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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED AUG 29 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is allowed from 11:00am to 5:00pm.
WEATHER DISCUSSION:
An upper level trough of low pressure is moving through Oregon this morning. Satellite
pictures show the clouds associated with the trough are not extensive and radar is
showing just a few widely scattered light showers over the northern coast range.
Temperatures were generally running a few degrees below where they were at the same time
yesterday morning.
The morning Salem sounding showed temperatures aout 3-5 degrees cooler than yesterday
from the surface up to about 10,000 feet. The cooler air aloft should help push mixing
heights to well above 5000 feet again this afternoon.
Winds on the sounding were light southerly to about 3500 feet where they turned light
southwesterly.
The morning ODF surface pressure analysis shows an inverted trough extending from central
California into southwestern Oregon. This should turn transport flow to an unfavorable
northerly direction this afternoon.
Pressure gradients are rather flat. Pressure gradients as of 8am included: Newport to
Salem, 0.3 mb onshore; Salem to Redmond, 0.2 mb onshore; and Portland to Medford, 1.1 mb
northerly.
Pibals will confirm wind directions this afternoon. We will monitor conditions for any
changes but at this point it does not look favorable for open field burning.
TODAY'S FORECAST:
Salem's high temperature today will be near 74.
Relative humidity drops to 50% by 2pm. Minimum relative humidity 40%.
Surface winds: Northwest to north-northwest 3-8 mph.
Transport winds: Northerly 4-8 mph, increasing during the evening.
Maximum mixing height: 5500 feet.
Sunset tonight: 7:53 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 62 70 74 68
Relative Humidity: 70% 49% 40% 53%
Surface Wind Direction: 310 330 340 330
Surface Wind Speed: 3 6 8 7
Transport Wind Direction: 350 350 360 010
Transport Wind Speed: 4 7 8 13
Estimated Mixing Height: 1200 4900 5500 1500
Ventilation Index: 5 34 44 20
EXTENDED DISCUSSION:
Low marine clouds will push inland tonight and tomorrow morning as another upper level
trough drops southward from British Columbia and into the northeastern Pacific. Clouds
should clear by afternoon for warm (but slightly below average) temperatures.
The trough will continue to dig south to off the California coast tomorrow night and
Friday. The thermal trough will intensify and increase northerly transport flow for the
valley Friday but also bring warmer temperatures and more sunshine.
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:290812:0834
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