[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 9:25am.

...Corrected temperatures on extended forecast...

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 Arizona through Nevada,
     and Eastern Oregon, northwestward to SE Alaska this morning.  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
     advanced 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 today, 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 more westerly transport winds.  Once again, limiting factors
     could be the presence of showers/t-storms 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.  50/80

Mon (22 Sep):  Increasing Clouds.  Chance of Showers Late.  50/75

ODA Meteorologist
weather at oda.state.or.us