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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED SEP 05 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming not allowed.
WEATHER DISCUSSION:
An uppper level ridge extends from west of the Washington coastline northward into the
Yukon with its axis along about 135 degrees west. To the south of the ridge a weak upper
level low pressure area is off the California coast. An upper short wave is moving
southeastward along the spine of the Rockie Mountains in Alberta/British Columbia.
At the surface gradients are weak. A thermal trough extends from the central valley of
California into southwestern Oregon and then weakly northweard into the Willamette
Valley.
The morning Salem sounding shows that yesterday's inversion aloft has lowered and
strengthened. This will hold mixing heights lower until later in the day. Mixing
heights won't reach 3000 feet until about 3pm this afternoon.
Winds on the sounding were light but mostly northerly or northeasterly up to about 10,000
feet. Computer models indicate a light, nearly due northerly, transport wind today.
The short wave dropping southeastward in Canada is likely too far away and is moving too
slowly to turn winds more northwesterly for a burn opportunity today, but conditions will
need to be monitored with pibals this afternoon.
TODAY'S FORECAST:
Sunny and warm.
Salem's high temperature today will be near 85.
Relative humidity drops to 50% by 12pm. Minimum relative humidity 30%.
Surface winds: Northerly 4-6 mph, becoming westerly mid-evening.
Transport winds: Northerly 5-7 mph, increasing late.
Mixing height reaches 3000 feet by 3pm, and increases to 4100 feet by 5pm.
Sunset tonight: 7:40 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 72 81 85 74
Relative Humidity: 51% 37% 30% 46%
Surface Wind Direction: 360 350 350 290
Surface Wind Speed: 4 5 6 4
Transport Wind Direction: 350 350 350 360
Transport Wind Speed: 5 5 7 9
Estimated Mixing Height: 2200 2900 4100 4500
Ventilation Index: 11 14 29 40
EXTENDED DISCUSSION:
The upper level ridge in the northeastern Pacific will continue the dry and warm weather
through Friday. Offshore winds will likely develop tomorrow afternoon for no burn
opportunity. The thermal trough will strengthen and expand northward Friday, intensifying
the offshore flow. Flow goes back onshore Saturday. There may be a burn opportunity
ahead of the switch to onshore flow but this will depend on timing.
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:050912:0830
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