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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT TUE SEP 11 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
Satellite pictures show sunny skies over all of Oregon as of late morning. Temperatures
are fairly close to where they were yesterday at the same time, but with more sunshine
this afternoon expect temperatures to top out several degrees higher than yesterday.
The surface pressure pattern shows high pressure nosing into western Washington while a
surface thermal trough extends from the central valley of California into SW Oregon and
northward along the southern Oregon coast. Flow is easterly across the Cascades and
should become increasingly easterly across the Coast Range and the Willamette Valley as
well.
As of 11:00am The Portland to Medford gradient was 5.1mb northerly, while the Redmond to
Salem gradient was 1.1mb offshore. Newport to Salem was still weakly onshore.
Gradients are unfavorable for open field burning today.
THIS AFTERNOON'S FORECAST:
Sunny but breezy. Temperatures near average for mid September.
Salem's high temperature today will be near 75.
Relative humidity drops to 50% by 12pm. Minimum relative humidity 26%.
Surface winds: Northerly 7-12 mph with higher gusts.
Transport winds: North to northeast 15-25.
Maximum mixing height: 4800 feet.
Sunset tonight: 7:28 pm.
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 68 75 66
Relative Humidity: 37% 26% 37%
Surface Wind Direction: 010 010 020
Surface Wind Speed: 11 13 8
Transport Wind Direction: 030 030 020
Transport Wind Speed: 19 22 25
Estimated Mixing Height: 4000 4800 2000
Ventilation Index: 76 106 50
EXTENDED DISCUSSION:
The surface thermal trough will advance up the coast then shift into the Willamette
Valley then to east of the Cascades where it dissipates. The latest NAM model indicates
the shift to the Valley on Friday with a potential sea-breeze setting up for Friday
afternoon. Earlier models had left open a possibility for a sea-breeze on Thursday
afternoon. It is not unusual for computer models to vary on the timing of such a shift
and at this point it is best to keep options open and plan for a possible burn opportunity
either Thursday or Friday. Each model run will likely have a slightly different solution
and all we can do this far ahead is monitor the models as they run.
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:110912:1147
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