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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE AUG 09 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from 11:00am to 2:00pm with a 50 acre limit.
Propane flaming is allowed from 11:00am to 5:00pm.
WEATHER DISCUSSION:
The Pacific is under the influence of weak westerly flow aloft this morning. Once again
today a deep marine layer covers western Oregon from the coast to the Cascades and from
the Columbia to southern Lane County. While the marine layer is deep the actual cloud
layer appears thinner today versus yesterday. A quicker burn-off means slightly higher
temperatures today.
The Salem sounding today showed considerable cooling from about 2000 feet to 5500 feet
above ground. Yesterday morning the temperature at 5000 feet was 55 degrees (fahrenheit)
but today had dropped to just 40. This cooling means mixing heights should climb rapidly
today. An inversion just above 5000 feet will ultimately limit afternoon mixing heights.
While surface winds were light northwesterly from about 500 feet to 3500 feet winds were
light northerly or northeasterly. Above 5000 feet good solid westerly flow prevailed.
The morning surface analysis shows a thermal trough extending from central California
into southwestern Oregon. The north-south gradient has relaxed somewhat today but is
still about two-and-a-half millibars from Portland to Medford giving the area a tendency
toward northerly winds. Computer models indicate actual transport winds will be slightly
northwesterly, marginally favorable for some open field burning this afternoon.
Gradient stacking to the east with its implied subsidence will be an issue today. At 8am
Newport to Salem was just 0.8 mb onshore while Salem to Redmond was a considerably larger
4.5 mb. It is unlikely, but not impossible, that that will even out by the end of the
afternoon. We will monitor.
TODAY'S FORECAST:
Morning clouds, gradual afternoon clearing.
Salem's high temperature today will be near 79.
Relative humidity drops to 50% by 10am. Minimum relative humidity 32%.
Surface winds: NNW 4-6 mph becoming NW 4-6 late.
Transport winds: NNW 4-6 mph slowly backing to NW 5-8 late afternoon.
Maximum mixing height: 5200 feet.
Sunset tonight: 8:27 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 69 76 79 72
Relative Humidity: 47% 36% 32% 43%
Surface Wind Direction: 340 320 330 290
Surface Wind Speed: 5 6 6 7
Transport Wind Direction: 340 340 330 300
Transport Wind Speed: 3 7 9 10
Estimated Mixing Height: 4700 5200 5200 1000
Ventilation Index: 14 36 47 10
EXTENDED DISCUSSION:
Long range charts indicate dry weather through next Monday with a chance for some rain on
Tuesday. Otherwise the pattern shows onshore flow for morning clouds, a tendency to
northerly transport winds, and a situation where day-by-day monitoring may reveal small
windows of burning opportunity.
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:090811:0820
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