[willamette-fcst] Forecast - Wednesday, November 19th, 2008

Wed Nov 19 09:16:09 PST 2008

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, November 19th, 2008 at 9:00am.

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

Weather Discussion:
     A very weak cold front spread considerable middle and high
     clouds across Oregon Tuesday but was too weak to break the
     strong low-level temperture inversion over the Willamette
     Valley.  Areas of morning fog gave way to filtered sunshine
     Tuesday afternoon.  Winds remained light and valley highs
     climbed into the mid to upper 50s.

     An upper-level ridge began rebuilding over Oregon last night
     with the surface cold front stalling over Oregon.  Fog and
     low clouds redeveloped across much of the Willamette Valley
     overnight and in some of the valleys of southwestern and
     eastern Oregon.  Southwesterly flow aloft was continuing to
     spread middle and high clouds across most of the state this
     morning.

     The ODA surface analysis showed increasing offshore flow
     across Oregon, and easterly winds were gusting to near 30
     mph at Troutdale, in the western Columbia Gorge, by
     mid-morning.  That should help to keep dense fog out of the
     extreme north valley today, although some fog was being
     reported in western parts of the north valley, near
     Hillsboro.  Easterly winds were also keeping fog away from
     the coastline.

     Satellite imagery showed middle and high clouds covering
     most of the state.  A very weak upper-level disturbance is
     forecast to move over Oregon today, in the southwesterly
     flow aloft, so only some filtered sunshine can be expected,
     once the areas of morning fog begin to lift.  The offshore
     flow will increase across western Oregon with continued dry
     easterly outflow, from the Columbia Gorge, fanning out
     across the north valley.  Winds should remain light in the
     central and south valley.

     The sounding over Salem this morning was similar to Tuesday
     morning.  Warm air aloft is creating a very stable air mass
     with continued poor ventilation conditions over Oregon. 
     Surface temperatures this morning ranged from the upper 20s
     to the low 40s east of the Cascades and were generally in
     the 40s across western Oregon.  Highs today should be mostly
     in the mid 50s across the Willamette Valley and along the
     coast.  Central and Eastern Oregon temperatures will climb
     into the 50s and low 60s.

     An approaching cold front will further increase the offshore
     flow tonight, which should keep fog from forming in the
     north valley.  Southeastery winds may develop enough to keep
     fog from forming in the central and south valley.  In any
     case, middle and high clouds, from the approaching weather
     system, will begin spreading onshore by Thursday morning
     with rain possible by daybreak.

Surface Winds:
     N 0-3 this morning, N 0-3 this afternoon.
Transport Winds:
     N 3 this morning, N 3 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 500 feet.  Ventilation index 2.
High Temperature:
     Salem\'s high temperature today will be near 54.
Humidities:
     Minimum relative humidity will be near 71%.
Sunrise/Sunset:
     Salem sunset tonight: 4:39pm; sunrise tomorrow: 7:17am.

Extended Outlook:
     Rain will likely spread across western Oregon Thursday
     morning, with cooler air aloft improving ventilation
     conditions.  Rain will be heaviest in the north valley,
     where amounts could top one-half inch.  Even with
     significant cooling aloft, valley highs will only fall to
     near normal.  Snow levels will quickly drop to about 4000
     feet, by midday Thursday, with a few inches of snow likely
     over the Cascade passes Thursday afternoon and night.

     A warm front will spread clouds across western Oregon
     Friday.  Rainfall appears as if it will stay north of
     Oregon, until a cold front approaches the coastline Friday
     evening.  The cold front will sweep across western Oregon
     early Saturday with a few showers lingering into Saturday
     night.  Rainfall from this system will generally be less
     than one-quarter of an inch across the Willamette Valley
     with the greatest totals in the northern Cascade foothills.

     A strong upper-level ridge will likely bring a return of dry
     weather Sunday through Tuesday with a little
     sunshine...mainly in the north valley.  Low-level
     temperature inversions will make for poor ventilation
     conditions and lead to valley fog formation. Outflow, from
     the Columbia Gorge, should help keep widespread fog from
     forming in the north valley, but fog could become persistent
     in the central and south valley.

     The ridge is forecast to begin breaking down about Tuesday
     of next week with increasing southwesterly flow aloft. 
     Middle and high clouds will likely spread over the region
     Tuesday with a threat of light rain moving onshore by
     Tuesday night.  A weak cold front may bring light rain to
     the valley next Wednesday.  This system may be cold enough
     to bring some snow to the ski areas, just in time for the
     Thanksgiving holiday weekend.

Tomorrow (20 Nov):  Rain Likely.  Snow Level Dropping to 4-5000 Feet.  45/51

Fri (21 Nov):  Mostly CLoudy.  Chance of Light Rain North. Snow Level 5000 Feet.  36/52

Sat (22 Nov):  Light Rain AM...Decreasing Showers PM.  Snow Level 4-5000 Feet.  41/51

Sun (23 Nov):  Areas of Fog...Mainly Central and South Valley.  Partly Sunny.  35/55

Mon (24 Nov):  Areas of Fog...Mainly Central and South Valley.  Partly Sunny.  37/52

Tue (25 Nov):  Areas of Fog...Increasing Clouds.  37/54

Wed (26 Nov):  Chance of Light Rain and Mountain Snow.  Snow Level 4000 Feet.  41/49

ODA Meteorologist
weather at oda.state.or.us