254 
fxus66 ksew 091104 cca 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
238 am PST Tuesday Feb 9 2010 


Synopsis...an upper level ridge will bring areas of morning fog and 
afternoon sunshine to western Washington today. An approaching cold 
front will bring rain to the area by late Wednesday afternoon. A 
stronger and wetter frontal system will affect the area late 
Wednesday night and Thursday. Stronger southwest flow aloft over the 
region will sent additional weather systems across the area Friday 
through the weekend. 


&& 


Short term...the longwave ridge over The Rockies will be the 
dominant feature today. Supporting the longwave ridge is the 
offshore shortwave ridge approaching 135w. The upper level shortwave 
trough near 130w is all that is left of the splitting system that 
fell apart just offshore last evening. Clouds from the associated 
frontal band just offshore dissipated rapidly after 03z Monday 
evening so that areas of IFR ceilings and/or visible have developed at the 
majority of ob sites. Otherwise most other spots are clear. GFS/NAM 
have this remnant upper level shortwave trough moving over the coast 
around 00z with little effect. 


NAM time-height sections seem to have a lot of moisture around 
3000-5000 feet overnight and this morning that does not show up in 
the GFS time-height sections. NAM does not seem to have caught up to 
the drying shown on infrared imagery so will follow the GFS solution for 
sky condition today. 


All models are in good agreement showing the upstream frontal system 
currently along 140w charging east and reaching the area Wednesday. 
Should see cirrus increasing at least late tonight. 00z model 
guidance appears to have shifted to a faster solution...so all model 
and MOS guidance now support at least likely probability of precipitation for the interior 
Wednesday afternoon. Will raise probability of precipitation for the coast to at least 
likely Wednesday morning. Snow levels will be low enough for snow to 
reach the passes...though it may be close at Snoqualmie. NAM 12 hour 
precipitation amounts are below Snow Advisory criteria for the mountains. 
The precipitation should ease shortly after 00z as the shortwave trough 
moves through. The surface front may end up dissipating over the 
area. 


European model (ecmwf)/GFS are in good agreement with the next stronger system late 
Wednesday night and Thursday. A warm front will affect the area late 
Wednesday night and Thursday morning followed by the cold front 
Thursday afternoon/evening. Snow levels will be higher...especially 
over the Olympics...and precipitation heavier. Winds are below advisory 
criteria except for the marine areas. Kam 


Long term...the hemispheric pattern shows minor changes between 
early this week and late this week...but this will impact the 
Pacific northwest. 5-wave charts show the mean longwave upper level 
ridge currently over the Pacific northwest shifting east over The 
Rockies by Thursday. Simultaneously the upper level trough over the 
NE Pacific deepens slightly and shifts a little further east...to 
around 150w or 145w. This is enough to set up a more active SW flow 
aloft over the Pacific northwest that will probably persist into early next 
week. GFS/European model (ecmwf) show a fast-moving cold front arriving 
Friday...followed by a stronger warm front/cold front combo Saturday 
through early Sunday morning. Likely probability of precipitation for rain at times seems 
reasonable for the extended period. Kam 


&& 


Hydrology...a slight shift in the hemispheric pattern will allow 
stronger SW flow aloft to develop over West Washington from middle-week Onward. A 
cold front on Wednesday will be the first system to reach the area. 
A stronger warm front/cold front combo may bring some heavier rain 
to the area late Wednesday night/Thursday...with a short period of 
45-50kt SW winds aloft over the Olympics. 06z NAM 24 hour precipitation 
shows over 2.5 inches over the Olympics ending Thursday evening. 
This probably won't cause any flooding but will cause rises on 
olympic rivers. 


A fast moving cold front on Friday probably won't add significant 
precipitation to the area. However a stronger system arriving this weekend 
has more potential for heavy rain. Expect another round of 50-60kt 
SW flow aloft. This system looks stronger and wetter than the 
Thursday system...with a longer duration over West Washington. European model (ecmwf) is a 
little quicker. 


Like previous wet systems this season...the strong SW flow aloft 
with the two systems mentioned above will affect mainly the 
Olympics...and to a lesser extent the north Cascades. The probability of 
flooding on the Green River...in the central Cascades is low. Kam 


&& 


Aviation...areas of fog or low stratus are over western Washington...they 
will likely disperse and mix out midday...with a few of the more 
stubborn locations like Olympia possibly in the afternoon. The flow 
surface and aloft is light and not until around daybreak Wednesday does the 
flow aloft pick up. The models show rather a lot of moisture spreading 
into western Washington Wednesday morning...so there is a good chance that after 
patchy fog tonight skies turn cloudy rather rapidly Wednesday morning. 19 




Ksea...current taf is the best guess for the morning fog...in some 
places the fog has lifted to a low stratus deck but that wont help 
ksea up on the hill. Skies should clear midday as the fog disperses 
and mixes out. As southerly flow develops Wednesday morning clouds will quickly 
increase and so I do not expect a repeat of the fog Wednesday morning. 


&& 


Marine...a series of progressively stronger frontal systems will 
affect the area starting Wednesday...coastal southerly gales are fairly certain 
by late in the week. Today will be a quiet day with light offshore 
flow...morning fog...and then light southerly flow increasing on Wednesday with 
the first front. 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...none. 




$$ 


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