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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT MON AUG 15 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from 11:00am to 1:00pm with a 50 acre limit.
Propane flaming is allowed from 11:00am to 5:00pm.
WEATHER DISCUSSION:
Pacific Northwest weather is dominated by a broad upper level trough over the region
today. This will keep clouds and cooler than average temperatures in the area.
Satellite pictures show low clouds covering much of the northern Willamette Valley this
morning. In addition a band of thicker clouds extends from northeastern Washington into
the north central Oregon Cascades, then trails back across Lane county. This band is
producing some rain over parts of eastern Washington and north-central Oregon. However
west of the Cascades rain in Oregon has been limited to just some spotty drizzle mainly
over the north coast range. There is still a small chance of some drizzle in the central
Willamette Valley through the morning hours, but mainly expect just cloudy skies and
relatively high humidities. Clouds will break up some this afternoon.
The surface analysis shows a well developed thermal trough in the Central Valley of
California extending northward into southwest Oregon.
The morning Salem sounding shows that the atmosphere above 4000 feet has cooled several
degrees since yesterday morning. This will allow for good mixing heights by this
afternoon - reaching 3000 feet by about 11am and topping out above 6000 feet this
afternoon. The sounding shows north or northeast winds at low levels.
Onshore gradients are weak and with the thermal trough in California this is a north wind
pattern. Computer models show a north wind today, possibly even north-northeast in some
locations in the central Willamette Valley. Thus today does not look like a day with any
open field burning opportunities.
TODAY'S FORECAST:
Mostly cloudy.
Salem's high temperature today will be near 77 (Average is 82).
Relative humidity drops to 50% by noon and the minimum relative humidity today should be
about 37%.
Surface winds: N or NNW 2-4 mph increasing to 5-8 later this afternoon.
Transport winds: NNW through NNE 3-5 mph increasing to 5-8 by early evening.
Maximum mixing height: 6100 feet.
Sunset tonight: 8:18 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 67 74 76 71
Relative Humidity: 59% 43% 37% 47%
Surface Wind Direction: 360 330 350 320
Surface Wind Speed: 2 4 6 7
Transport Wind Direction: 330 360 360 360
Transport Wind Speed: 3 3 5 8
Estimated Mixing Height: 2500 4800 6100 1000
Ventilation Index: 8 14 30 8
EXTENDED DISCUSSION:
Looking ahead, the upper trough will split, part moving east into central Canada, part
hanging back as a weak upper low off the coast of California. A dome of high pressure
now over the southwestern US will expand northwestward for warmer temperatures and more
sunshine beginning tomorrow and lasting into the weekend.
Temperatures will also warm in California enhancing the thermal trough and keeping the
Willamette Valley in a generally northerly transport wind pattern. Any westerly flow
would have to be associated with an afternoon sea breeze and would need to be monitored
on a day-to-day basis.
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:150811:0828
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