[willamette-fcst] Tue, 21 Sep 10

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT TUE SEP 21 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from noon to 5:00pm.
     Prep burning is allowed from noon to 5:00pm with a 50 acre limit.
     Propane flaming is allowed from noon to 5:00pm.

WEATHER DISCUSSION:

     Satellite pictures show patchy mid and high level clouds streaming across the
     region as of late morning. These are from an upper level disturbance now off the
     southern Oregon coast. This will move into northern California with no further
     effect on mid Willamette Valley weather.

     The morning Salem sounding showed a fairly unstable atmosphere up to above
     7000 feet.   There is an inversion from 7400 to 8000 feet with stable
     conditions above.  However, daytime heating should easily push the mixing
     height to the inversion level and maximum mixing heights this afternoon
     should be about 7200 feet.

     Winds on the morning Salem sounding were light and variable with a tendency
     to northerly up to about 5000 feet.  Computer models today are consistent in
     showing a northerly wind throughout the day today.  Trajectory forecasts bring
     any smoke from mid Marion County into central Lane County near Eugene and
     Springfield.

     For the mid Willamette Valley expect mostly sunny skies with a few high clouds
     from time to time. Winds will be light northerly becoming light northwesterly
     later this afternoon. Salem's high temperature today will be near 70. Relative
     humidity drops to 50% by 1pm. Minimum relative humidity 41%.

     Sunset tonight: 7:06 pm


THREE-HOURLY DATA:

                                     2pm     5pm     8pm
     Temperature:                     64      66      60
     Relative Humidity:               56%     48%     62%
     Surface Wind Direction:         350     350     310
     Surface Wind Speed:               5       6       5
     Transport Wind Direction:       360     360     330
     Transport Wind Speed:            10      10      10
     Estimated Mixing Height:       7100    7200     500
     Ventilation Index:               71      72       5


EXTENDED DISCUSSION:

     Tomorrow will be another drying day, however there likely will be areas of
     morning fog which may deposit some dew. There has been a new run of both the NAM
     and GFS models since the morning forecast. The NAM model is now dry for Thursday
     while the GFS is still showing some precip.

     Friday will see minor ridging for dry weather. Longer range, models have
     completely backed off on precip for the weekend.

     Given the flip-flops we have low confidence in forecasts from Thursday on.
     However, if Thursday remains dry, Thursday, Friday, and Saturday all have
     potential for some open field burning.

EXTENDED FORECAST:

     Wednesday: Areas of fog before 11am. Otherwise, partly sunny, with a high near
     72. Calm wind becoming west northwest between 5 and 8  mph.
     Thursday: Mostly cloudy, slight chance of light rain, 67. SSW 6-8 mph.
     Friday: Partly sunny, with a high near 74.
     Saturday: Mostly sunny, with a high near 76.
     Sunday: Slight chance of showers. Mostly cloudy, with a high near 72.
     Monday: A chance of rain. Mostly cloudy, 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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