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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED SEP 19 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
As of late morning the low clouds near Portland and near Eugene were beginning to break
up but low cloudiness was still solid along the coastline. A few mid and high clouds
were drifting northweard over the central Valley and central Cascades.
Temperatures are running considerably cooler than they were yesterday at this time most
areas were 8-12 degrees cooler. On Tuesday, for example, Salem was 72 degrees at 11am
but today was only 61.
With a very stable atmosphere, and surface temperatures slow to warm mixing heights will
be suppressed today. The maximum mixing height will struggle to reach 3000 feet.
The surface thermal trough has shifted east into the Cascades. There is little eastward
push through the moutnains - at 11am there was a 2.1mb onshore gradient Newport to Salem,
but just 0.2mb from Salem to Redmond.
While wind directions are marginally favorable today, the combination of low mixing
heights, little or no westerly gradient across the Cascades, and high levels of smoke
already in the airshed make today a poor candidate for open field burning.
THIS AFTERNOON'S FORECAST:
Sunny
Salem's high temperature today will be near 82. (Average: 75)
Relative humidity drops to 50% by 1pm. Minimum relative humidity 34%.
Surface winds: NW 5-9 mph.
Transport winds: NW 3-5 mph.
Maximum mixing height:
Sunset tonight: 7:13 pm.
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 72 80 70
Relative Humidity: 49% 39% 55%
Surface Wind Direction: 050 350 300
Surface Wind Speed: 4 6 3
Transport Wind Direction: 350 340 300
Transport Wind Speed: 5 5 14
Estimated Mixing Height: 2500 3000 1100
Ventilation Index: 12 15 15
EXTENDED DISCUSSION:
Computer models continue to show aan upper level low drifting north through Friday night,
then turning southeast and moving onshore over southwest Oregon. This would indicate
increasing morning marine clouds but afternoon sunshine the tomorrow and Friday. There
will also could be a chance for a shower as the upper low kicks through. However, there
are no obvious indications of any burn opportunities through the end of the week.
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:190912:1137
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