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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED SEP 14 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:
The upper air pattern of a high pressure area to the north and a weak low to the south is
slowly changing as both of those features move to the east today.
The late morning surface pressure analysis shows a thermal trough extends from
central California into SW Oregon and another trough extends from a low in central
Washington back into central Oregon, then into SW Oregon.
Satellite pictures show clouds still hanging tough in the Willamette Valley as of the
noon hour. The Salem Airport reported a layer of broken clouds at 2600 feet and an
overcast layer at 3300 feet. A pilot report received around 10am this morning from near
Eugene indicated the top of the overcast layer was at 4000 feet, higher than earlier
thought. None the less, the clouds should break up a little earlier than yesterday.
Pressure gradients as of 11am included: Newport to Salem, 1.0 mb onshore; Salem to
Redmond, 4.5 mb onshore; Newport to Redmond, 5.5 mb onshore; and Eugene to Medford, 3.7
mb northerly.
Computer models continue to show a little more of a northwesterly transport wind this
afternoon. Pibals will indicate if there is enough westerly to be favorable, but with
the
gradient stacking to the east still large, open field burning is not likely this
afternoon.
THIS AFTERNOON'S FORECAST:
Cloudy, becoming mostly sunny later this afternoon.
Salem's high temperature today will be near 72.
Relative humidity drops to 50% by 1pm. Minimum relative humidity 47%.
Surface winds: N 6-8 mph.
Transport winds: N 6-8 mph.
Maximum mixing height: 3500 feet.
Sunset tonight: 7:25 pm,
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 71 71 68
Relative Humidity: 49% 47% 60%
Surface Wind Direction: 340 340 290
Surface Wind Speed: 7 7 7
Transport Wind Direction: 360 350 320
Transport Wind Speed: 6 8 12
Estimated Mixing Height: 3500 3500 1600
Ventilation Index: 21 21 19
EXTENDED DISCUSSION:
Changes are coming up after today. A weak upper level trough moves through the region
tomorrow afternoon. Latest models are a little faster with this feature. There may be a
burning opportunity ahead of this system tomorrow during the afternoon.
On Friday another impulse moves through to the north with another potential burn
opportunity.
A more significant system reaches the Pacific Northwest on Saturday. This looks to be a
rain producer for the Willamette Valley. Timing on this system is the key, however if
rain holds off long enough it should provide a good burn opportunity. Latest models are
similar in timeing to earlier runs. Expect rain for sure by Saturday night and another
Pacific system should bring an additional surge of moisture into the region Sunday into
Monday morning.
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:140911:1155
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