[willamette-fcst] Forecast - Thursday, January 22nd, 2009

[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, January 22nd, 2009 at 9:00am.

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

Weather Discussion:
     Washington and Oregon are still under the influence of a
     weak southwesterly flow aloft today.  The Salem sounding
     this morning showed continued cooling aloft, between about
     2000 and 20,000 feet.  The freezing level was 7300 feet
     early this morning, which is down 2000 feet from Wednesday
     afternoon.  The strong low-level temperature inversion over
     the Willamette Valley is starting to weaken.  3000-foot
     temperatures have cooled below 50 degrees.  However, mixing
     heights will remain very low today with continued poor
     ventilation conditions.

     The late-morning ODA surface analysis continued to show
     offshore flow across western Oregon, with high pressure over
     eastern Washington and Oregon and a thermal trough of low
     pressure just off the coastline.  Offshore pressure
     gradients have continued to weaken.  Easterly winds were
     still gusting to 25 mph at Troutdate and to near 45 mph in
     Corbett, at the west end of the Columbia Gorge.  Winds were
     calm at the Portland Airport and throughout most of the
     Willamette Valley.

     Considerable middle and high clouds kept overnight
     temperatures a little warmer in the Willamette Valley,
     although they were still below normal.  Corvallis, Salem,
     and Hillsboro fell to 28 degrees, Eugene diipped to 31, and
     McMinnville dropped to the freezing mark.  There was not
     much in the way of fog this morning across the valley with
     skies remaining mostly cloudy.  Dry easterly winds kept fog
     off the coast this morning, where minimum temperatures were
     held in the mid 30s to low 40s by mostly cloudy skies.
     Mostly cloudy skies also kept overnight temperatures warmer
     east of the Cascades, where minimums this morning were
     mostly in the 20s.

     A weakening weather system moved onshore last night and was
     strong enough to produce some sprinkles across southwestern
     Oregon and a light wintry mix of precipitation across
     sections of eastern Oregon.  Amounts were very light.
     Infrared satellite imagery showed the center of that weak
     weather system near San Francisco, CA.  Middle and high
     clouds covered most of the western third of the
     country...including Oregon.  Doppler radar showed some rain
     in Northern California but only some sprinkles and very
     light showers making it north into southern and eastern
     Oregon.  No rain was reported overnight in the Willamette
     Valley.  Medford was reporting some light rain at
     mid-morning.

     Skies will remin mostly cloudy across Oregon today.  There
     is a slight chance of sprinkles or very light showers across
     mainly southern and eastern Oregon.  With cold air trapped
     near the surface in the valleys of eastern Oregon and in the
     Columbia Gorge, any precipitation could be in the form of a
     wintry mix, but amounts should be very light.  The freezing
     should stay around 7000 feet with Cascades pass temperatures
     climbing into the low 40s this afternoon.  A weak ridge is
     forecast to dry out the air mass tonight and Friday.  

Surface Winds:
     N 0-5 this morning, N 0-5 this afternoon.
Transport Winds:
     N 5 this morning, N 4 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 900 feet.  Ventilation index 5.
High Temperature:
     Salem\'s high temperature today will be near 46.
Humidities:
     Minimum relative humidity will be near 65%.
Sunrise/Sunset:
     Salem sunset tonight: 5:07pm; sunrise tomorrow: 7:41am.

Extended Outlook:
     An upper-level ridge is forecast to build northward, into
     the Gulf of Alaska, and drive a cold but mostly dry weather
     system southward into the Pacific Northwest this weekend. 
     That will lower snow levels to near 2000 feet Saturday with
     a chance of showers returning to the region.  The cool
     upper-level trough will further weaken the low-level temperature
     inversion and improve ventilation conditions Saturday.

     It appears likely now that some colder Arctic air will spill
     into Washington late Saturday and into northern Oregon
     Sunday, but it may not be accompanied by much
     moisture...which would limit the low-elevation snow
     potential.  Temperatures should start out in the upper 30s,
     on Sunday, then hold steady or fall during the day, as
     colder air moves south across the Willamette Valley.  The
     transition to colder air will likely be accompanied by snow
     flurries with accumulations likely remaining below an inch. 
     This pattern will need to be monitored closely, however,
     since a slight change to a more over-the-water track of this
     system could bring a couple of inches of snow to the floor
     of the Willamette Valley.

     Skies should begin to clear Sunday night, as northerly winds
     usher colder and dry air southweard across western Oregon. 
     Willamette Valley minimums should drop into the low to mid
     20s and perhaps locally into the upper teens.  Skies will
     continue to clear Monday, as a cold high pressure system
     settles over the area.  High temperatures will struggle into
     the low to mid 30s.  After a cold night Monday night, mostly
     sunny skies and a buiding ridge of high pressure aloft will
     begin to moderate temperatures Tuesday.  A weak weather system
     will bring some clouds and perhaps a few sprinkles Wednesday,
     followed by dry conditions Thursday with areas of fog.

Tomorrow (23 Jan):  Patchy AM Fog.  Partly Cloudy.  31/47

Sat (24 Jan):  Increasing Chance of Showers.  Snow Level dropping to 2000 Feet.  32/45

Sun (25 Jan):  Light Rain Turning to Light Snow...Colder.  Minor Accumulations.  32/38

Mon (26 Jan):  Slight Chance of Light Snow Early.  Partly Cloudy and Cold.  25/34

Tue (27 Jan):  Mostly Sunny and Cold.  22/38

Wed (28 Jan):  Mostly Cloudy.  Chance of Sprinkles.  29/41

Thu (29 Jan):  Areas of AM Fog.  Partly Sunny.  30/45

ODA Meteorologist
weather at oda.state.or.us