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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT THU AUG 16 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
Upper level charts show a northwesterly flow across southern British Columbia and
counterclockwise flow around an upper level low located several hundred miles off the
California coast. Oregon is in between in an area of light and variable flow aloft.
Satellite pictures show low clouds banked up along the coast and a band of high level
moisture over southern Oregon moving northward. This higher moisture is associated with
the low west of California.
A surface thermal trough extends from the central valley of California northward into the
Willamette Valley to about Salem. The axis of this trough then extends north-northwest
to Astoria and into Washington coastal waters.
There was a fairly strong surface-based inversion on the morning Salem sounding,
extending to about 2200 feet. This will mix out and with temperatures approaching 100
degrees today the maximum mixing height will likely reach to above 5000 feet.
With the trough directly over the region winds will be light and variable today with a
tendency toward northerly.
Fire Marshal burn-ban conditions are likely today due to high temperatures and low
humidities.
TODAY'S FORECAST:
Sunny and hot.
Salem's high temperature today will be near 100.
Relative humidity drops to 50% by 11am. Minimum relative humidity 23%.
Surface winds: Variable tending toward northerly 4-6, becoming NW 8 this evening.
Transport winds: Light/variable becoming NW 8 this evening.
Maximum mixing height: 5800 feet.
Sunset tonight: 8:15 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 87 98 99 82
Relative Humidity: 40% 26% 24% 41%
Surface Wind Direction: 110 350 290 250
Surface Wind Speed: 6 5 7 5
Transport Wind Direction: 320 010 320 300
Transport Wind Speed: 2 4 1 8
Estimated Mixing Height: 1500 4300 5800 1700
Ventilation Index: 3 17 6 14
EXTENDED DISCUSSION:
The thermal trough will remain over the Willamette Valley Friday then shift eastward on
Saturday. At the same time increasing southerly flow will bring a more moist air mass
northward for showers and thunderstorms. Most will be over the Cascades, but
precipitation in the valley cannot be ruled out for Saturday. Temperatures Saturday will
depend on cloud cover and timing of any showers but should be significantly cooler than
Friday. By Sunday onshore flow will bring temperatures back down into the low 80’s.
Early next week sees a trough off the coast, southwesterly flow aloft and temperatures
near average. This is a weather pattern much more likely to produce burn opportunities.
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:160812:0846
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