[willamette-fcst] Tue, 14 Sep 10

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE SEP 14 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is allowed from 1:00pm to 5:00pm with a 50 acre limit.
     Propane flaming is not allowed.


WEATHER DISCUSSION:

     A weak upper level ridge covers western Oregon this morning. To the east a small
     but active upper level disturbance is pulling out and to the west a strong but
     slow moving upper level low is developing.

     Satellite pictures show low clouds over portions of the Willamette Valley, but
     the cloud deck is not as extensive as yesterday. By afternoon the entire valley
     should be sunny with temperatures climbing into the low 80's which is about 5
     to 7 degrees above the seasonal average for this time of year.

     The morning Salem sounding temperature profile looked similar to yesterday's.
     There was a weak inversion between 2200 feet and 2900 feet. Below the inversion
     winds were light north-easterly, above light north-westerly. The inversion should
     easily mix out and the maximum mixing height today should go above 4000 feet.

     The morning ODF surface analysis shows higher pressure to the north, lower
     pressure to the south for northerly winds today. Unfortunately this is not a
     favorable direction for burning. Computer models indicate a transition to
     southerly winds early this evening, but no indication of any sustained transport
     flow with a westerly component for a burn opportunity. Pibals this afternoon
     will confirm or deny the modeled winds.

     For the mid Willamette Valley look for mostly sunny skies today with light north
     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:

                                 11am     2pm     5pm     8pm
     Temperature:                 69       78      81      73
     Relative Humidity:           61%      42%     35%     48%
     Surface Wind Direction:     360      340     360     280
     Surface Wind Speed:           2        3       4       4
     Transport Wind Direction:   350      350     340     340
     Transport Wind Speed:         3        5       5       3
     Estimated Mixing Height:   2500     3300    4500     500
     Ventilation Index:            8       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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