[willamette-fcst] Forecast - Wednesday, December 10th, 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: 
     Wednesday, December 10th, 2008 at 9:00am.

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

Weather Discussion:
     A strong upper-level ridge of high pressure remains over
     Oregon today with very warm air aloft trapping cool air near
     the surface.  The Salem sounding this morning showed a steep
     low-level temperature inversion with surface temperatures in
     the mid 30s and 2000-foot temperatures near 50 degrees. 
     That inversion will cap mixing heights at or below 500 feet
     again today, making for poor ventialtion conditions over the
     Willamette Valley.

     A very weak frontal system moved southward across Washington
     yesterday and was near the northwest corner of Oregon this
     morning.  Satellite imagery showed a band of middle and high
     clouds across northern Oregon with low clouds along the
     north coast and over the western valleys.  Visible satellite
     imagery showed that the south coast, and the higher inland
     locations across Oregon had mostly sunny skies this morning.
     Low clouds also covered many of the valley locations in
     central and easten Oregon, due to the stagnant conditons.

     The Willamette Valley had cloudy and/or foggy conditions
     this morning with temperatures mostly in the 30s.  Eugene
     had icy fog with a temperature of just 32 degrees.  There
     have been a few sprinkles in the valley this morning with
     some very light rain along the north coast, at Astoria. 
     Skies should stay mostly cloudy today with just a slight
     chance of sprinkles.  Winds will be light and temperaures
     will edge up into the mid 40s across most of the valley.

Surface Winds:
     SE 3 this morning, Var 3 this afternoon.
Transport Winds:
     S 5 this morning, Var 3 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 79%.
Sunrise/Sunset:
     Salem sunset tonight: 4:31pm; sunrise tomorrow: 7:41am.

Extended Outlook:
     The cold front is forecast to dissipate across Oregon on
     Thursday.  Skies should stay mostly cloudy over the
     Willamette Valley with a slight chance of sprinkles.  A
     little cooling aloft will improve ventilation conditions but
     not likely enough to allow for stack burning. Partial clearing
     Thursday evening may allow areas of fog to reform overnight.

     A major change in the weather pattern will begin Friday, as
     our protective ridge of high pressure shifts westward, into
     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 and windy weather onto the Oregon Coast by
     around midday Friday and into the Willamette Valley Friday
     afternoon.  Brisk southerly winds should lift the fog from
     all of the western valleys, with rapid cooling aloft greatly
     improving ventilation conditions.

     Although this system will initially bring rain, even to
     fairly high high elevations, conditions will rapidly change
     during the day Friday.  As the strong cold front moves into
     the Cascades, Friday afternoon, much colder air aloft will
     lower to snow level to the passes with strong winds and
     heavy snow making for extremely difficult travel conditions.

     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 several inches likely 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 by Sunday
     afternoon.  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.  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 floors in several years.

     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 north valley with
     northerly winds in the central and south valley.  The next
     weather system is forecast to drop down the British Columbia
     coastline and into the cool dome of air over the region
     around the middle of next week.  If it is able to tap into
     enough Pacific moisture, then it would bring more snow the
     the Willamette Valley.  That is still too far in the future
     to predict at this time.

Tomorrow (11 Dec):  Areas of AM Fog.  Mostly Cloudy.  Slight Chance of Sprinkles.  37/48

Fri (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.  33/39

Sun (14 Dec):  Rain or Snow...Turning to 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):  Chance of Snow.  17/29

ODA Meteorologist
weather at oda.state.or.us