[willamette-fcst] Forecast - Friday, September 12th, 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 12th, 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:
     A strong upper-level ridge of high pressure remained over
     Oregon this morning with weak northwesterly flow aloft.  The
     ODA surface analysis showed the thermal trough had shifted
     east of the coastal range into the western valleys
     overnight.  Pressure gradients were weak over the valley
     this morning with light winds.  Even with a warm air mass
     over the region, clear skies and longer nights allowed valley
     temperatures to locally drop into the upper 40s this morning.

     The Salem sounding this morning continued to show very warm
     air aloft, so mixing heights will stay below 3000 feet until
     mid to late afternoon.  There was some cooling above 10,000
     feet, where winds had backed from northeasterly to
     northwesterly (onshore).  That shift to an onshore wind
     direction will drop closer to the surface as the day
     progresses.  By late this afternoon, the air aloft should
     cool enough to lift mixing heights to about 4000 feet.

     The air mass is still very dry this morning, with dew-point
     temperatures in the mid to upper 40s.  If the weak onshore
     flow does not moisten the low-level air mass, relative
     humidities may drop to near or below 30 percent this
     afternoon.  North-northwesterly winds may locally reach 15
     mph, so State Fire Marshal conditions are not out of the
     question, again, this afternoon...especially in the south valley.

     A weak upper-level disturbance is forecast to flatten the
     ridge, as it slides across Southern British Columbia this
     afternoon.  That will push the surface thermal trough into
     Eastern Oregon and induce weak onshore flow west of the
     Cascades.  That should cap high temperatures in the mid 80s,
     after reaching 90 degrees Thursday.  Although unlikely,
     transport winds may back enough northwesterly to allow for
     open burning late this afternoon in the north valley. 
     Transport winds are forecast to stay northerly south of
     about Salem.

     The upper-level ridge is forecast to rapidly build back over
     the region with surface winds becoming northerly overnight. 
     Little to no marine clouds are forecast to penetrate east of
     the coast range with this weak influx of marine air.


Surface Winds:
     N 5-10 this morning, NNW 7-14 north valley; N 8-16 south valley this afternoon.
Transport Winds:
     NNE 8 this morning, N 10 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 4000 feet.  Ventilation index 40.
High Temperature:
     Salem\'s high temperature today will be near 84.
Humidities:
     Relative humidity drops to 50% by 11am.
     Minimum relative humidity will be near 30%.
Sunrise/Sunset:
     Salem sunset tonight: 7:28pm; sunrise tomorrow: 6:49am.

Extended Outlook:
     A strong upper-level ridge is forecast to build over the
     Pacific Northwest this weekend for sunny and warmer weather.
     Northerly surface and transport winds are forecast
     Saturday.  By Sunday, a strong thermal tough along the coast
     should turn transport winds offshore, with valley
     temperatures warming into the low 90s.  Little change in the
     weather pattern is expected Monday, with continued warming
     aloft leading to very low mixing.

     A light southwesterly flow aloft is forecast, beginning
     Tuesday. A weak upper-level trough, off the northern
     California Coast, may send enough moisture northward over
     southern Oregon for a slight chance of mainly mountain
     thunderstorms.  Otherwise, dry conditions are forecast to
     continue with a slow cooling trend, as the surface thermal
     trough shifts into Eastern Oregon and eventually into Idaho.

     Increasing onshore flow may create some limited open burning
     opportunities during the second half of next week or at
     least improve ventilation conditions enough for some prep burning.

Tomorrow (13 Sep):  Sunny and Warm.  North Winds.  48/84

Sun (14 Sep):  Sunny and Very Warm.  Northeast Winds.  49/92

Mon (15 Sep):  Mostly Sunny and Continued Very Warm.   53/93

Tue (16 Sep):  Mostly Sunny and Warm.  53/88

Wed (17 Sep):  Mostly Sunny.  A Little Cooler.  53/84

Thu (18 Sep):  Mostly Sunny.  50/82

Fri (19 Sep):  Mostly Sunny.  50/82

ODA Meteorologist
weather at oda.state.or.us