[willamette-fcst] (no subject)

[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, September 17th, 2008 at 9:00am.

Burn Advisory:
      Agricultural burning is not recommended.
      Preparatory burning is not allowed.
      Propane flaming is not allowed.
      Stack burning is not allowed.

Weather Discussion:
      The strong ridge of high pressure that has been 
giving
      Oregon a late-summer hot spell is moving to the east 
and
      will allow a weak upper-level trough to move 
northward,
      across Oregon, today.  That will destabilize the 
atmosphere
      and lead to thunderstorm development today...mainly 
from the
      Cascades eastward.  South-southeasterly flow aloft 
is
      continuing to circulate California wildfire smoke 
over
      Oregon, so we will see a mix of clouds and hazy 
sunshine
      over the state today.

      The ODA surface analysis showed the surface thermal 
trough
      still over Western Oregon at mid-morning, but it is 
forecast
      to shift east of the Cascades by this afternoon. 
 Increasing
      onshore flow should cap Willamette Valley 
temperatures in
      the low 80s this afternoon.  The air aloft has 
cooled some
      since Tuesday morning, but mixing heights are 
forecast to
      remain below 2000 feet this morning and below 3000 
feet
      until mid-afternoon.  The upper-level trough will 
continue
      to cool the air aloft this afternoon with transport 
winds
      veering from southeasterly to southwesterly.  Mixing 
heights
      should climb to near 4000 feet by late this 
afternoon.

      This transition may create an open burning 
opportunity for
      the mid-to-late afternoon period.  However, there 
are some
      potential limiting factors.  Southerly flow aloft 
may keep
      mixing heights suppressed below 3000 feet, and 
thundershower
      development over and east of the Cascades may not 
allow for
      good evacuation of smoke over the Cascades (poor
      gradient-stacking conditions).  There is also a 
slight
      chance of a thundershower rolling off the Cascades 
into the
      Willamette Valley, because the flow above 10,000 
feet is
      forecast to remain southeasterly today.

Surface Winds:
      SW 5 this morning, SW 5-12 this afternoon.
Transport Winds:
      SE 6 this morning, SW 9 this afternoon.
Atmospheric Ventilation Conditions:
      Maximum mixing height today will be near 4000 feet. 
 Ventilation index 36.
High Temperature:
      Salem\'s high temperature today will be near 83.
Humidities:
      Relative humidity drops to 50% by 12pm.
      Minimum relative humidity will be near 35%.
Sunrise/Sunset:
      Salem sunset tonight: 7:18pm; sunrise tomorrow: 
6:55am.

Extended Outlook:
      Another upper-level low pressure system is forecast 
to
      approach the Northern California Coastline Thursday
      afternoon with increasing south-southwesterly flow 
aloft and
      continued onshore flow at the surface.  This system 
should
      bring a little more cooling to Western Oregon with a 
slight
      chance of showers, as it comes onshore late Thursday 
and
      Friday.  This system will likely bring showers to 
Southern
      and Eastern Oregon.  It may also create favorable 
conditions
      for burning.

      A split-flow jet stream pattern is forecast by 
Saturday,
      with one branch to our south and the other to our 
north,
      leaving Oregon in a dry weather pattern with weak 
onshore
      flow making for seasonal temperatures.  That could 
create
      burning opportunities over the weekend.  The next 
weather
      system is forecast to approach the coast late Sunday 
and may
      bring some light rain to the valley by Monday 
afternoon.  A
      ridge is forecast to begin building back over the 
region
      next Wednesday.

Tomorrow (18 Sep):  AM Clouds...Mostly Sunny.  52/78

Fri (19 Sep):  Mostly Cloudy.  Slight Chance of a 
Shower...Mainly South.  50/75

Sat (20 Sep):  Partly Cloudy.  50/75

Sun (21 Sep):  Mostly Sunny.  Increasing Clouds Late. 
 49/75

Mon (22 Sep):  Increasing Chance of Light Rain and Cooler. 
 48/68

Tue (23 Sep):  Partly Cloudy.  Slight Chance of Showers. 
 48/70

Wed (24 Sep):  Mostly Sunny.  47/72

ODA Meteorologist
weather at oda.state.or.us