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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT THU AUG 25 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from now until 2:00pm with a 50 acre limit.
Propane flaming is allowed from now until 5:00pm.
WEATHER DISCUSSION:
Upper air charts show a strong ridge over much of the central US centered over the
Oklahoma panhandle. A weak trough lies off the west coast, and a stronger trough aloft
covers the north-central Gulf of Alaska. Minor disturbances are rippling northeastward
out of the offshore trough and have produced areas of showers or thunderstorms over the
state.
Another one of those ripples will move into Oregon from the west-southwest this evening
for a renewed chance of showers or thunderstorms in northwestern Oregon.
Surface pressure analysis shows a thermal through in central California, extending
northward into southwest Oregon, then northward into the Willamette Valley.
The coast-to-valley gradient was onshore, while the valley-to-central Oregon gradient was
offshore. Pressure gradients as of 11am showed Newport to Salem at 2.0 mb onshore while
Salem to Redmond was 0.4 mb offshore. This confirms the position of the thermal trough
in
the Willamette Valley.
Computer models continue to show northerly transport winds this afternoon.
Unfavorable transport winds and an offshore flow across the Cascades means little chance
for burning this afternoon.
THIS AFTERNOON'S FORECAST:
Sunny and warm.
Salem's high temperature today will be near 89.
Relative humidity drops to 50% by 11am. Minimum relative humidity 33%.
Surface winds:
Transport winds:
Maximum mixing height:
Sunset tonight: 8:02 pm.
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 87 89 80
Relative Humidity: 39% 35% 47%
Surface Wind Direction: 360 010 300
Surface Wind Speed: 5 5 6
Transport Wind Direction: 360 360 330
Transport Wind Speed: 8 11 16
Estimated Mixing Height: 4900 5000 1000
Ventilation Index: 39 55 16
EXTENDED DISCUSSION:
The upper disturbance now offshore moves through tomorrow morning followed by a second
weak upper impulse moving through SW Oregon on Saturday. Neither of these features look
to give a burning opportunity. A trough moving across the Gulf of Alaska on Monday should
turn flow more westerly aloft and help turn pressure gradients more onshore for a
potential opportunity 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:250811:1234
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