[willamette-fcst] Thursday, August 26th

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT THU AUG 26 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 12:00pm to 6:00pm.
     Prep burning is allowed from 12:00pm to 6:00pm with a 50 acre limit.
     Propane flaming is allowed from 12:00pm to 6:00pm.


WEATHER DISCUSSION:

     Upper level ridge has moved off to the east this morning as an eastern Pacific
     trough moves into the state today. Marine push came in modestly last evening
     with variable low-level marine clouds along the coast and in the Coast Range.
     Upper level trough will continue to move into the state bringing cooler
     temperatures aloft. Axis of trough moves to the coast late this afternoon and
     then will broaden and settle over the entire state overnight tonight. Onshore
     pressure gradients have developed with the main gradient stacking over the
     Cascades and eastward. Newport to Salem is currently 2.5 mb onshore while Salem
     to Redmond is 5.2 mb onshore. Wind flow will increase from the W to NW today to
     10 - 18 mph.

     Morning Salem sounding showed decent cooling aloft from yesterday's sounding.
     Temperature inversion is minor this morning with temperatures staying about the
     same from surface to 5000 ft. Thus the air mass is still quite stable. However,
     with cooling aloft and warming at the surface the air mass will destabilize well
     today with mixing heights rising to near 3000 ft by around 11 a.m. and near 5000
     ft by 2 p.m.

     As the upper trough continues to move in today, and temperatures cool aloft,
     cumulus clouds should develop in the Willamette Valley and shut off heating to
     the lower to mid 70s this afternoon. In addition, W to NW wind flow will be on
     the increase. However, with gradient stacking to the east, will have exercise
     caution as to how much can be burned today.

     Mid-Willamette Valley temperatures will peak out at about 74 degrees this
     afternoon. Relative humidity will drop to about 50% between 11 a.m. and noon
     with the minimum humidity around 35%


     Sunset tonight: 7:54 pm


THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 68       72      73      64
     Relative Humidity:           53%      41%     35%     46%
     Surface Wind Direction:     310      280     290     290
     Surface Wind Speed:           6        7      12      11
     Transport Wind Direction:   310      290     290     300
     Transport Wind Speed:         8       11      14      12
     Estimated Mixing Height:   3000     4800    5000    2500
     Ventilation Index:           24       53      70      30



EXTENDED DISCUSSION:

     Upper level trough will remain over the state throughout the day tomorrow with
     cool, marine air over the region. Expect more marine low clouds over the
     Willamette Valley tomorrow than today. Mixing will be mostly good. Gradient
     stacking will be more over the Coast Range than the Cascades or at least even.
     Winds will be NW to N during the morning turning more NW'erly during the
     afternoon. This could produce some decent burning conditions in the afternoon.

     Upper level trough deepens Saturday as another disturbance moves through the
     flow. This trough will gradually move to the east later Saturday and through the
     remainder of the weekend. Mixing heights remain good but wind flow may turn a
     little more northerly with the most NW'erly flow occurring during the
     afternoon. Flat ridging moves into the state Monday and Tuesday bringing modest
     warming.

EXTENDED FORECAST:

     Friday: Morning low clouds and afternoon clearing, with a high near 71. Calm wind
during the morning becoming NW'erly between 8 and 12 mph during the afternoon.
     Friday Night: Partly to mostly cloudy, with a low around 45. NW wind between 5 and 8
     mph becoming calm.
     Saturday: Mostly cloudy, with a high near 68. Calm wind becoming northwest
     between 7 and 10 mph.
     Saturday Night: Partly to mostly cloudy, with a low around 47.
     Sunday: Morning low clouds and afternoon clearing, with a high near 70.
     Sunday Night: Partly cloudy, with a low around 47.
     Monday: Morning clouds and afternoon clearing, with a high near 70.
     Monday Night: Mostly lear, with a low around 49.
     Tuesday: Mostly sunny, with a high near 74.
     Tuesday Night: Clear, with a low around 51.



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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