[willamette-fcst] Forecast - Wednesday, September 17th, 2008

[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: 
     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