[willamette-fcst] Forecast - Thursday, August 28th, 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, August 28th, 2008 at 9:00am.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from 2:00pm until 7:00pm.
     Preparatory burning is not allowed.
     Propane flaming is not allowed.
     Stack burning is not allowed.

Weather Discussion:
     A west-to-east oriented cold front unexpectedly stalled and
     weakened right across the northern Willamette Valley
     Wednesday afternoon.  That created excellent ventilation
     conditions over the valley and allowed for the burning of
     some dry fields.  High pressure began building into Western
     Oregon Wednesday evening...warming the air aloft with a
     sinking northwesterly flow.  That cleared skies overnight
     and allowed temperatures to drop into the upper 40s and low
     50s this morning.

     The Salem sounding this morning showed strong northwesterly
     flow aloft and considerble warming above 3000 feet.  That
     will keep mixing heights from climbing above 3000 feet until
     about 2pm, with maximum mixing heights today only 3-4000
     feet.  The ODA surface analysis showed high pressure centers
     over both the Willamette Valley and East-Central Oregon with
     very weak pressure gradients over the Willamette Valley. 
     Much of the valley had calm winds this morning.

     In response to a flat ridge of high pressure building over
     Oregon, the jet stream migrated northward across southern
     Canada overnight.  A warm front was bringing some light rain
     to the Washinton Coast and the interior of NW Washington
     this morning.  Satellite imagery showed cloudy skies across
     all of Western Washington and extreme NW Oregon (from
     Astoria to Portland).  The remainder of Oregon had mostly
     clear skies.  Precipitation is forecast stay north of the
     Oregon border today, except for possibly along the extreme
     north Oregon Coast.

     Some clouds will likely persist in the extreme north valley
     today, while the south valley enjoys a full day of sunshine.
     Temperatures will range from the upper 70s in the Portland
     area to the mid 80s in the south valley near Eugene.
 
     Transport winds were light northerly this morning and should
     increase, from the north, this afternoon, as the upper-level
     ridge axis moves directly over Western Oregon.  The ridge is
     forecast to shift east of the Cascades overnight with the
     flow aloft becoming west-southwesterly by Friday morning.


Surface Winds:
     Var 0-5 this morning, N 10 this afternoon.
Transport Winds:
     N 5 this morning, N 13 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 4000 feet.  Ventilation index 52.
High Temperature:
     Salem\'s high temperature today will be near 82.
Humidities:
     Relative humidity drops to 50% by 2pm.
     Minimum relative humidity will be near 41%.
Sunrise/Sunset:
     Salem sunset tonight: 7:55pm; sunrise tomorrow: 6:31am.

Extended Outlook:
     An upper-level trough is forecast to approach the coastline
     Friday afternoon with the flow aloft becoming southwesterly.
     Valley temperatures should start off warmer than today but
     get capped in the low to mid 80s by increasing onshore flow
     in the afternoon.  The increasing onshore flow should back
     the transport winds from northerly to northwesterly Friday
     afternoon...especially in the north valley, with cooling
     aloft leading to higher mixing heights than we will have today.

     It is likely that the combination of northwesterly transport
     winds and 4000-foot-plus mixing heights will provide a
     burning opportunity for dry north valley fields.  Transport
     winds may stay too northerly to allow for burning in the
     south valley.  Mechanical fluffing of damp fields is
     required, if you plan to take advantage of the potential
     Friday afternoon burning opportunity.

     A strong upper-level trough is forecast to move over the
     region Saturday with increasing onshore flow cooling valley
     temperatures back into the 70s.  Some drizzle is
     possible...especially along the coast and over the north
     valley.  The upper-level trough is forecast to remain over
     the region through Sunday for mostly cloudy and cool
     condtions with a chance of light showers.

     The trough is forecast to push east of the region Monday
     with the flow aloft turning northwesterly That will begin to
     clear skies and warm temperatures, with transport winds
     turning northerly.  A weak weather system is forecast, in
     the northwesterly flow aloft, for Tuesday, which could
     create a burning opportunity.  An upper-level ridge of high
     pressure will likely begin building over the region
     Wednesday, which could last through the remainder of next week.

Tomorrow (29 Aug):  Mostly Sunny.  Increasing High Clouds Late.  53/82

Sat (30 Aug):  Mostly Cloudy and Much Cooler.  Chance of Drizzle or Light Showers.  55/70

Sun (31 Aug):  Mostly Cloudy and Cool.  Chance of Drizzle or Light Showers.  48/69

Mon (01 Sep):  Becoming Mostly Sunny and Warmer.  47/76

Tue (02 Sep):  Increasing Clouds.  Slight Chance of Showers North.  49/77

Wed (03 Sep):  Patchy Morning Clouds.  Mostly Sunny and Warmer.  50/79

Thu (04 Sep):  Sunny and Warmer.  50/82

ODA Meteorologist
weather at oda.state.or.us