 
moves rapidly eastward with the help of a 140 knots jet. It will move 
through the forecast area tonight. 


Heavy snow in the mountains looks like a sure bet. The snow level 
will rise only to around 3000 feet as this feature moves through...and 
precipitation amounts will be impressive. Liquid amounts of 3 to 4 
inches in the Olympics are possible through Sunday morning -- 
translating roughly to 30 to 40 inches of snow. 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. Water 
equivalents in the Cascades will likely be in the 1 to 3 inch 
range...for snowfall totals of 10 inches to 2.5 feet. The Winter Storm 
Watch will be upgraded to a warning with the early morning forecast 
package. 


One matter that is somewhat uncertain is the track of the surface 
low. The most likely scenario is that it will reach the central 
Washington coast around midnight tonight...track northeast across 
the olympic peninsula and the north part of the Puget Sound sound region 
to around Montana Baker at 4 am...then head rapidly into British 
Columbia. It will produce breezy to windy conditions...but there 
appears to be little threat of a wind storm. 


Post-frontal showers will taper off Sunday afternoon and evening... 
and an upper ridge will move inland Sunday night. The ridge will 
quickly head east of the Cascades Monday and another frontal system 
just upstream of the ridge will arrive. This system could initially 
bring decent precipitation amounts to the coast and olympic 
peninsula Monday afternoon...but it will weaken as it moves inland 
Monday night. By recent standards this will be an uneventful system 
and it will mark the beginning of a period of quieter...though not 
necessarily dry...weather. Mcdonnal 


Long term...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...the system coming in today through midday Sunday ... 
while it will produce impressive amounts of precipitation...will 
have minimal effect on rivers except possibly for 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. With the 
snow level only rising to around 3000 feet...it will be a mountain 
snow producer not a flooding rain producer. 


The Skokomish River is forecast to fall below flood stage about noon 
today. There will be a planned release of 3000 cfs from Cushman dam 
#2 into the North Fork Skokomish River today. This release will keep 
the Skokomish River running high. The forecast calls for the river 
to remain just below flood stage tonight and Sunday. 


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 is 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 should move through quickly and be too 
weak to cause any flooding...even with rising snow levels. For the 
remainder of the week...the air mass over the northwest 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. Mcdonnal 


&& 


Aviation...a weak...upper ridge over the region will move east 
tonight...allowing a strong...upper trough to move over the area. 
Increasing westerly flow aloft...subsiding and becoming northwest late tonight. 


At the surface...a frontal system over the eastern Pacific will rapidly approach 
western Washington during the day today...resulting in increased southerly winds. The 
occluded front will move across the area this evening. A secondary 
surface low is anticipated to develop off the far SW Washington or far northwest Oregon 
coast this evening...and track northeastward across the area later tonight. 
Thus expect the winds to back behind the occluded front this 
evening...and this should raise the potential for low level wind shear since winds 
above the surface will remain southerly. Winds will pick up dramatically from 
the west or SW late tonight or early Sunday once the surface low moves east of 
the area. 


Meanwhile...a Puget Sound sound convergence zone /pscz/ was roughly 
located between kpae and kawo at this time. This feature should 
drift north and dissipate by midmorning as its support wanes. 


Ksea...winds will remain southerly today...becoming more Ely tonight. 
There will be a potential for low level wind shear between 0000 and 1200 UTC Sunday 
due to Ely winds at the surface and strong southerly winds near 2k feet. Have 
held off on mentioning this in the taf until we see another model run. 
The current model solutions were quite different than the previous 
run due to a more vigorous system now expected to affect the area. 
Winds will increase dramatically from the SW sometime between 1000 
UTC and 1200 UTC Sunday. 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. 


&& 


Marine...there was a significant change in the forecast from the 
previous one due to a more vigorous system expected to affect the 
area today. Southerly winds are still anticipated to increase during the 
day...ahead of a fast-moving frontal system still well offshore... 
for gale or Small Craft Advisory wind speeds. The occluded front 
will sweep across the area this evening. Due to somewhat low 
confidence have went with a Small Craft Advisory for the northern 
inland waters and East Entrance to the Strait of Juan Delaware fuca today 
instead of upgrading the previous gale watch to a warning. 


Another surface wave or low is expected to develop off the far SW 
Washington or far northwest Oregon coast tonight...and this will cause the 
winds to back and drop off...especially across the inland waters. 
The low will move NE across interior western Washington overnight. 
Winds will kick back up late tonight or early Sunday in response to 
strong onshore or westerly flow developing. Confidence was fairly 
high that gale force winds will develop over the Strait of Juan Delaware 
fuca early Sunday...thus have issued a gale watch for the central 
and eastern Strait. There is a possibility that winds may briefly 
reach storm force...near 50 knots...over the central Strait Sunday 
morning with the initial push. 


Anticipate the onshore flow to weaken and become southerly Sunday 
night. The next frontal system will affect the waters on Monday. 


&& 


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. 


Pz...a Gale Warning remains in effect for the coastal waters and 
West Entrance to the Strait of Juan Delaware fuca through this 
evening. 


A Small Craft Advisory is in effect for the remaining 
waters. 


A Small Craft Advisory for rough Grays Harbor bar conditions 
is in effect until noon Monday. 


$$ 


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For an illustrated version of the forecast discussion...please see 
www.Weather.Gov/Seattle/gafd/latest_webafd.Html. 

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