[willamette-fcst] September 10

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT FRI SEP 10 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 1:00pm to 5:30pm.
     Prep burning is not allowed.
     Propane flaming is not allowed.


WEATHER DISCUSSION:

     Weak upper level ridge will build into the state today but will quickly slide
     to the east later tonight as a minor trough moves in. Weather pattern
     is fairly innocuous with little pressure gradient and little wind.

     Morning sounding showed a fairly strong surface-based temperature inversion up
     to a few hundred feet then a modestly stable temperature sounding above. Based
     on the sounding the current mixing height is near 2500 ft. Afternoon heating
     will gradually lift mixing levels up to 4000 - 5000 ft.

     Expect a mostly fair day with morning clouds slowly giving way to
     afternoon sunshine as clouds clear from the south to north. Pressure gradients
     are weak this morning with the Newport to Salem gradient .6 mb onshore and the
     Salem to Redmond gradient .8 mb onshore. With surface heating today that
     pressure gradient will become increasingly onshore with fairly even stacking
     between the Coast Range and Cascades.

     Salem's high temperature today will be near 72. Relative humidity drops to 50%
     by 1 pm. Minimum relative humidity near 40%. Wind flow is currently light
     SW'erly but should turn northwesterly during the afternoon. Some models show a
     nearly northerly flow. Will have to monitor with pibals this afternoon to
     determine if there will be any usuability to the wind for burning.

     Sunset tonight: 7:27 pm


THREE-HOURLY DATA:

                                      2pm     5pm     8pm
     Temperature:                      69      72      65
     Relative Humidity:                45%     39%     54%
     Surface Wind Direction:          300     340     310
     Surface Wind Speed:                5       6       5
     Transport Wind Direction:        320     320     340
     Transport Wind Speed:              7       7       7
     Estimated Mixing Height:        4200    4800    1000
     Ventilation Index:                29      34       7



EXTENDED DISCUSSION:

     Weak troughing moves in over the state early tomorrow as an upper level low
     moves by to the north. Trough then moves by to the east during the afternoon.
     This will have little effect on the weather with another pleasant warm day and
     mostly sunny skies after some morning low clouds. Pressure gradients will be
     mostly weak with a slight onshore flow in the afternoon.

     Another minor trough moves by to the north on Sunday with a ridge moving in
     later Monday and Tuesday. Temperatures will gradually warm up through the
     extended period. Upper ridge generally holds through the period as well. Pressure
     gradients and wind flow will be rather weak with light northerly to offshore
     flow in the morning and northerly to onshore flow in the afternoon.


EXTENDED FORECAST:

     Saturday: Partly sunny, with a high near 74. North northwest wind between 3 and
     8 mph.
     Saturday Night: Partly cloudy, with a low around 48. North northwest wind
     between 4 and 7 mph becoming calm.
     Sunday: Partly sunny, with a high near 78. Calm wind becoming north northwest
     between 4 and 7  mph.
     Sunday Night: Partly cloudy, with a low around 51.
     Monday: Sunny, with a high near 79.
     Monday Night: Mostly clear, with a low around 49.
     Tuesday: Sunny, with a high near 80.
     Tuesday Night: Mostly clear, with a low around 50.
     Wednesday: Mostly sunny, with a high near 79.
     Wednesday Night: Partly cloudy, with a low around 52.
     Thursday: Mostly sunny, with a high near 80.













Notes:
     1.  Mixing height, as used here, is the lowest height at which the
         potential temperature exceeds the potential temperature at the surface.
         As a practical matter it is the approximate height to which a
         smoke plume will rise assuming good ignition, dry fuels, and
         winds less than about 15mph.
     2.  Transport winds are a layer average through the mixing height,
         weighted slightly toward the winds at the top of the layer.
     3.  Ventilation Index is the height of the mixing layer times
         the transport wind speed divided by 1000.
     4.  Surface wind direction is the general expected wind direction.
         At a specific point surface winds are highly dependent on
         local terrain conditions.

This forecast is provided under an agreement between the Oregon Department of
Agriculture and the Oregon Department of Forestry.  For information contact ODA
at 503-986-4701.





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