[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 12:00pm.

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
     Eastern Oregon, today.  That will destabilize the atmosphere and lead to thunderstorm
     development today...mainly east of the Cascades.  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 late-morning ODA surface analysis showed a broad surface thermal trough extending
     from SW Oregon northward through the Eastern Willamette Valley to near The Dalles.  It
     is slowly shifting eastward and is forecast to move into Central Oregon this afternoon.
     Midday valley temperatures were near 70 degrees with hazy sunshine.  Increasing onshore
     flow should cap Willamette Valley temperatures in the low 80s this afternoon.

     Satelliete imagery showed low clouds along the entire Washington and Oregon Coastlines.
     The low clouds had not penetrated inland, beyond the coastal range, due to the presence
     of the thermal trough in the Willamette Valley.  Lots of clouds were circulating
     northward across the eastern half of the state and may develop into scattered showers
     and thunderstorms this afternoon.

     The air aloft has cooled some since Tuesday morning, but mixing heights are forecast to
     remain below 3000 feet until mid-afternoon.  As the upper-level trough sweeps northward,
     across Eastern Oregon this afternoon, transport winds should veer from southeasterly to
     south-southwesterly.  Mixing heights may climb to near 4000 feet later this afternoon,
     but some minor waring aloft may hold mixing heights near or just below 3000 feet.

     There may be an open burning opportunity during the mid-to-late afternoon period, after
     the thermal trough shifts out of the Willamette Valley and into Central Oregon.  However,
     transport winds may be too southerly to allow for burning with mixing heights perhaps no
     higher than 3000 feet.  Thundershowers appear as if they will stay mainly in Eastern
     Oregon, so they should be less of a factor than previously forecast.

Surface Winds:
     Becoming SW 5-12 this afternoon.
Transport Winds:
     S to SW 9 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be 3-4000 feet.  Ventilation index 36.
High Temperature:
     Salem\'s high temperature today will be near 83.
Humidities:
     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 more 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 weak ridge is forecast to
     build back over the region starting Tuesday afternoon.

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