[willamette-fcst] Forecast - Thursday, September 18th, 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: 
     Thursday, September 18th, 2008 at 9:00am.

Burn Advisory:
     Agricultural burning is not recommended.
     Preparatory burning is allowed from 12:00pm until 2:00pm with a 100 acre limit.
     Propane flaming is allowed from 12:00pm until 5:00pm.
     Stack burning is not allowed.

Weather Discussion:
     A weak upper-level disturbance turned the flow aloft
     southeasterly Wednesday and carried smoke from Oregon
     Cascade wildfires over the Willamette Valley. That system
     moved into Idaho overnight with onshore flow ushering in a
     shallow layer of cooler marine air into the valley.
     Willamette Valley nephelometer readings dropped considerably
     overnight...indicating improved air quality. 

     The Salem sounding this morning showed that the flow aloft
     had become south-southeasterly again this morning, in
     response to an approaching weather system about 200 miles
     off the Southern Oregon Coast. That may direct more wildfire
     smoke over Western Oregon today. However, computer models
     forecast the flow aloft to turn more southerly this
     aftenoon, which may keep the bulk of the wildfire smoke east
     of the Willamette Valley. 

     Low-level onshore flow overnight was strong enough to push
     coastal low clouds inland across the Willamette Valley. 
     Satellite imagery showed low clouds extending from the coast
     to the west slopes of the Cascades.  Based on surface and
     pilot reports, the low clouds are only about 1500 feet
     thick, with clouds tops below 3500 feet.  That would imply
     an early afternoon break-out into sunshine with temperatures
     recovering into the mid 70s. Cooler air aloft and forecast
     southwesterly transport winds may improve ventilation
     conditions enough for limited open burning this afternoon.
     


Surface Winds:
     Var 0-4 this morning, SW 5-10 this afternoon.
Transport Winds:
     SW 5 this morning, SW 7 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 3700 feet.  Ventilation index 26.
High Temperature:
     Salem\'s high temperature today will be near 77.
Humidities:
     Relative humidity drops to 50% by 1pm.
     Minimum relative humidity will be near 39%.
Sunrise/Sunset:
     Salem sunset tonight: 7:16pm; sunrise tomorrow: 6:56am.

Extended Outlook:
     The next weather system is forecast to move onto the
     Southern Oregon/Northern California Coastline aroudn midday
     Friday.  It will likely spread some showers and possibly
     thundershowers from SW Oregon across much of Central and
     Eastern Oregon Friday and Friday night.  There is a chance
     of shower activity making it as far north as the Willamette
     Valley Friday, but amounts would not be impressive.  This
     system will further cool the air aloft, which has the
     potential to raise mixing heights, if precipitation stays
     south of the valley.  Forecast south-southwesterly transport
     winds may allow for limited open burning.

     In the wake of the system Friday, strong onshore flow will
     likely bring a thicker marine layer into the Willamette
     Valley by Saturday morning.  A drier westerly flow aloft
     should clear skies Saturday afternoon, but a deep marine
     layer and strong gradient-stacking may not allow for open
     burning.  The next weather system is forecast to approach
     the coast late Sunday, which could create a burning
     opportunity Sunday afternoon, if the air mass stays dry
     enough.  High pressure is forecast to build over the region
     starting Monday afternoon with the potential of a return to
     sunny and warm conditions during the middle of next week.

Tomorrow (19 Sep):  Mostly Cloudy.  Slight Chance of a Shower...Mainly South.  50/75

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

Sun (21 Sep):  Partly sunny.  Increasing Cloudy Late.  49/74

Mon (22 Sep):  Chance of Showers Early...Afternoon Clearing.  48/68

Tue (23 Sep):  Beconing Mostly Sunny.  46/72

Wed (24 Sep):  Sunny and Warmer.  45/82

Thu (25 Sep):  Sunny and Warm.  47/82

ODA Meteorologist
weather at oda.state.or.us