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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT THU SEP 15 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
A weak Pacific cold front is off the coast this morning. Satellite pictures show this
system progressing eastward at a steady pace. It will pass through the Willamette Valley
this afternoon keeping cloud cover over the area but likely not producing any rain.
Satellite pictures also show marine stratus clouds covering much of the Willamette Valley
this morning.
The morning Salem sounding showed considerable cooling above 3500 feet since yesterday.
At 4200 feet, for example, the temperature had dropped from 57F yesterday to 41F today.
This cooling destabilizes the atmosphere and allows higher mixing heights. Mixing
heights should reach 3000 feet by 11am and peak at 5200 feet this afternoon.
The sounding also showed light north-northwesterly winds to about 4000 feet, backing to
westerly from 5000 to 7000 feet, then backing to west-southweterly above. Models
indicate a light northwest transport wind persisting.
Pressure gradients as of 8am included: Newport to Salem, 1.2 mb onshore; Salem to
Redmond, 3.1 mb onshore; and Newport to Redmond, 4.3 mb onshore.
With good mixing heights, generally favorable winds and an approaching front, today
potentially presents the best burning opportunity for quite some time. This will be an
early opportunity. As the front moves through we may need to shut down burning earlier
than normal as well.
A pibal is tentatively scheduled for 11 am. If this develops as expected growers should
be ready to jump on the opportunity!
TODAY'S FORECAST:
Mostly cloudy...a few sunbreaks.
Salem's high temperature today will be near 68.
Relative humidity drops to 50% by 3pm. Minimum relative humidity 49%.
Surface winds: WNW through W 4-7 mph.
Transport winds: Light SW becoming light W this afternoon, becoming NW 6 by evening.
Maximum mixing height: 5200
Sunset tonight: 7:22 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 65 67 67 62
Relative Humidity: 60% 52% 49% 62%
Surface Wind Direction: 310 280 330 300
Surface Wind Speed: 5 5 6 5
Transport Wind Direction: 300 300 320 330
Transport Wind Speed: 3 3 10 14
Estimated Mixing Height: 3000 5000 5200 1200
Ventilation Index: 9 15 52 17
EXTENDED DISCUSSION:
Models show a weak impulse moving by to the north Friday which may present a burn
opportunity during the day tomorrow.
Models have backed off on the intensity of the system due in on Saturday. It now looks
like precipitation will be very limited with this feature. For open field burning
purposes, however, it still looks like a potential opportunity.
Longer range: A couple of minor systems move through early in the week, then upper level
ridging builds for a return to warm, dry weather mid to late next 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:150911:0831
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