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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED AUG 15 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
In the upper atmosphere a weak disturbance pulled out of southeastern British Columbia
and northeastern Washington this morning, moving east. A ridge of high pressure will
build in from the southeast over the course of the next couple of days for dry weather
and increasing heat.
The morning ODF surface pressure analysis shows a well developed thermal trough extending
from the central valley of California into southwest Oregon. Easterly gradients are
beginning to develop across the Cascades. The Redmond to Salem pressure gradient was
1.5mb offshore and the Salem to Newport gradient was 0.8mb onshore. Portland to Medford
was 2.3mb northerly.
Satellite pictures show low marine clouds over portions of Linn, Marion, Clackamas and
Multnomah counties. These should dissipate this morning and with easterly flow
developing temperatures should rise fairly rapidly.
The central Willamette Valley will be on the threshold of Fire Marshal burn ban
conditions late this afternoon.
The morning Salem sounding showed a temperature trace fairly similar to yesterday
morning’s. There were shallow inversions from 1900 to 2200 feet, 3100 to 3700 feet and
from 4900 to 5400 feet. Only the lower of these will completely mix out and maximum
mixing heights will top out at about 4500 feet this afternoon. Transport level winds
were generally north or north-northeast on the sounding and speeds were approaching 16mph
at just 600 feet above ground level.
With a northeast component to transport flow and potential Fire Marshal conditions, today
will likely not be an open burning day.
TODAY'S FORECAST:
Becomming sunny and hot.
Salem's high temperature today will be near 95.
Relative humidity drops to 50% by 12pm. Minimum relative humidity 26%.
Surface winds: Northerly to north-northeasterly 6-10 mph.
Transport winds: Northerly 6-10 becoming northeasterly 12-18 late.
Maximum mixing height: 4500 feet.
Sunset tonight: 8:17 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 81 92 95 82
Relative Humidity: 47% 31% 27% 41%
Surface Wind Direction: 010 010 010 020
Surface Wind Speed: 7 7 7 6
Transport Wind Direction: 360 020 030 030
Transport Wind Speed: 6 10 12 18
Estimated Mixing Height: 2000 3200 4500 2000
Ventilation Index: 12 32 54 36
EXTENDED DISCUSSION:
As the upper level ridge builds so does the heat. The surface thermal trough will be
over the valley through Friday and the next two days should see temperatures topping near
100 degrees. The thermal trough finally shifts east on Saturday allows temperatures to
drop back into the 80’s. Fire Marshal conditions and easterly pressure gradients across
the Cascades should preclude burning the rest of the week.
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:150812:0846
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