[willamette-fcst] TUE, 14 Sep 10

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PDT TUE SEP 14 2010

BURN ADVISORY:

     Agricultural burning is not recommended.
     Prep burning is allowed from noon to 5:30pm.
     Propane flaming is allowed from noon to 4:30pm.

WEATHER DISCUSSION:

     Satellite pictures still show some low clouds over portions of the Willamette
     Valley. There is a small patch of clouds over western Linn County, otherwise the
     clouds are mainly over extreme northern Marion County, western Clackamas County
     and most of Polk and Yamhill Counties.

     There is a weak upper level ridge over western Oregon today. About 600 miles
     west of Oregon a strong upper level low continues to develop.

     The late morning ODF surface analysis shows higher pressure to the north, lower
     pressure to the south for northerly winds today. Pressure gradients as of 11am
     included: Newport to Salem, 0.9 mb onshore; Salem to Redmond, 0.5 mb onshore;
     Newport to Redmond, 1.4 mb onshore; Portland to Eugene, 0.3 mb northerly; and
     Portland to Medford, 1.5 mb northerly.

     The morning Salem sounding temperature profile looked similar to yesterday's.
     There was a weak inversion between 2200 feet and 2900 feet. This inversion will
     mix out with a surface temperature of 74F and the maximum mixing height today
     should climb to above 4000 feet.

     For the mid Willamette Valley look for mostly sunny skies this afternoon with
     light northeast to northwest winds. Salem's high temperature today will be near
     81. Relative humidity drops to 50% by 12pm. Minimum relative humidity 35%.


     Sunset tonight: 7:19 pm

THREE-HOURLY DATA:

                                     2pm     5pm     8pm
     Temperature:                     78      81      73
     Relative Humidity:               42%     35%     48%
     Surface Wind Direction:         340     360     280
     Surface Wind Speed:               3       4       4
     Transport Wind Direction:       350     340     340
     Transport Wind Speed:             5       5       3
     Estimated Mixing Height:       3300    4500     500
     Ventilation Index:               16      22       2

EXTENDED DISCUSSION:

     Today may be the last of summer weather for 2010. The upper low to the west will
     continue to spin off the coast sending surges of moisture inland until the low
     itself finally weakens and moves across the region next Monday.

EXTENDED FORECAST:

     Wednesday:  Mostly cloudy,rain developing by evening. 74. Wind SW 3-8
     Thursday: Mostly cloudy, periods of light rain. 76.  Wind SW 3-8
     Friday: A 50 percent chance of rain. Mostly cloudy, with a high near 73.
     Saturday: A chance of rain. Mostly cloudy, with a high near 70.
     Sunday: A chance of rain. Mostly cloudy, with a high near 71.
     Monday: A chance of rain. Mostly cloudy, with a high near 71.


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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