[willamette-fcst] TUE, 21 Sep 10

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM 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:

     Morning upper air charts show a trough to the north, over southern British
     Columbia and northern Washington, and another trough to the southwest, about
     500 miles west of Brookings.  The northern trough will pull out to the east
     today and the one to the southwest should push into northern California.
     This will leave the Willamette Valley in the middle for dry weather today
     and tomorrow.

     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 today expect mostly cloudy skies this
     morning with some sunshine this afternoon.  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:

                                 11am     2pm     5pm     8pm
     Temperature:                 62       69      70      64
     Relative Humidity:           65%      45%     41%     52%
     Surface Wind Direction:     360      340     340     300
     Surface Wind Speed:           3        4       6       5
     Transport Wind Direction:   010      360     360     330
     Transport Wind Speed:        10       10      10      10
     Estimated Mixing Height:   2800     7100    7200     500
     Ventilation Index:           28       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.  Both the NAM and
     the GFS models are showing measurable precipitation for Thursday
     afternoon.  Friday will see minor ridging for dry weather.  Longer
     range, models are less pessimistic than they were earlier regarding
     the weekend.   A deep upper low in the Pacific may remain far
     enough off the coast that moisture will rotate northward offshore
     through much of the weekend.  At this point Saturday looks dry,
     but we will mention a possibility of rain for Sunday and Monday,
     but low confidence beyond Saturday.

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: A 40 percent chance of rain after 11am. Mostly cloudy, with a high
     near 67. South southwest wind between 6 and 8 mph.
     Friday: Partly sunny, with a high near 74.
     Saturday: Mostly sunny, with a high near 76.
     Sunday: A 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.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20100921/97395d97/attachment.html>