[willamette-fcst] Forecast - Tuesday, August 4th, 2009

[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: 
     Tuesday, August 4th, 2009 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 pressure center was about 250 miles off
     the central California coastline and drifting slowly
     eastward this morning. It will continue to bring weak
     southerly flow aloft to the Oregon today, along with a
     chance of afternoon and evening thunderstorms...mainly south
     and from the Cascades eastward. A drier and more stable weak
     westerly flow aloft prevailed over Washington.  The surface
     thermal trough extended from just east of The Dalles to near
     Redmond to near Medford, with continued weak onshore flow
     across the Willamette Valley.

     The air mass aloft is slowly cooling, but still very warm,
     with weak onshore flow slowly moderating temperatures west
     of the Cascades. Some morning clouds managed to make their
     way into the southern Willamette Valley again this morning,
     but they should evaporate by midday. There are also some mid
     and high clouds circulating over wesern Oregon, via the
     southerly flow aloft. Willamette Valley highs cooled into
     the upper 80s and low 90s Monday, and another 2-3 degrees of
     cooling is expected today.  The record streak of 10 straight
     days of 90+ degree weather for Portland should come to an
     end today.  The old record was set August 12-19, 1967. 
     Salem had 9 straight days of 90 degrees or warmer until the
     89-degree high yesterday.

     The Salem sounding this morning was very similar to
     yesterday morning, with continued very warm air aloft
     keeping mixing heights below 3000 feet until this aftenoon. 
     Weak onshore flow is slowly improving air quality and
     ventilation conditions across western Oregon. Mixing heights
     should climb to around 4000 feet by late this afternoon.
     Forecast light westerly transport winds may create another
     limited open burning opportunity this afternoon, but
     thunderstorm development over central Oregon will need to be
     closely monitored.  That could prevent smoke evacuation over
     the Cascades. 

     As expected, the thunderstorms of the past few days have
     started a few widefires around the region.  The largest fire
     in western Oregon is a 4500 acre blaze near highway 138 in
     the western Cascades of Douglas County.  The wildfire
     potential will increase over the next couple of days, as the
     upper-level trough moves onshore and increases the
     thundershower activity...mainly from the Cascades eastward.

Surface Winds:
     SW 0-8 this morning, WSW 5-10 this afternoon.
Transport Winds:
     SW 4 this morning, W 7 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 4000 feet.  Ventilation index 28.
High Temperature:
     Salem\'s high temperature today will be near 86.
Humidities:
     Relative humidity drops to 50% by 12pm.
     Minimum relative humidity will be near 31%.
Sunrise/Sunset:
     Salem sunset tonight: 8:34pm; sunrise tomorrow: 6:03am.

Extended Outlook:
     The upper-level ridge axis is forecast to shift eastward, to
     over western Montana, on Wednesday, as an upper-level
     low-pressure center slowly approaches the northern
     California coastline.  The flow aloft will remain southerly
     over Oregon with the potential for afternoon and evening
     thunderstorm development continuing...mainly from the
     Cascades eastward.  By Wednesday evening, enough moisture
     may feed far enough north and west for a chance of showers
     and thunderstorms even over the Willamette Valley.  Weak
     onshore flow will continue to slowly improve ventilation
     conditions and cool temperatures across western Oregon.

     The upper-level trough is forecast to move onshore Thursday,
     mainly across northern California, bringing much stronger
     onshore flow and a significant cool-down across all of
     Oregon. The threat of showers and thunderstorms will mainly
     shift easat of the Cascades, as a more stable westerly flow
     aloft invades western Oregon.  The trough is forecast to
     continue eastward, into Idaho and Nevada, Friday with a
     drier northwesterly flow aloft spreading across Oregon.  A
     stronger weather system may bring some rain to the
     Willamette Valley by the end of this weekend.

Tomorrow (05 Aug):  Partly Cloudy to Mostly Cloudy.  Chance of Showers or T-Storms.  52/85

Thu (06 Aug):  Mostly Cloudy and Cooler.  Slight Chance of Showers.  52/77

Fri (07 Aug):  AM Clouds.  Parly Sunny.  52/77

Sat (08 Aug):  AM Clouds.  Parly Sunny.  52/80

Sun (09 Aug):  Mostly Cloudy.  Increasing Chance of Rain.  53/77

Mon (10 Aug):  Mostly Cloudy.  Chance of Showers.  53/76

Tue (11 Aug):  Mostly Cloudy.  Slight Chance of Showers.  53/79

ODA Meteorologist
weather at oda.state.or.us