[willamette-fcst] Forecast - Monday, September 15th, 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: 
     Monday, September 15th, 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:
     A broad upper-level ridge of high pressure built over the
     Pacific Northwest during the weekend with the low-level flow
     turning offshore by Sunday and helping warm valley
     temperatures to near 90 degrees.  The ridge axis extended
     from Ariznon through Nevada and Eastern Oregon,
     northwestward to SE Alaska this morning.  It was so broad
     that it was producing generally clear skies over the western
     third of the U.S. and most of British Columbia.

     The ODA surface analysis showed a thermal trough extending
     from the Southern Oregon to the Northern Washington Coast
     with offshore flow continuing to bring clear skies to most
     of Western Oregon.  The exception was along the Central and
     Southern Oregon Coast, where onshore flow was slowly
     advancing a blanket of low clouds and fog northward.  It had
     advance about to Newport by mid-morning.

     The sounding over Salem this morning showed further warming
     aloft since Sunday, so valley temperatures will top out
     several degrees warmer today.  Highs should climb into the
     mid 90s this afternoon, which will threaten some daily
     records.  The very warm air aloft will keep mixing heights
     below 3000 feet today with north-northeasterly transport
     winds.  In additions, State Fire Marshal conditions may be
     reached late this afternoon due to the hot temperatures and
     low relative humidities.

Surface Winds:
     N 3-8 this morning, N 5-12 this afternoon.
Transport Winds:
     NE 6 this morning, NNE 8 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 2800 feet.  Ventilation index 22.
High Temperature:
     Salem\'s high temperature today will be near 95.
Humidities:
     Relative humidity drops to 50% by 11am.
     Minimum relative humidity will be near 20%.
Sunrise/Sunset:
     Salem sunset tonight: 7:22pm; sunrise tomorrow: 6:53am.

Extended Outlook:
     The upper-level ridge is forecast to slowly shift eastward
     Tuesday with light south-southwesterly flow aloft
     developing.  The broad thermal trough is forecast to move
     over the Willamette Valley for another very warm afternoon. 
     Some marine air may seep into the valley late in the
     afternoon...capping high temperatures a couple of degrees
     cooler than on Monday, but daily records will, once again,
     be threatened.  Mixing heights will remain very low due to
     warm air aloft.

     A weak upper-level low pressure system is forecast to move
     onto the Northern California Coast Wednesday, with
     increasing south-southwesterly flow aloft over Oregon. 
     Increasing mid and high level moisture will combine with
     daytime heating to trigger scattered shower and
     thundershower activity...mainly over the Cascades.  The
     surface thermal trough is forecast to shift eastward to over
     Eastern Oregon and Idaho.  That will bring cooler onshore
     flow into Western Oregon.  There may be a burning opportunity
     with the transition to cooler conditions Wednesday.  However,
     southerly flow aloft and the possible development of 
     thunderstorms over the Cascades makes that questionable.

     A stronger upper-level low pressure system is forecast to
     take a similar track and approach the Northern California
     Coastline by Thursday afternoon.  South-southwesterly flow
     aloft will increase over Oregon with a chance of showers
     moving into Western Oregon late in the day.  This system
     also has the potential to produce a burning opportunity with
     increasing onshore flow possibly turning transport winds
     westerly.  Once again, limiting factors could be the
     presence of showers and too much southerly flow aloft.

     The computer models have considerable disagreement,
     beginning late this week, so my confidence in the extended
     forecast details is below average.  The second upper-level
     low pressure system appears as if it will slide across
     Northern California Thursday, with a split-fow jet stream
     pattern Friday and Saturday for Oregon.  That would bring
     dry conditions to Western Oregon with weak onshore flow.

     The next weather system is forecast to approach the coastal
     waters late Sunday and Monday, with the flow aloft becoming
     more southwesterly over Oregon.  Depending on the timing and
     strength of that system, it may present a burning
     opportunity either Sunday or Monday.

Tomorrow (16 Sep):  Sunny.  53/93

Wed (17 Sep):  Increasing Clouds.  Slight Chance of T-Storms.  53/81

Thu (18 Sep):  Mostly Cloudy.  Chance of Showers Late.  53/75

Fri (19 Sep):  Partly Cloudy.  50/76

Sat (20 Sep):  Partly Cloudy.  50/77

Sun (21 Sep):  Partly Cloudy.  49/76

Mon (22 Sep):  Increasing Clouds.  Chance of Showers Late.  48/75

ODA Meteorologist
weather at oda.state.or.us