[willamette-fcst] Willamette Valley Field Burning Forecast

Wed Aug 1 11:43:05 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT WED AUG 01, 2012

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 12:00pm to 2:00pm with a 50 acre limit.

Propane flaming is allowed from 12:00pm to 5:00pm.

WEATHER DISCUSSION:

Weak upper level trough to zonal-like pattern remains over the Pacific Northwest today. Satellite picture shows marine low clouds dissipating out of the Willamette Valley as temperatures warm.

Expect little change in the upper air pattern today with only a minor disturbance moving down from the north later today and tonight.  Mixing heights rise to similar levels as yesterday reaching about 3000 ft around noon and 4500 ft by late afternoon.  Transport and surface winds will be mostly a light NW to N.

Pressure gradient stacking has increased to the east with gradients 1.3 mb onshore from Newport to Salem and 3.3 mb onshore from Salem to Redmond.  Portland to Medford gradient is 3.9 mb northerly. Gradients should increase some over the Coast Range this afternoon but probably not enough to overtake the gradient stacking over the Cascades.

TODAY'S FORECAST:

Sunny and warm

Salem's high temperature today will be near 81.

Relative humidity drops to 50% by 1:00 pm.  Minimum relative humidity 35%.

Surface winds:  NNW to N at 4 - 8 mph

Transport winds: NNW at 6 - 10 mph during the afternoon

Maximum mixing height: 4500 feet

Sunset tonight: 8:37 pm

THREE-HOURLY DATA:

                                 2pm     5pm     8pm

     Temperature:                 77      81      70

     Relative Humidity:           42%     35%     49%

     Surface Wind Direction:     350     350     320

     Surface Wind Speed:           7       8       8

     Transport Wind Direction:   350     340     320

     Transport Wind Speed:         7       8      11

     Estimated Mixing Height:   3800    4500    2000

     Ventilation Index:           27      36      22

EXTENDED DISCUSSION:

Upper level trough moves down from the north and may increase the onshore flow tomorrow afternoon.  It's still questionable, however, whether it will be enough for a burning opportunity.  This trough will move to the east Friday giving way to an upper ridge. Expect warming conditions through the weekend with mostly northerly to offshore flow.

The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text

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

ODF Meteorologist

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