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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT TUE AUG 30 2011
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:
An upper level trough continues to deepen over the Pacific Northwest today. Satellite
pictures show clouds still covering most of the Willamette Valley as of late morning.
The surface analysis shows a thermal trough in central California extending into SW
Oregon.
An earlier pilot report indicated the top of the cloud deck was at 5300 feet and the
Salem airport is showing the base of the overcast layer is at 4500 feet. With just 1000
feet of clouds to go some breakup of the cloud cover is likely later this afternoon.
Gradient stacking to the east is still strong. Pressure gradients as of 11am included:
Newport to Salem, 0.6 mb onshore; Salem to Redmond, 4.3 mb onshore; and Portland to
Medford, 3.9 mb northerly.
With the strong northerly gradient across Oregon transport level winds should be mostly
northerly or north-northwesterly this afternoon. Latest computer modeling shows
transport winds at about 347 degrees at 2pm and 333 degrees at 5pm.
THIS AFTERNOON'S FORECAST:
Cloudy, but partial afternoon clearing, becoming mostly sunny by late afternoon.
Salem's high temperature today will be near 73.
Minimum relative humidity near 38%.
Surface winds: NNW 8-18 mph becoming NW 10 by early evening.
Transport winds: NNW increasing to 12-18 by early evening.
Maximum mixing height: 5600 feet.
Sunset tonight: 7:53 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 73 74 66
Relative Humidity: 41% 36% 48%
Surface Wind Direction: 350 340 310
Surface Wind Speed: 8 10 10
Transport Wind Direction: 350 330 340
Transport Wind Speed: 9 15 18
Estimated Mixing Height: 5500 5500 2500
Ventilation Index: 50 82 45
EXTENDED DISCUSSION:
The upper level trough axis moves to the east of the area during the day tomorrow after
one last weak short wave impulse moves through. This will leave the Willamette Valley
under a dry northwesterly flow aloft. This should produce more sunshine and warmer
afternoon temperatures. It should also give a better chance for afternoon sea breezes to
develop and, depending on timing, could produce some burn opportunities. This will need
to be assessed on a day-to-day basis.
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:300811:1146
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