[willamette-fcst] Forecast - Wednesday, January 21st, 2009

Wed Jan 21 09:00:44 PST 2009

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, January 21st, 2009 at 9:00am.

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

Weather Discussion:
     A very strong upper-level ridge has brought unseasonably dry
     and stagnant weather conditions to the region since about
     the 9th of January.  The weather pattern is undergoing a
     major change.  The protective ridge is splitting, with the
     southern section now over the Rockies and the northern
     section building offshore, into Alaska.  That has left
     Washington and Oregon under a weak southwesterly flow aloft.

     The Salem sounding this morning showed a couple more degrees
     of cooling, compared with Tuesday morning, between about
     3000 and 17,000 feet.  The freezing level had dropped
     slightly to 9300 feet.  There was still a strong low-level
     temperature inversion over the Willamette Valley with
     surface temperatures in the 20s and the 3000-foot
     temperature warming to 54 degrees.  That will lead to very
     low mixing heights and poor ventilation conditions again today.

     The morning ODA surface analysis continued to show offshore
     flow, with high pressure over eastern Washington and Oregon
     and a thermal trough of low pressure just off the coastline.
      Offshore pressure gradients did not weaken much overnight,
     and easterly winds were still gusting to over 30 mph at
     Troutdate and to near 60 mph in Corbett, at the west end of
     the Columbia Gorge.  However, the east winds had stopped at
     the Portland Airport, which is a sign of the relaxing
     pressure gradients.

     Calm winds and dry air in the Willamette Valley this morning
     allowed minimum temperatures to fall well into the 20s. 
     Eugene dipped to 23 degrees, while Salem and McMinnville
     fell to 24.  Aurora and and Corvallis bottomed out at 25. 
     Some fog was able to form in the south valley, near Eugene,
     while the rest of the valley saw increasing middle and high
     clouds, from the southwesterly flow aloft. Dry easterly
     winds were still dropping off the coastal range, to the
     coastline, keeping fog off the coast but also holding
     minimum temperatures there in the 30s to low 40s.  Astoria
     did manage to briefly dip to the freezing mark this morning,
     but Newport never fell below 40 degrees.

     Valley fog and low clouds were widespread again this
     morning, east of the Cascades, along with increasing middle
     and high clouds.  It was another seasonably cold morning
     with minimums mostly in the teens to mid 20s.

     Infrared satellite imagery showed a splitting and weakening
     weather system a couple of hundred miles offshore with
     middle and high clouds advancing inland across mainly
     California and Oregon.  Clouds were just starting to rapidly
     increase across southwestern Washington.  Doppler radar
     showed some light showers across northwestern California,
     with a few sprinkles possibly circulating as far north as
     Lane County, in Oregon.

     Continued cooling aloft and increasing cloud-cover should
     hold afternoon temperatures down a few degrees from Tuesday
     across western Oregon.  Valley highs will be in the mid 40s
     with coastal readings in the low to mid 50s.  Mild Cascades
     pass temperatures will continue, with highs today warming
     into the low to mid 40s. Some locations, in the western
     Cascade foothills, could climb into the low 50s.

     A weakening weather system will move into California today
     with southwesterly flow aloft bringing increaing clouds to
     Oregon.  Some very light shower activity could make it into
     southwestern Oregon today, and eventually into the southern
     Willamette Valley by this evening.  The air aloft will
     continue to cool but not likely enough to eliminate the
     strong low-level temperature inversion, so  ventilation
     conditions should remain poor this afternoon.

Surface Winds:
     N 0-5 this morning, N 3-7 this afternoon.
Transport Winds:
     N 8 this morning, N 5 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 700 feet.  Ventilation index 6.
High Temperature:
     Salem\'s high temperature today will be near 44.
Humidities:
     Minimum relative humidity will be near 55%.
Sunrise/Sunset:
     Salem sunset tonight: 5:05pm; sunrise tomorrow: 7:41am.

Extended Outlook:
     There is a slight chance of sprinkles or very light showers
     tonight and Thursday morning across all of Oregon,
     especially for the southern half of the state.  With cold
     air trapped near the surface in the Willamette Valley, some
     local freezing drizzle could make for slick roadways.
     However, increasing clouds will make for a warmer night
     tonight, with minimums hovering near the freezing mark.  The
     air aloft will continue to slowly cool with the snow level
     dropping to around 5000 feet Thursday.  That should begin to
     improve ventialtion conditions over the Willamette Valley.

     Any moisture riding over the trapped cold air in the
     Columbia Gorge and eastern valley locations could result in
     a wintry mix of precipitation tonight and Thursday, but
     amounts should be very light.  A weak ridge is forecast to
     dry out the air mass Thursday afternoon and Friday, but snow
     levels should stay around 5000 feet.

     It appears that the upper-level ridge of high pressure will
     rebuild in the Gulf of Alaska with a colder
     north-northwesterly flow aloft developing over the Pacific
     Northwest beginning this weekend.  That would bring much
     lower snow levels with some rain and snow to the region.  It
     appears likely now that some Arctic air will spill into the
     region by Sunday, but it may not be accompanied by much
     moisture...which would limit the low-elevation snow potential.

Tomorrow (22 Jan):  Slight Chance of Drizzle or Freezing Drizzle Early.  Clearing PM.  31/46

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

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

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

Mon (26 Jan):  Light Snow Ending Early.  Becoming Partly Cloudy and Cold.  25/32

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

Wed (28 Jan):  Increasing Clouds Late.  24/41

ODA Meteorologist
weather at oda.state.or.us