[willamette-fcst] Forecast - Monday, September 29th, 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 29th, 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 upper-level ridge over the region is slowly shifting east and was centered over
     Idaho late this morning.  South-southeasterly flow aloft was circulating mid and high-level
     moisture into South-Central and Southwestern Oregon with Doppler radar indicating some light
     shower activity.  Satellite imagery showed clouds making it northward, across Lane County,
     into Southern Linn County.  Most of Eastern Oregon and all of Northern Oregon continued to 
     have sunny skies as of late this morning.  Valley temperatures were running about 10 degrees
     warmer than at the same time on Sunday and were mostly in the 70s.

     Some changes in the weather pattern were beginning to show up on the late-morning ODA surface
     analysis.  The thermal trough had shifted inland, into Southwestern Oregon, and extended
     northwestward to off the Washington Coast.  Onshore flow was advancing northward along the
     coast, with a blanket of low clouds extending as far north as Tillamook County.  The north
     coast still had sunny skies, with offshore flow, but that will not last long.

     The thermal trough will likely shift inland, into the Willamette Valley, this afternoon,
     with low clouds moving northward along the entire length of the Oregon Coast.  Some marine
     air may begin spilling into the Southern Willamette Valley by this evening, but prevailing
     winds in the valley should be light northerly this afternoon.  Valley temperatures will
     approach record warm readings this afternoon, especially north, with high in the upper 80s.

     The increasing southeasterly flow aloft will continue to circulate middle and high clouds
     northward today across the Willamette Valley.  That will block the sun and keep
     temperatures a few degrees cooler in the south valley today.  Increasing mid-level
     instability may combine with daytime heating to trigger shower/thundershower development as
     far north as the Willamette Valley this afternoon and evening.  The best chance of showers 
     will be in the south valley.  This is a classic summer-time thunderstorm pattern, for the
     Willamette Valley, but the sun angle is lower this time of the year, and that limits the
     amount of low-level energy available to fuel the storms.     

Surface Winds:
     N 3-8 this afternoon...possibly becoming NW 5-12 mph south valley late this afternoon.
Transport Winds:
     N 8 this afternoon...possibly becoming NW 8 south valley late this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 2300 feet.  Ventilation index 18.
High Temperature:
     Salem\'s high temperature today will be near 88.
Humidities:
     Minimum relative humidity will be near 24%.
Sunrise/Sunset:
     Salem sunset tonight: 6:55pm; sunrise tomorrow: 7:10am.

Extended Outlook:
     As the upper-level ridge continues to shift east, south-southeasterly flow aloft will
     continue to spread middle and high-level moisture northward, across Oregon, Tuesday.
     Daytime heating may, once again, help trigger shower and thundershower development...mainly
     from the Cascades eastward.  increased cloud-cover and the thermal trough shifting inland
     will begin a general cool-down, west of the Cascades, Tuesday.  That may present a burning
     opportunity Tuesday afternoon, as mixing heights climg to near 3000 feet with forecast
     northwesterly transport winds.

     By Wednesday afternoon, the thermal trough is forecast to shift well east of the Cascades,
     with increasing onshore flow into the Willamette Valley.  Transport winds are expected to
     increase from the south-southwest, presenting another possible burning opportunity.
     
     The first in a series of progressively wetter weather systems is forecast to come onshore
     Wednesday night with a good chance of light rain by Thursday morning...turning to showers
     in the afternoon.  A more potent autumn cold front is forecast to come onshore late Friday
     with a soaking rain forecast for Friday night.  Rain will turn to showers Saturday with
     total rainfal amounts possibly exceeding one inch in the valley.  There is every reason
     to believe that will bring a damp end to the 2008 burning season.

Tomorrow (30 Sep):  Increasing Middle & High Clouds.  Slight Chance Shower.  52/82

Wed (01 Oct):  Mostly Cloudy.  Increasing Chance of Rain Late.  52/78

Thu (02 Oct):  Cloudy and Much Cooler.  Light Rain Likely at Times.  55/66

Fri (03 Oct):  Rain Increasing Late...Possibly Heavy at Times.  52/66

Sat (04 Oct):  Rain Turning to Showers.  51/63

Sun (05 Oct):  Mostly Cloudy.  Chance of Light Rain.  49/67

Mon (06 Oct):  Mostly Cloudy.  Chance of Rain Late.  50/68

ODA Meteorologist
weather at oda.state.or.us