[willamette-fcst] August 27th

Willamette Valley Field Burning Forecast willamette-fcst at listsmart.osl.state.or.us
Fri Aug 27 08:38:27 PDT 2010


SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT FRI AUG 27 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 11:00am to 6:00pm.
     Prep burning is allowed from 11:00am to 4:00pm with a 50 acre limit.
     Propane flaming is allowed from 11:00am to 5:00pm.


WEATHER DISCUSSION:

     Upper level trough is firmly entrenched over the state today with higher level
     clouds over the region associated with the nearby jetstream. A disturbance will
     rotate through the flow bringing increasing clouds later today and tonight.
     Pressure gradients are rather flat this morning with the Newport to Salem
     gradient at .3mb onshore and Salem to Redmond .9mb onshore. With modest heating
     this afternoon pressure gradients should increase a little faster over the Coast
     Range than the Cascades. However, the Medford draw will be prevalent and keep
     winds fairly northerly today.

     Morning sounding showed a minor stable layer near the surface up to about 1000
     ft. This will mix out with daytime heating. Mixing levels will rise nicely today
     with the mixing height above 4000 ft by 11 a.m. and above 5000 ft around noon.
     Low-level winds are rather variable up about 1500 ft then turn N to NE at 5 to
     10 mph.

     Expect partly to mostly cloudy skies today. High temperature will rise to near
     70. Relative humidity drops below 50% by 1 p.m. with the minimum near 40%. Wind
     flow will be light this morning, becoming NW to N at 6 - 10 mph. Will have
     release pibals to see whether enough NW'erly flow can be "squeezed out" for a
     decent burning opportunity.

    Sunset tonight: 7:52 pm


THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 61       68      71      65
     Relative Humidity:           62%      42%     35%     50%
     Surface Wind Direction:     360      340     340     310
     Surface Wind Speed:           5        6       8       7
     Transport Wind Direction:   350      340     340     350
     Transport Wind Speed:         7       10      11      12
     Estimated Mixing Height:   4500     5500    6000    4000
     Ventilation Index:           32       55      66      48



EXTENDED DISCUSSION:

     Upper level trough deepens tonight as a disturbance moves into the
     state overnight. This quite possibly could produce some light rain over the
     region on Saturday, mainly to the south. Pressure gradients will be light
     northerly early, then turn more onshore during the day with NW'erly flow by mid
     to late afternoon. Temperatures will remain cool under mostly cloudy skies and
     may not reach 70.

     Upper trough moves off to the east Sunday but skies will remain cloudy and
     temperatures cool. Wind flow turns more westerly. Another disturbance will ride
     down the back side of the trough later Sunday and Monday and bring another
     chance of some light rain. Wind flow will be SW'erly on Monday. Trough moves
     off to the east Tuesday and Wednesday with a flat ridge moving in. Temperatures
     will warm and the air mass will dry out.


EXTENDED FORECAST:

     Saturday: Mostly cloudy, with a high near 69. Calm wind becoming NW'erly
     between 8 and 11  mph.
     Saturday Night: Mostly cloudy, with a low around 48. NW'erly wind
     between 8 and 11 mph becoming calm.
     Sunday: Mostly cloudy, with a high near 68. Calm wind becoming westerly
     between 5 and 10 mph.
     Sunday Night: Mostly cloudy, with a low around 47.
     Monday: A 30 percent chance of showers. Mostly cloudy, with a high
     near 67.
     Monday Night: A chance of showers. Mostly cloudy, with a low around 49.
     Tuesday: Partly sunny, with a high near 74.
     Tuesday Night: Mostly cloudy, with a low around 50.
     Wednesday Night: Partly cloudy, with a low around 52.
     Thursday: Mostly sunny, with a high near 82.













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.



Nick Yonker
Meteorology Manager
Oregon Department of Forestry
503-945-7451


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