[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.

SPECIAL UPDATE...Extended Preparatory burning until 4 pm with a 200 acre limit.

Issued: 
     Thursday, September 18th, 2008 at 1:30pm.

Burn Advisory:
     Agricultural burning is not recommended.
     Preparatory burning is allowed from now until 4:00pm with a 200 acre limit.
     Propane flaming is allowed from 12:00pm until 5:00pm.
     Stack burning is not allowed.

Weather Discussion:
     A weak upper-level disturbance moved into Idaho overnight with onshore flow ushering
     cooler marine air into the valley.  The Salem sounding this morning clearly showed the
     marine layer extending from the surface to about 3500 feet with south-southeasterly
     flow aloft.  Visible satellite imagery showed low clouds had penetrated from the coast
     to the crest of the Cascades this morning.     

     The ODA surface analysis showed that the thermal trough had just pushed far enough
     east, early this afternoon, to allow marine air to spill into Central Oregon via blustery
     west winds.  Prior to that, the cool pool of air had already deepened, to over 4000 feet,
     in the Western Valleys.  Low clouds were just starting to break up in the south valley at
     1:30pm with some brightening of the sky in the north valley.  With a mid to late-afternoon
     cloud break-up expected, high temperatures will likely only climb into the low 70s.

     Infrared satellite imagery showed clouds from an approaching weather system about 175
     miles off the Southern Oregon Coast. That system appears to be forcing the retreat of
     the southern edge of the marine clouds in the Willamette Valley and will aid in their
     break-up this afternoon.  With a deep marine layer in place over Western Oregon,
     gradient-stacking issues may inhibit open burning this afternoon.  However, ventilation 
     conditions have improved enough to allow for an expansion of preparatory burning.

Surface Winds:
     SW 5-10 this afternoon.
Transport Winds:
     SW 7 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 3500 feet.  Ventilation index 25.
High Temperature:
     Salem\'s high temperature today will be near 72.
Humidities:
     Relative humidity drops to 50% by 4pm.
     Minimum relative humidity will be near 47%.
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 around 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 another marine
     layer into the Willamette Valley by Saturday morning.  A drier westerly flow aloft
     should clear skies Saturday afternoon, but 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.  That system may bring a
     little rain to the valley Sunday night and early Monday.

     High pressure is forecast to build over the region, starting Monday afternoon, with the
     potential of warm conditions, with offshore flow, 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 Clouds Late.  49/74

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

Tue (23 Sep):  Becoming 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