[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 12:00pm.

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 remains over Oregon with weak northwesterly
     flow aloft.  Satellite imagery showed mostly clear skies over Oregon late this morning
     with the exception of low clouds blanketing the immediate coastline.  The late-morning
     ODA surface analysis showed the thermal trough over the Willamette valley was beginning
     to shift eastward.  Winds in the valley were still light northerly.  Temperatures had
     already warmed into the low to mid 70s.

     The Salem sounding this morning continued to show very warm air aloft, so mixing heights
     will stay below 3000 feet until mid 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.  An
     11:40am pibal reading over Salem showed mostly northerly winds below 5000 feet with a
     shift to northwesterly above 5000 feet.  The northwesterly winds aloft should drop closer
     to the surface by late this afternoon with cooling aloft lifting mixing heights above
     4000 feet.

     The air mass is still very dry, but dew-point temperatures are beginning to rise in
     the north valley and may come up enough to avoid Fire marshal conditions this afternoon.
     The south valley will be drier, and increasing northerly winds this afternoon could put
     areas of the south valley into Fire Marshal conditions.

     Satellite imagery showed a band of high clouds stretching from NE Washington to
     North-Central Oregon.  Those clouds are associated with a weak upper-level
     disturbance that is sliding across Southern British Columbia.  It is responsible for
     the eastward shift of the surface thermal trough late this morning.  The thermal
     trough will push into Eastern Oregon by late this afternoon and induce weak onshore
     flow west of the Cascades.  That should cap high temperatures in the mid 80s.

     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.  We will begin pibal readings in the north and south valley at 2 pm.
     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...becoming NNW 7-14 north valley; N 8-16 south valley this afternoon.
Transport Winds:
     N 10-15...becoming NNW 10-15 north valley this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 4500 feet.  Ventilation index 45.
High Temperature:
     Salem\'s high temperature today will be near 84.
Humidities:
     Minimum relative humidity will be near 30% (likely a little above north and below south).
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 poor ventialtion conditions expected.

     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 and eastern 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