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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE AUG 21 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 2:00pm to 5:00pm.
WEATHER DISCUSSION:
Upper air charts show a ridge axis along the Rockey Mountains with a realtive weak trough
running from Vancouver Island southwestward to about 250 miles west of San Fransisco.
The trough axis will progress steadily eastward today and run from central British
Columbia through about Klamath Falls by midnight tonight. This will keep a few clouds in
the Willamette Valley today, along with cooler temperatures and a more westerly or
northwesterly transport flow.
Morning satellite pictures show areas of clouds in poarts of the Willamette Valley, but
the cloud cover is not extensive and there are numerous areas seeing sunshine.
Temperatures are actually slilghtly warmer than yesterday morning, but a cooler
atmosphere today with the upper level trough should limit highs to a few degrees lower
than yesterday.
The Salem sounding this morning showed slightly warmer tempertures than yesterday up to
about 2500 feet then slightly cooler temperatures above 4000 feet. This makes the
conditions less stable than yesterday and should promote smoke dispersion through a
deeper layer. Winds on the sounding were very light and variable although tending toward
northerly at levels below about 3000 feet, above there was a light to moderate westerly
flow.
Trajectory models show a generally northwesterly transport flow for this afternoon.
Gradient stacking to the east will be a limiting factor today. Pressure gradients as of
8am included: Newport to Salem, 1.0 mb onshore; Salem toRedmond, 2.8 mb onshore for a
total onshore gradient Newport to Redmond of 3.8 mb onshore. The total Portland to
Medford gradient was 1.8 mb northerly.
TODAY'S FORECAST:
Mostly sunny and cooler.
Salem's high temperature today will be near 78.
Relative humidity drops to 50% by 1pm. Minimum relative humidity 38%.
Surface winds: North-northwest backing to west-northwest in the afternoon.
Speed increasing to 8-10 in the afternoon.
Transport winds: Becoming west-northwest 8-12 in the afternoon.
Maximum mixing height: 4600 feet.
Sunset tonight: 8:07 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 69 75 78 70
Relative Humidity: 59% 45% 38% 51%
Surface Wind Direction: 340 320 310 300
Surface Wind Speed: 2 6 9 10
Transport Wind Direction: 340 310 310 320
Transport Wind Speed: 3 5 10 15
Estimated Mixing Height: 2500 3100 4600 3000
Ventilation Index: 8 164 46 45
EXTENDED DISCUSSION:
Models show transport flow returning to an unfavorable northerly direction tomorrow.
Another upper level system will move by to the north Thursday which should shift
transport winds back to more westerly. Any burn opportunity would depend on timing.
Friday should see dry weather as higher pressure aloft begins to build.
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:210812:0848
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