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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE AUG 07 2012
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:
Satellite pictures show the marine low cloudiness has penetrated farther inland than
yesterday. Marion County, however, is mostly free of low clouds. Satellite pictures
also show a few high clouds associated with a weak disturbance moving northeastward well
offshore.
Pressure gradients are onshore this morning with 8am readings showing a Newport to Salem
gradient of 0.7mb onshore while the pressure difference from Salem to Redmond was 2.5mb.
The morning Salem sounding shows that temperatures have cooled an average of about 3
degrees from the surface 5000 feet. Low level winds were mostly light west-southwest to
west-northwest. There was some variability to the winds from 5000 to 10,000 feet but
above that level winds were generally southerly to southwesterly.
Forecast trajectory models show a generally westerly transport wind today. This would be
favorable for open field burning. Gradient stacking to the east will be a limiting
factor today as this is an indication of subsidence which would hold smoke low and
prevent good evacuation to the east.
The key will be early dissipation of the low cloudiness in the Willamette Valley to allow
good heating and lowering of valley barometric pressures to equalize the pressure
gradients coast-to-valley and valley-to-central-Oregon.
TODAY'S FORECAST:
Decreasing clouds, mostly sunny this afternoon. Warm.
Salem's high temperature today will be near 83.
Relative humidity drops to 50% by 1pm. Minimum relative humidity 38%.
Surface winds: Light winds becoming westerly 5-7 in the afternoon
Transport winds: Light westerly becoming west 5-10 in the afternoon
Maximum mixing height: 3600 feet.
Sunset tonight: 8:29 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 73 81 82 71
Relative Humidity: 55% 41% 40% 57%
Surface Wind Direction: 340 230 320 280
Surface Wind Speed: 3 4 6 5
Transport Wind Direction: 260 260 290 290
Transport Wind Speed: 3 5 10 12
Estimated Mixing Height: 1800 3000 3600 1500
Ventilation Index: 5 15 36 18
EXTENDED DISCUSSION:
There will be a good influx of marine air overnight tonight and tomorrow should be cloudy
and cooler with partial afternoon cooling. The first day after a marine push is usually
a poor open field burning day and tomorrow looks like no exception. Expect cloudy skies
with some afternoon clearing and temperatures limited to the upper 70’s. There will be
strong gradient stacking to the east and unfavorable northerly transport winds.
Thursday and Friday will see morning clouds, with afternoon sun and a slow warming trend
in temperatures. There may be small windows of opportunity during the afternoon hours.
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:070812:0837
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