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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED SEP 14 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from 12:00pm to 2:00pm with a 50 acre limit.
Propane flaming is allowed from 12:00pm to 5:00pm.
WEATHER DISCUSSION:
Upper air charts show a ridge to the north, moving slowly eastward this morning. To the
south, a weak upper low is centered over about Las Vegas, Nevada. Oregon is in between
these features with a generally light WSW flow at higher elevations.
The surface pressure analysis is very similar to yesterday. 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.
This pattern will give a generally northerly transport wind pattern today.
Pressure gradients as of 8am included: Newport to Salem, 1.2 mb onshore; Salem to
Redmond, 3.2 mb onshore; Newport to Redmond, 4.4 mb onshore; and Eugene to Medford, 2.7 mb
northerly.
The morning Salem weather balloon sounding shows warmer temperatures, compared to
yesterday, in the lowest 2000 feet of the atmosphere. The inversion layer is higher today
than yesterday, with a base at 3400 feet and a top at about 6200 feet.
In addition the moist layer is not as thick. The Salem airport reports a cloud base at
3100 feet and from the sounding it appears that the top of the cloud layer is just a
couple of hundred feet above that. Thus we should see an earlier burn off of the clouds
today.
With more and earlier sunshine in the Willamette Valley today, pressure gradients should
even out, but the limiting factor on open field burning will be transport wind direction.
Models are consistent in showing persistent unfavorable northerly transport winds over
central Marion county through the day. Pibals will either confirm or deny the model
forecast.
TODAY'S FORECAST:
Mostly cloudy this morning, becoming mostly sunny 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 5 mph becoming NW 5-8 late afternoon.
Transport winds: N 5-7 mph.
Maximum mixing height:
Sunset tonight: 7:24 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 66 71 71 65
Relative Humidity: 63% 49% 47% 60%
Surface Wind Direction: 360 350 330 290
Surface Wind Speed: 5 5 7 6
Transport Wind Direction: 360 360 350 320
Transport Wind Speed: 5 6 8 12
Estimated Mixing Height: 3400 3500 3500 1600
Ventilation Index: 17 21 28 19
EXTENDED DISCUSSION:
Changes are coming up after today. A weak upper level trough moves through the region
tomorrow evening. There may be a burning opportunity ahead of this system tomorrow
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. Expect rain for
sure by Saturday night and another Pacific system should bring an additional surge of
moisture into the region Sunday night 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:140911:0825
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