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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT TUE AUG 23 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is allowed from 12:00pm to 5:00pm.
WEATHER DISCUSSION:
The upper level pattern remains much the same as yesterday: A large upper level high
pressure system remains over much of the continental United States today
while a broad upper level trough covers much of the Gulf of Alaska. Minor impulses are
periodically rippling out of the trough and into Canada around the top of the US ridge.
Satellite pictures show that this morning's clouds are retreating to the north. Marion
county is mostly cloud free except for some puffy, fair-weather cumulus.
Pressure gradients remain quite flat. Pressure gradients as of 11am included: Newport
to Salem, 0.7 mb onshore and Salem to Redmond, 0.3 mb offshore.
Surface maps show the California thermal trough intensifing and eventually the Valley
should move into a northerly wind flow pattern. Computer models show this transition
occuring during the course of the afternoon with northerly or even slightly northeasterly
transport winds.
A late morning sounding from near Silverton showed southerly winds to about 4000 feet
then westerly above.
THIS AFTERNOON'S FORECAST:
Mostly sunny.
Salem's high temperature today will be near 85.
Relative humidity drops to 50% by 1pm. Minimum relative humidity 42%.
Surface winds: Becoming NW 2-6 this afternoon.
Transport winds: Becoming northwesterly then northerly this afternoon 3-8 mph.
Maximum mixing height: 6000 feet.
Sunset tonight: 8:05 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 81 85 79
Relative Humidity: 46% 42% 52%
Surface Wind Direction: 310 350 310
Surface Wind Speed: 2 6 5
Transport Wind Direction: 300 360 020
Transport Wind Speed: 3 6 9
Estimated Mixing Height: 4900 6000 1000
Ventilation Index: 15 42 9
EXTENDED DISCUSSION:
Latest computer models show a disturbance in the upper atmosphere approaching the area
from the southwest. If this arrives soon enough it may be enough to trigger a weak
marine
push and give a burn opportunity. Timing is important and it may be too late in the day,
however. This feature will need to be monitored closely tomorrow.
Another upper disturbance moving more from the west on Thursday has the same potential -
and the same timing problems.
Finally, weak troughiness aloft on Friday could also provide a burn opportunity, once
again, depending on timing.
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:230811:1149
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