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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED SEP 07 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
It appears that the smoke aloft from Cascade wildfires is dense enough to block enough
sunlight to keep temperatures below the Fire Marshal burn-ban criteria. We have revised
the high temperature for this afternoon downward several degrees.
The smoke is mostly aloft as Valley nephelometer readings are not particularly high.
However closer to the Cascade foothills readings are a bit higher. Late morning readings
included 1.05 b-scat at Lyons and 1.22 at Sweet Home. Salem was .93 b-scat.
A strong ridge in the upper atmosphere continues to cover much of the western US today.
Our surface pressure analysis shows a thermal trough in the Willamette Valley. As of
11am the gradient between Newport and Salem was 1.2mb onshore while the Redmond to Salem
gradient was 3.2mb offshore.
This pattern gives a light northeasterly transport wind pattern to the area.
With unfavorable wind directions and smoke already in the air today is not an open field
burning day.
THIS AFTERNOON'S FORECAST:
Hazy sunshine. Warm.
Salem's high temperature today will be near 90.
Relative humidity drops to 50% by 8am. Minimum relative humidity 18%.
Surface winds: N 5-8 mph.
Transport winds: Northerly 6-10 mph, increasing to 8-16 mph late this afternoon.
Maximum mixing height: 3300 feet.
Sunset tonight: 7:38 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 86 90 78
Relative Humidity: 29% 25% 38%
Surface Wind Direction: 360 010 290
Surface Wind Speed: 7 7 6
Transport Wind Direction: 010 010 020
Transport Wind Speed: 6 8 16
Estimated Mixing Height: 2800 3300 2200
Ventilation Index: 17 26 35
EXTENDED DISCUSSION:
Latest models show little change from earlier runs for the extended period.
A weak upper level disturbance now located off the northern California coast will slowly
undercut the upper level ridge during the next couple of days. This will alter the wind
flow aloft over the region bringing northeasterly winds to the upper atmosphere over
Oregon and Washington beginning Friday. This will only serve to enhance the dry weather
pattern and cut off any chance for mountain thunderstorms.
Dry weather and unfavorable transport winds are likely into the weekend.
Longer range computer models are indicating a change to a more favorable westerly
transport wind pattern by Monday.
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:070911:1145
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