[willamette-fcst] Willamette Valley Agricultural/Burning Forecast

[Willamette Valley Ag/Burning 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