[willamette-fcst] Monday - Aug 23, 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT MON AUG 23 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 12:00pm to 6:00pm.
     Prep burning is not allowed.
     Propane flaming is not allowed.


WEATHER DISCUSSION:

     Upper air charts show a cool trough moving east over eastern Oregon this morning
     with an upper ridge of high pressure building off the coast. The morning ODF
     surface analysis shows a thermal trough from central California extending
     northwestward to the SW Oregon coast. Weak surface high pressure covers the
     Puget Sound region.

     Satellite pictures showed nearly cloud-free skies over Oregon with just a
     few low clouds banked up against the lower west slopes of the Cascades from
     Multnomah County southward to Marion County. These should dissipate quickly.

     The morning Salem sounding showed the atmosphere had warmed considerable above
     about 4000 feet since yesterday, with models showing additional warming
     throughout the day today. This will stabililze the atmosphere and suppress
     mixing heights. The maximum mixing height today in the mid-Willamette
     Valley will likely not reach 3000 feet. Winds on the sounding were northeasterly
     to about 4000 feet, then northerly and finally north-northwesterly above about
     8000 feet.

     As of 8am this morning there was a 0.9mb offshore gradient Redmond to Newport
     and the Portland to Medford gradient was northerly at 3.8mb. The offshore flow
     should increase this afternoon.

     Temperatures this morning were running 2-3 degrees cooler than yesterday
     morning, but by this afternoon high temperatures should be 6-8 degrees warmer
     than they were yesterday at the same time.

     With the low mixing heights and northeasterly transport winds today, there will
     likely be no open field burning this afternoon.

     Salem's high temperature today will be near 83. Relative humidity drops to 50%
     by 11am. Minimum relative humidity 24%.

     Sunset tonight: 7:59 pm


THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 67       77      83      77
     Relative Humidity:           52%      33%     24%     32%
     Surface Wind Direction:     360      360     010     010
     Surface Wind Speed:           5        8      12       6
     Transport Wind Direction:   010      010     010     010
     Transport Wind Speed:         7       11      12       5
     Estimated Mixing Height:   1500     2700    2800     500
     Ventilation Index:           10       30      34       2

EXTENDED DISCUSSION:

     The upper level ridge off the coast this morning will strengthen and move over
     the Oregon and Washington tomorrow for hot weather and gusty, dry offshore winds.
     Temperatures will be around 10-12 degrees warmer than today. Burning is not
     likely Tuesday.

     The next opportunity for open field burning may be Wednesday as the upper ridge
     axis moves east and marine push ends the minor warm spell. Timing is the key. If
     the marine push comes in too late there will be no burn opportunity. Models are
     inconsistent on the onset of westerly flow in the Valley some showing a burn
     possibility by mid-afternoon, others holding off the push until the evening, too
     late for burning.

     Thursday will be the first day after a marine push, which usually means low
     mixing heights and gradient stacking (subsidence aloft) to the east and thus not
     likely a good burn day. Friday will be partly sunny and the GFS model shows an
     unfavorable northerly transport flow over the region.


EXTENDED FORECAST:

     Tuesday: Sunny, with a high near 94. Wind ENE 5-15, gusts to 20.
     Wednesday: Sunny, with a high near 91. E 10mph becoming NNW, possibly WNW late.
     Thursday: Partly sunny but chance of sprilkles. Hihg 78.
        (First day after a marine push)
     Friday: Partly sunny, 72.
     Saturday: Mostly cloudy, 72.
     Sunday: Mostly cloudy, chance of showers. High 68.

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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