[willamette-fcst] Wed, 08 Sep 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED SEP 08 2010

BURN ADVISORY:

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

WEATHER DISCUSSION:

     An upper level low pressure area is situated over south central Washington at
     late morning. This is making the atmosphere unstable and scattered showers cover
     much of the Pacific Northwest. As surface temperatures warm, instability in the
     atmosphere will be released and additional showers will develop. Some of these
     showers will be quite significant with locally heavy rain and possibly thunder
     and lightning.

     The ODF late morning surface analysis shows onshore low level flow with gradient
     stacking to the east. Gradient stacking will likely continue throughout the day.

     The morning Salem sounding showed temperatures had cooled slightly since
     yesterday at all levels from the surface to above 14,000 feet. The
     temperature/moisture profile was conditionally unstable - supporting a strong
     shower or thunderstorm forecast for this afternoon.

     For the mid Willamette Valley, expect showers, heavy at times, for this
     afternoon with the possibility of a thunderstorm. Winds will be light westerly
     except gusty near showers. Salem's high temperature today will be near 66.
     Minimum relative humidity will be about 60%.

     Sunset tonight: 7:30 pm

THREE-HOURLY DATA:

                                     2pm     5pm     8pm
     Temperature:                     65      66      59
     Relative Humidity:               60%     60%     72%
     Surface Wind Direction:         210     270     270
     Surface Wind Speed:               5       5       5
     Transport Wind Direction:       220     280     270
     Transport Wind Speed:             4       9       6
     Estimated Mixing Height:       5000    5000     500
     Ventilation Index:               20      45       3


EXTENDED DISCUSSION:

     The upper level low will move east of the area during the day tomorrow for a
     decreasing chance of showers. Models indicate a continuing onshore flow for
     mostly cloudy skies and poor field drying conditions. In addition models
     indicate gradient stacking to the east continuing. For Friday a flat ridge
     builds for some sun but the gradient stacking will likely continue.

     Over the weekend another upper level low will move down the coast of British
     Columbia and help carve out a weak upper trough over the Pacific Northwest. At
     this point it looks like most moisture should stay just north of the mid
     Willamette Valley. Onshore flow will keep temperatures a bit below average and
     there will be a mixture of clouds and sun.

EXTENDED FORECAST:

     Thursday: Mostly cloudy, with a high near 67. Wind W 6.
     Friday: Partly sunny, with a high near 69. Wind NW 6.
     Saturday: Partly sunny, with a high near 70.
     Sunday: Mostly cloudy, with a high near 70.
     Monday: Partly sunny, with a high near 73.
     Tuesday: Partly sunny, with a high near 74.

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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