[willamette-fcst] Forecast - Tuesday, December 9th, 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: 
     Tuesday, December 9th, 2008 at 9:00am.

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

Weather Discussion:
     The Salem sounding this morning showed strong warming
     warming in response to a building upper-level ridge of high
     pressure.  That stabilized the air mass overnight and
     allowed widespread fog to form throughout the Willamette
     Valley.  Visibilities were generally about one-quarter of a
     mile or less across the valley this morning.  A warm front
     was spreading middle and high clouds across all of
     Washington and much of Oregon.  Doppler radar showed light
     rain falling across sections of northwesterln Washington and
     extending southwestward to just off the northern Oregon Coast.

     There were no surface reports of precipitation across Oregon
     as of mid-morning.  The ODA surface analysis showed high
     pressure centered over southeastern Oregon with
     southeasterly pressure gradients across both Oregon and
     Washington.  Winds were around 5 mph or less across all of
     Oregon, except for along the coast, where Newport was
     getting southeasterly winds of 10 mph.  The moist low-level
     air mass was helping to keep temperatures above freezing
     across most of the Willamette Valley at mid-morning  Aurora
     and Eugene were right near the freezing mark.  Eugene
     briefly dropped to 26 degrees early this morning, before the
     fog formed.

     Fair skies allowed temperatures to get considerably colder
     east of the Cascades this morning, with virtually all
     locations dropping below freezing.  Lakeview and Rome
     dropped down to 9 degrees this morning, and Burns fell to 10
     degrees.  Pendleton was the warmest spot I could find, east
     of the Cascades, with a low of 30 degrees.  Even The Dalles
     dropped to 29 thanks to calm winds.  Mid-morning
     temperatures were still near the morning minumims.

     The warm front will spread light rain southward today across
     most of western Washington and possibly extreme northern
     Oregon.  All of Oregon will see increasing amoutns of middle
     and high clouds.  The western valleys will have a tough time
     getting rid of the morning fog, but some afternoon sunbreaks
     are possible, especially in the south valley.  Ventilation
     conditions will be poor today, due to cool surface
     temperatures and unseasonably warm air aloft.  The freezing
     levels over Salem and Medford this morning were measured at
     11,600 feet and 12,500 feet respectively.

Surface Winds:
     SE 3 this morning, SE 3 this afternoon.
Transport Winds:
     Var 4 this morning, SE 6 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 500 feet.  Ventilation index 3.
High Temperature:
     Salem\'s high temperature today will be near 46.
Humidities:
     Minimum relative humidity will be near 68%.
Sunrise/Sunset:
     Salem sunset tonight: 4:31pm; sunrise tomorrow: 7:40am.

Extended Outlook:
     An west-to-east oriented cold front will sag southward into
     Washington tonight and possibly as far south as extreme
     northern Oregon Wednesday.  That will keep skies mostly
     cloudy tonight and Wednesday across the Willamette Valley
     with areas of overnight fog reforming...mainly in the
     central and south valley.  Light winds will allow
     temperatures to locally drop to the freezing mark overnight,
     but Wednesday morning minimums should be a couple of degrees
     warmer than this morning.  Continued warming aloft may add a
     couple of degrees to afternoon high temperatures, especially
     in the south valley.

     The cold front is forecast to dissipate, as it sags
     southward, into the central Willamette Valley, on Thursday. 
     Skies should stay mostly clouds with at least a slight
     chance of light rain...mainly north.  A little cooling aloft
     will improve ventilation conditions but not likely enough to
     allow for stack burning. Skies should at least partially
     clear Thursday evening with areas of fog reforming.

     A major change in the weather pattern is forecast to begin
     Friday, as our protective ridge of high pressure shifts
     westward, to the Gulf of Alaska, and directs a series of
     cold weather systems southward, along the British Columbia
     Coastline, into the Pacific Northwest.  The first system
     will likely spread rain onto the Oregon Coast by around
     midday Friday and into the Willamette Valley Friday
     afternoon.  Increasing southerly winds should lift the fog
     from all of the valley, with rapid cooling aloft greatly
     improving ventilation conditions.

     Big changes are in store for the mountains, beginning
     Friday, with significant snow accumulations likely beginning
     later Friday.  The cold front will drop snow levels in the
     Cascades to pass elevations by late Friday afternoon with
     winter driving conditions likely by Friday evening and
     night.  A cold and unstable northwestery flow aloft will
     prevail Saturday and Sunday with locally heavy mountain
     snows likely.

     As the weekend progresses, colder Arctic air will be sagging
     southward, across southern British Columbia, Washington, and
     eventually northern Oregon.  Initially, that cold air will
     be modified, as it is pulled out over the warmer ocean and
     then returned inland in the form of cold showers.  Snow
     levels will get progressively lower with each band of
     showers...eventually reaching the valley floors. 
     Eventually, Arctic air that has not been out over the warmer
     ocean waters, will begin spilling into the Willamette
     Valley, via the Columbia Gorge...dropping snow levels to the
     surface.  That transition to a drier and cold air mass may
     bring significant snow accumulation to the valley, if enough
     moisture is still available in the northwesterly flow aloft.

     Drier and cold Canadian air will continue to move into the
     region later Sunday and Monday, with snow showers tapering
     off and temperatures dropping well below freezing.  Brisk
     eastly winds will develop in the north valley with northerly
     winds in the central and south valley.  Winter will have arrived!

Wed (10 Dec):  Areas of AM Fog...Mainly South.  Mostly Cloudy.  Chance of Rain North.  34/49

Thu (11 Dec):  Areas of AM Fog...Mainly South.  Mostly Cloudy.  Chance of Light Rain.  37/50

Fri (12 Dec):  Rain Developing.  Snow Level Dropping to 4000 Feet.  35/46

Sat (13 Dec):  Rain or Snow Showers.  Snow Level Dropping to near Valley Floor.  33/39

Sun (14 Dec):  Snow Likely...Possible Accumulations.  28/34

Mon (15 Dec):  Partly Cloudy and Cold.  21/30

Tue (16 Dec):  Mostly Sunny and Very Cold.  14/27

ODA Meteorologist
weather at oda.state.or.us