[willamette-fcst] Forecast - Thursday, December 11th, 2008

[Willamette Valley Ag/Burning Forecast]

Daily Smoke Management Forecast




Oregon Department of Agriculture
Smoke Management Program
Weather Outlook and Field Burning Advisory for Willamette Valley Growers and Fire Districts.

Issued: 
     Thursday, December 11th, 2008 at 9:00am.

Burn Advisory:
     Agricultural burning is not recommended.
     Stack burning is not allowed.

Weather Discussion:
     The strong upper-level ridge, that has generally been over
     the Pacific Northwest since mid-November, is showing signs
     of shifting westward this morning with increasing
     northwestery flow aloft.  Infrared satellite imagery showed
     an area of middle and high clouds, associated with a weak
     upper-level disturbance, covering most of Washington and
     Oregon and extending a couple of hundred miles offshore. 
     That helped to keep dense fog from forming in the Willamette
     Valley overnight, but the valley was socked in with low
     clouds and a light fog this morning.

     Astoria, on the north coast, picked up one-quarter of an
     inch of rain Wednesday morning, with a few hundredths of an
     inch falling as far south and east as Tillamook and
     Scappoose. Mostly dry conditions prevailed across the
     remainder of Oregon Wednesday with valley temperatrues
     climbing to near 50 degrees in the afternoon.  Mostly cloudy
     skies overnight kept dense fog from forming in the valley,
     but light fog was prevalent.

     No surface stations in Oregon were reporting any
     precipitation this morning, but Doppler radar was showing
     some sprinkles moving onto the central coast.  Temperatures
     were warmer this morning with minimums staying above
     freezing across the Willamette Valley.  Minimums east of the
     Cascades were mostly in the 20s and 30s.  The coldest
     readings were in south-central and southeastern Oregon. 
     Lakeview dropped to 16 degrees and Burns dipped to 17. 
     Mid-morning temperatures were in the low to mid 40s across
     the Willamette Valley with low clouds and light fog.

     The Salem sounding this morning showed a little cooling
     aloft, but the freezing level was still up at 9700 feet. 
     Mixing heights will likely climb above 1000 feet this
     afternoon.  However, ventilation is still too poor to allow
     stack burning.  In addition, north-northeasterly transport
     winds are not conducive to good smoke evacuation from the
     valley.

     The ODA surface analysis showed a weak thermal trough along
     the coast with light north-northeastery gradients across
     western Oregon.  Little change in that pressure pattern is
     expected this afternoon.  Morning low clouds and fog will
     get way to mostly cloudy and mild conditions this afternoon.
     There is a slight chance of a sprinkle...mainly in the
     south valley.  Highs should, once again, climb into the
     upper 40s and perhaps locally reach 50.  Today is the calm
     before the storm.  More on that in the extended
     discussion...

Surface Winds:
     N 5-10 this morning, N 5-10 this afternoon.
Transport Winds:
     NNE 11 this morning, NNE  11 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 1200 feet.  Ventilation index 13.
High Temperature:
     Salem\'s high temperature today will be near 50.
Humidities:
     Minimum relative humidity will be near 66%.
Sunrise/Sunset:
     Salem sunset tonight: 4:31pm; sunrise tomorrow: 7:41am.

Extended Outlook:
     A major change in the weather pattern will begin Friday, as
     the upper-level ridge of high pressure continues to shift
     weastward and builds north into the Gulf of Alaska. 
     Increasing northwesterly flow aloft will direct a series of
     cold weather systems southward, along the British Columbia
     Coastline, into the Pacific Northwest.  That will greatly
     improve ventilation conditions across western Oregon.

     The first system will likely bring rain and strong southerly
     winds to the northern Oregon Coast around midday Friday and
     into the Willamette Valley Friday afternoon.  A High Wind
     Watch has been issued along the Oregon Coast for southerly
     gusts to 65 mph Friday.  South winds may gust to 40 mph in
     the Willamette Valley Friday afternoon and evening, as the
     cold front moves onshore.  

     Although this system will initially bring rain, even to
     fairly high high elevations, snow levels will rapidly drop
     Friday afternoon and night.  Snow levels will start near
     7000 feet, Friday morning, and drop to near 1000 feet, in
     the wake of the cold front, Friday night.  Heavy snow and
     strong winds will make for blizzard conditions in the
     Cascade passes by late Friday, where a Winter Storm Watch
     has been issued.  The coastal passes may see accumulating
     snow as early as Friday afternoon with snowy travel likely
     by Friday night.

     In the wake of the cold front, cold and unstable
     northwestery flow aloft will bring heavy snow showers to
     both the coastal and Cascade passes Saturday and Sunday.
     Total snow accumulations could exceed two feet over the
     Cascade passes with up to a foot of snow over the coast
     range passes.  Be prepared for extreme winter driving
     conditions over any mountain passes this weekend.

     Colder Arctic air will slowly sag southward, into
     Washington, Saturday and northern Oregon as early as
     Saturday night.  Ahead of the Arctic cold front, The cold
     air will be pulled out over the warmer ocean and then
     returned inland in the form of rain and snow showers, with
     the snow levels progressively lowering.  Sticking snow will
     probably get as low as 500 feet Saturday, and perhaps to the
     valley floors by Saturday night...especially north.  The
     snow level will drop to the valley floors, from north to
     south, on Sunday, as the cold Arctic air begins pouring
     directly into the valley, via the Columbia Gorge.  This
     transition, into a much coler air mass, has the potential to
     bring the most significant snow accumulations to the
     Willamette Valley floor since January, 2004.  It is starting
     to look more and more as if the valley will get measurable
     amounts of snow on the valley floor Sunday.

     Drier and cold Canadian air will continue to move into the
     region Sunday night and Monday, with snow showers tapering
     off and temperatures dropping well below freezing.  Brisk
     easterly winds will develop in the Portland area with
     northerly winds in the remainder of the valley.  The next
     weather system is forecast to drop down the British Columbia
     coastline around the middle of next week.  With cold air
     already in place, across the Willamette Valley,
     precipitation from that system will fall in the form of
     snow.  It is still too far out to forecast how much snow may
     fall, but it is looking more likely that the valley will get
     measurable snowfall late Wednesday and/or Thursday of next week.

Tomorrow (12 Dec):  Increasing Rain and Wind.  PM Snow Level Dropping to 2000 Feet.  35/46

Sat (13 Dec):  Rain or Snow Showers.  Snow Level 500 Feet or Lower.  35/39

Sun (14 Dec):  Snow.  Valley Accumulations Likely.  30/34

Mon (15 Dec):  Decreasing Snow Showers.  Clearing and Turning Much Colder.  23/30

Tue (16 Dec):  Mostly Sunny and Cold.  18/29

Wed (17 Dec):  Increasing Chance of Snow.  17/29

Thu (18 Dec):  Snow Likely Early...Decreasing Late.  22/28

ODA Meteorologist
weather at oda.state.or.us