[willamette-fcst] Forecast - Friday, September 19th, 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: 
     Friday, September 19th, 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:
     An upper-level low presure system, just off the Northern
     California Coast this morning, with increasing the southerly
     flow aloft and sending more California and Oregon wildfire
     smoke over the region...making for hazy conditions. As that
     system moves onshore today, it will aid in the break-up of
     the marine low clouds over the Willamette Valley.  However,
     even if the marine low clouds clear from the Willamette
     Valley this afteroon, skies will remain mostly cloudy.

     The sounding over Salem this morning showed a shallowing
     marine layer, now less than 3000 feet thick, with
     northeasterly transport winds.  Increasing southerly winds
     are present above the marine layer with continued quite warm
     air aloft.  A strong temperature inversion was present at
     the top of the marine layer with quite warm air between 3000
     and 10,000 feet.  That will keep mixing heights below 3000
     feet until the air mass aloft begins cooling later this
     afternoon.  Temperatures aloft support Willamette Valley
     highs climbing into the 80s this afternoon, but the
     combination of the low-level marine air and cloud-cover
     should hold valley highs in the low 70s.

     The ODA surface analysis showed a stalled thermal trough
     extending from East-Central Washington, through Central
     Oregon, to SW Oregon.  It should finally progressed eastward
     today in response the the upper-level system coming onshore.
      Scattered showers are likely, with a possible
     thundershower, from SW Oregon across Central and Eastern
     Oregon later today and tonight, as the weather system slowly
     pushes eastward across Southern Oregon. It does not appear
     that showers will make it as far north as the Willamette
     Valley, but that possibility can not be entirely ruled
     out...especially for the south valley. 

     Cooler air aloft and northwesterly transport winds are
     forecast for Western Oregon later this afternoon. If surface
     temperatures warm enough this afternoon, mixing heights
     could reach 4000 feet or higher. Therefore, it is possible,
     although unlikely, that ventilation conditions could improve
     enough to allow for limited open burning this afternoon.  We
     plan to monitor the changing conditions this afternoon. 


Surface Winds:
     Var 0-5 this morning, NW 4-8 this afternoon.
Transport Winds:
     NE 9 this morning, NW 7 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 4500 feet.  Ventilation index 41.
High Temperature:
     Salem\'s high temperature today will be near 72.
Humidities:
     Relative humidity drops to 50% by 3pm.
     Minimum relative humidity will be near 43%.
Sunrise/Sunset:
     Salem sunset tonight: 7:14pm; sunrise tomorrow: 6:57am.

Extended Outlook:
     Onshore flow is likely Friday night and Saturday, as the
     upper-level low pressure suystem moves across Eastern Oregon
     and into Idaho.  That should bring a solid deck of marine
     low clouds back into the Willamette Valley Saturday morning.
      A more westerly flow aloft should finally help to clear the
     wildfire smoke from Western Oregon by Saturday afternoon. 
     Some afternoon sunshine is possible, as the marine clouds
     break-up.  If there is enough surface warming, mixing
     heights and transport winds may be sufficient to allow for
     limited open burning in the late afternoon, but that is unlikely.

     A transitory ridge is forecast to move over Western Oregon
     Saturday night with the next weather system, in westerly
     flow aloft, forecast to approach the Sunday afternoon.  That
     system has the potential to create a burning opportunity
     Sunday, if the air mass is able to warm enough ahead of it. 
     The timing of that system and the amount of marine air
     already in the valley makes that questionable.

     Some light showers are possible, beginning Sunday, through
     Monday, as a stronger upper-level trough moves over the
     state, via the westerly flow aloft.  Total rainfall amounts
     are forecast to be less than one-tenth of an inch with the
     greatest chance of rain in the north valley near the Cascade
     Foothills.

     High pressure is forecast to build over the region Tuesday
     with the onshore flow turning more northerly in the
     afternoon.  It appears that the upper-level ridge will
     strengthen over the region Wednesday for continued warming
     and drying of the air mass.  Transport winds will likely be
     northerly and may become offshore.  The ridge is forecast to
     weaken, by some of the long-range computer models, late next
     week with the possibility of a weather system creating a
     burning opportunity by Friday.  However, that is getting
     beyond the scope of the weather forecast models this time of
     the year.

Tomorrow (20 Sep):  Cloudy in the Morning.  Partial Afternoon Clearing.  51/72

Sun (21 Sep):  Mostly Cloudy.  Increasing Chance of Light Showers.  51/70

Mon (22 Sep):  Mostly Cloudy.  Chance of Showers and Cool.  48/65

Tue (23 Sep):  Becoming Partly Cloudy in the Afternoon.  45/71

Wed (24 Sep):  Sunny and Warmer.  47/78

Thu (25 Sep):  Sunny and Warm.  48/78

Fri (26 Sep):  Increasing Clouds and Cooler.  49/72

ODA Meteorologist
weather at oda.state.or.us