<pre><!DOCTYPE HTML PUBLIC -//W3C//DTD HTML 4.0 Transitional//EN>
<html>
<head>
</head>
<body>
<pre>
SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT MON AUG 22 2011
HEADLINES:
Potential burn opportunity this afternoon!
After a couple of weeks of mostly northerly winds in the Willamette Valley, weather
patterns have changed enough to give a more westerly or southwesterly transport flow.
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is allowed from 12:00pm to 5:00pm.
WEATHER DISCUSSION:
In the upper atmosphere strong high pressure remains anchored over the central United
States, with the center of the high pressure over Texas. Meanwhile a broad trough covers
the Gulf of Alaska and the northeastern Pacific Ocean. A disturbance rotating through
that broad trough supports a Pacific cold front that is now moving into northwestern
Washington. Heavy rain has been reported there. Farther south, in the central
Willamette Valley, the front will likely produce only clouds.
Satellite pictures show middle and high clouds out ahead of the front covering parts of
the Willamette Valley. In addition, low marine clouds cover the coast and are pushing
into the coast range and into parts of the western Valley.
Pressure gradients are onshore. Gradients as of 8am showed stacking to the east: Newport
to Salem was 0.6 mb onshore while Salem to Redmond was 3.1 mb onshore. Meanwhile the
north-south gradient was not a factor with Portland to Medford 0.2 mb northerly.
The morning Salem sounding showed the lower atmosphere had cooled quite a bit since
yesterday. At 1800 feet the temperature fell from 73F to 57F or 16 degrees. At 8000
feet, however, the temperature had cooled only 3 degrees. In relative terms that means
it's still fairly warm aloft but has cooled at lower elevations. And that means a more
stable atmosphere. The sounding did show several shallow inversions between the ground
and 5000 feet. The lower ones should mix out depending on surface temperatures.
Items to watch today will be: 1) Surface temperatures in the valley as we will need about
76 degrees to give a 3000 foot mixing height, and 2) Pressure gradient stacking to the
east with its implied subsidence. If these criteria become favorable, growers should be
prepared to get a quick start today.
TODAY'S FORECAST:
Partly cloudy and cooler.
Salem's high temperature today will be near 82.
Relative humidity drops to 50% by 1pm. Minimum relative humidity today about 40%.
Surface winds: SSW 8-10 mph.
Transport winds: SW 7-9 mph veering to W 12-14 this afternoon.
Maximum mixing height: 3900 feet.
Sunset tonight: 8:06 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 70 78 82 74
Relative Humidity: 61% 47% 40% 51%
Surface Wind Direction: 200 210 230 250
Surface Wind Speed: 8 8 10 8
Transport Wind Direction: 230 250 270 270
Transport Wind Speed: 08 10 13 11
Estimated Mixing Height: 1900 3500 3900 1000
Ventilation Index: 15 35 51 11
EXTENDED DISCUSSION:
The front moving into Washington should stall overnight then the remains will push through
Oregon early tomorrow. By that time it should be considerably weakened. If it induces a
strong marine push early tomorrow then tomorrow would not be a good burn day. If its
effect is minimal then tomorrow could work out as a pretty good burn day. This will need
to be monitored closely.
The remains of the front will be east of the Cascades by Wednesday and temperatures should
pop up a few degrees and skies should be sunny through the end of the week. However this
also means a return to a more northerly transport wind pattern and more unfavorable burn
conditions.
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:220811:0839
</pre>
</body>
</html>