[willamette-fcst] TUE, Aug 31st, 2010
Willamette Valley Field Burning Forecast
willamette-fcst at listsmart.osl.state.or.us
Tue Aug 31 11:51:24 PDT 2010
SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT TUE AUG 31 2010
BURN ADVISORY:
Recommended burn times for agricultural burning are now until 6:00pm.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
Upper air charts show a broad trough covering the western US today. The axis of
the trough remains along the Canadian Rockies, through central Idaho then
curving southwestward to southern California. This leaves the Pacific Northwest
in a generally west-northwesterly flow aloft. This flow is steering a Pacific
weather system at Washington and northwest Oregon.
Rain began falling early this morning along the northern Oregon coast, and by
late morning had spread to a line from Portland to McMinnville to Pacific City
on the coast. Rain will likely continue to slowly spread southeastward this
afternoon.
The morning Salem sounding showed that the air mass had warmed somewhat in the
past 24 hours. This will keep mixing heights lower than yesterday when the
maximum mixing height was about 4000 feet. Look for a maximum today of about
3500 feet. Low level winds measured this morning were southwesterly, veering to
westerly by about 6000 feet above ground level. Computer models suggest that the
a WSW wind direction will continue through this afternoon. Winds and
mixing heights today will be favorable for burning.
If the rain holds off long enough there may be a window of opportunity between
the time that the mixing heights reach a favorable level and the time that the
rain moves in for some open field burning.
As of noon the estimated mixing height is 2000 feet with the mixing height
reaching 3000 feet about 3pm.
Afternoon forecast for the mid-Willamette valley: expect mostly cloudy skies, an
increasing chance of light rain, mainly northern sections. Salem's high
temperature today will be near 69. Minimum relative humidity 55%.
Sunset tonight: 7:45 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 64 64 60
Relative Humidity: 60% 67% 81%
Surface Wind Direction: 190 190 220
Surface Wind Speed: 11 8 8
Transport Wind Direction: 220 240 200
Transport Wind Speed: 15 18 11
Estimated Mixing Height: 2800 3500 1000
Ventilation Index: 42 63 11
EXTENDED DISCUSSION:
After Wednesday expect drier and warmer conditions as a flat upper level ridge
builds over the area. Temperatures should climb to near or slightly above
average for the time of year. Computer models suggest that another upper level
trough will move over the area sometime early in the weekend. Moisture looks
limited but temperatures will likely fall back below average for Saturday,
Sunday and the Labor Day holiday.
EXTENDED FORECAST:
Wednesday: Mostly cloudy, showers decreasing. High 74. Wind S 5-9 becomming WNW.
Thursday: Mostly sunny, with a high near 85. North wind between 3 and 10 mph.
Friday: Sunny, with a high near 86.
Saturday: Mostly sunny, with a high near 78.
Sunday: Partly sunny, with a high near 75.
Monday: Partly sunny, with a high near 76.
Notes:
1. Mixing height, as used here, is the lowest height at which the
potential temperature exceeds the 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.
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