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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT THU AUG 25 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:
Upper air charts show a strong ridge over much of the central US centered over the
Oklahoma panhandle. A weak trough lies off the west coast, and a stronger trough aloft
covers the north-central Gulf of Alaska. Minor disturbances are rippling northeastward
out of the offshore trough and have produced areas of showers or thunderstorms over the
state. (Yesterday there were over 8500 lightning strikes in the state.)
Another one of those ripples will move into Oregon from the west-southwest this evening
for a renewed chance of showers or thunderstorms in northwestern Oregon.
The morning ODF surface anlaysis shows the thermal trough extending northward from central
California, into the Rogue Valley of southwest Oregon, then farther north into the
southern Willamette Valley.
Gradients are onshore coast-to-valley, but offshore across the Cascades with Redmond's
pressure slightly higher than Salem's. As of 8 am Newport to Salem was 1.8 mb onshore
while Salem to Redmond was 0.1 mb offshore.
The morning Salem sounding shows temperatures have cooled several degrees above about 5000
feet, increasing instability over the area. Winds below 5000 feet are northerly, winds
above are south-southwesterly or southerly. Little change in wind direction is expected
this afternoon.
TODAY'S FORECAST:
Mostly sunny.
Salem's high temperature today will be near 89.
Relative humidity drops to 50% by 11am. Minimum relative humidity 33%.
Surface winds: N 3-5 mph becoming NW 4-6 late afternoon.
Transport winds: Northerly 8-12.
Maximum mixing height: 5000 feet.
Sunset tonight: 8:01 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 79 87 89 79
Relative Humidity: 52% 38% 33% 47%
Surface Wind Direction: 010 350 340 330
Surface Wind Speed: 3 5 6 6
Transport Wind Direction: 340 360 360 340
Transport Wind Speed: 5 8 11 16
Estimated Mixing Height: 1900 4900 5000 1000
Ventilation Index: 10 39 55 16
EXTENDED DISCUSSION:
The upper level disturbance lingers tomorrow with an increased threat of thunderstorms in
northwest Oregon. Low level wind flow remains northerly and unfavorable.
The flow aloft is forecast to turn more southwesterly this weekend which should stabilize
tha air mass and bring a return to mostly sunny and dry weather. Long range models are
still showing a possible burning opportunity for Monday as a system moving into southern
BC turns the flow aloft more westerly.
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:250811:0845
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