<pre><html xmlns:o="urn:schemas-microsoft-com:office:office"
xmlns:w="urn:schemas-microsoft-com:office:word"
xmlns="http://www.w3.org/TR/REC-html40">
<html>
<head>
</head>
<body>
<pre>
SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED SEP 28 2011
BURN ADVISORY:
Recommended burn times for agricultural burning are from now until 5:00pm.
Prep burning is not allowed.
Propane flaming is allowed.
WEATHER DISCUSSION:
Satellite pictures show sunny skies throughout the Willamette Valley as of late morning.
Temperatures are mostly in the mid 50's to about 60 degrees just slightly cooler than
yesterday at the same time.
Upper air charts show a ridge of high pressure extend west to east from central
California to central Utah. Upper wind flow over Oregon is westerly.
At the surface high pressure is centered over eastern Washington and a thermal trough
extends from central California into southwest Oregon then along the coastline to about
Astoria. This is giving an easterly low level flow to northwest Oregon.
Pressure gradients as of 11am included: Newport to Salem, 0.6 mb offshore; Salem to
Redmond, 2.1 mb offshore.
THIS AFTERNOON'S FORECAST:
Salem's high temperature today will be near 75.
Relative humidity drops to 50% by 1pm. Minimum relative humidity 34%.
Surface winds: N-NE 9-12 mph.
Transport winds: NE 12-15 mph.
Maximum mixing height: 4500 feet.
Sunset tonight: 6:58 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 64 71 65
Relative Humidity: 54% 39% 50%
Surface Wind Direction: 020 020 040
Surface Wind Speed: 9 12 5
Transport Wind Direction: 040 030 040
Transport Wind Speed: 12 13 15
Estimated Mixing Height: 4200 4500 1000
Ventilation Index: 50 58 15
EXTENDED DISCUSSION:
The thermal trough shifts into the valley tomorrow and be east of the Cascades by Friday.
Latest models are a little earlier with the shift eastward and give a southwesterly flow
by mid afternoon on Thursday. If this develops there may be an opportunity to finish up
remaining fields tomorrow afternoon. Timing will be critical.
Friday will see an onshore flow and likely gradient stacking to the east, however a small
number of acres may be possible Friday. Long range charts are now showing rain over the
weekend. In addition conditions look fairly wet through the early part of next week.
Models are not in good agreement on timing but do indicate significant rain off and on
Monday through Wednesday. This may bring an end to the 2011 open field burning system.
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:280911:1135
</pre>
</body>
</html>