[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT FRI AUG 5, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from noon until 5:00 p.m.



WEATHER DISCUSSION:



Late-morning visible satellite imagery showed low clouds continuing to cover the Willamette Valley and the north coast.  However, skies had cleared along the central coast and were beginning to clear over the northern and central coastal range.



The surface map showed high pressure building into northwestern Oregon with weak northerly gradients across western Oregon.  With a fresh supply of cool marine air and cloudy skies over the Willamette Valley, gradient-stacking has worsened slightly since this morning.  Newport-Salem was 0.4mb onshore with Salem-Redmond 3.5mb onshore.  It will take at least a couple hours of sunshine over the Willamette Valley to level those gradients.



The air aloft is still fairly cool, so mixing heights will be high, once we get some warming August sunshine to the valley floors.  Another limiting factor for open burning today will be the direction of the transport winds, which are forecast stay mostly northerly.



TODAY'S FORECAST:



Low clouds giving way to mostly sunny skies mid-afternoon.  Cooler.



Salem's high temperature today will be near 79 degrees (normal is 84).

Relative humidity drops to near 50% by 3 p.m.

Surface winds:  NNW 5-12 mph this afternoon.

Transport winds: NNW 10-15 mph this afternoon.

Mixing height: Rises to 5500 feet by 5 p.m.

Salem's sunset tonight: 8:33 p.m.





THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 73      78      72

     Relative Humidity:           52%     44%     55%

     Surface Wind Direction:     340     340     340

     Surface Wind Speed:           6       8      10

     Transport Wind Direction:   340     340     340

     Transport Wind Speed:        10      15      15

    Mixing Height:             4000    5500    2800

     Ventilation Index:           40      83      42



EXTENDED DISCUSSION:



Little change in the weather pattern is forecast through early next week.  Expect varying degrees of morning clouds with afternoon sunshine and moderate temperatures.  That may create some limited burning opportunities much like we have seen this week.



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.



Pete Parsons

ODF Meteorologist



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