[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:35 AM PDT WED AUG 8, 2012



BURN ADVISORY:



Recommended burn times for agricultural burning are from noon until 6:30 p.m.

Prep burning is allowed from noon until 2:00 p.m. with a 50 acre limit.

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



WEATHER DISCUSSION:



Late morning satellite imagery continued to show marine low clouds covering most of northwestern Oregon, but some breaks in the overcast were beginning to appear in the central Willamette Valley.  The low clouds should slowly break up this afternoon, with high temperatures likely staying in the 70s. Cooler air aloft will result in high afternoon mixing heights.  However, there continues to be significant pressure gradient stacking east of the Willamette Valley.  That is not conducive to good ventilation, for burning, and is unlikely to reverse this afternoon.



TODAY'S FORECAST:



Slowly clearing skies this afternoon. Cooler.



Salem's high temperature today will be near 78 degrees (normal is 83).

Relative humidity drops to near 40% by 5 p.m.

Surface winds:  SW 5-10; Becoming NNW 5-10 mph this afternoon.

Transport winds: SW 5-10; Becoming NNW 7-12 mph this afternoon.

Mixing height: Rising to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:28 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 75      78      71

     Relative Humidity:           47%     42%     57%

     Surface Wind Direction:     330     350     340

     Surface Wind Speed:           5       7       9

     Transport Wind Direction:   310     340     340

     Transport Wind Speed:         6       9      12

     Mixing Height:             4500    5000    3000

     Ventilation Index:           27      45      36



EXTENDED DISCUSSION:



Another weak upper-level trough will maintain the onshore flow pattern across western Oregon, on Thursday, with morning clouds giving way to afternoon sunshine.  Gradient stacking may, once again, be unfavorable for open burning, and transport winds will be mostly northerly.  A weak westerly flow aloft, on Friday, will maintain a pattern of morning clouds and afternoon sunshine.  Temperatures should stay near normal.  Northerly transport winds will keep the potential for burning opportunities low.  Little change to the weather pattern is forecast through early next 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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