[willamette-fcst] August 27th - noon

Willamette Valley Field Burning Forecast willamette-fcst at listsmart.osl.state.or.us
Fri Aug 27 11:55:30 PDT 2010


SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT FRI AUG 27 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from now to 6:00pm.
     Prep burning is allowed from now to 4:00pm with a 50 acre limit.
     Propane flaming is allowed from now 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 tonight. Pressure gradients
     are rather light with the Newport to Salem gradient at .7mb onshore and Salem to
     Redmond 1.7mb onshore. It appears that since there is less cloud cover east of
     the Cascade, pressures are lowering more on the east side of the mountains, thus
     gradient stacking is remain to our east. In addition, the Medford draw will
     remain prevalent and keep winds fairly northerly today.

     Morning sounding showed a minor stable layer near the surface up to about 1000
     ft. This has mostly mixed out due to daytime heating. Mixing levels have risen
     to above 5000 ft already. Low-level winds from the Portland wind profiler show
     due north winds at about 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 38%. Wind
     flow is mostly northerly and appears it may remain so through most of the
     afternoon.

    Sunset tonight: 7:52 pm


THREE-HOURLY DATA:

                                      2pm     5pm     8pm
     Temperature:                      68      71      65
     Relative Humidity:                42%     38%     50%
     Surface Wind Direction:          350     350     330
     Surface Wind Speed:                6       8       7
     Transport Wind Direction:        350     350     350
     Transport Wind Speed:             10      11      12
     Estimated Mixing Height:        5500    6000    4000
     Ventilation Index:                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 30 percent chance of showers, with a high near 70.
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 60 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: Mostly sunny, with a high around 80.
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.









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