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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT TUE AUG 21 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from now until 2:00pm with a 50 acre limit.
Propane flaming is allowed from now until 5:00pm.
WEATHER DISCUSSION:
A weak upper level trough is moving through the region today, producing just a few middle
and hight level clouds. Temperatures in the northern Willamette Valley range from 1-3
degrees cooler than yesterday at the same time in the north, to 6 to 8 degrees warmer in
the south.
Surface winds are still north or slightly north-northeast but models continue to show
winds becoming more northwesterly during the afternoon.
Gradient stacking to the east is still with us. Pressure gradients as of 11am included:
Newport to Salem, 1.3 mb onshore and Salem to Redmond, 3.3 mb onshore while Portland to
Medford was 3.0 mb northerly. Computer models are not optimistic.
We will monitor the gradients this afternoon.
THIS AFTERNOON'S FORECAST:
Mostly sunny and cooler.
Salem's high temperature today will be near 78.
Relative humidity drops to 50% by 1pm. Minimum relative humidity 38%.
Surface winds: North-northwest backing to west-northwest in the afternoon.
Speed increasing to 8-10 in the afternoon.
Transport winds: Becoming west-northwest 8-12 in the afternoon.
Maximum mixing height: 4600 feet.
Sunset tonight: 8:07 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 75 78 70
Relative Humidity: 45% 38% 51%
Surface Wind Direction: 320 310 300
Surface Wind Speed: 6 9 10
Transport Wind Direction: 310 310 320
Transport Wind Speed: 5 10 15
Estimated Mixing Height: 3100 4600 3000
Ventilation Index: 16 46 45
EXTENDED DISCUSSION:
Models show transport flow returning to an unfavorable northerly direction tomorrow.
Another upper level system will move by to the north Thursday which should shift
transport winds back to more westerly. Any burn opportunity would depend on timing.
Friday should see dry weather as higher pressure aloft begins to build.
The National Weather Service’s digital forecast is available at:
<a
href="http://forecast.weather.gov/MapClick.php?lat=44.905&lon=-122.810">http://forecast.weather.gov/MapClick.php?lat=44.905&lon=-122.810</a>.
Notes:
1. Mixing height, as used here, is the lowest height at which the
potential temperature exceeds the equivalent 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.
Jim Little
ODF Meteorologist
jlwx:210812:1145
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