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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED AUG 10 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:
In the upper atmosphere a weak disturbance is moving across southern British Columbia and
northern Washington today. Surface weather maps are very similar to yesterdays, but with
some subtile differences. The onshore pressure gradient is not as strong this morning
and the central Washington thermal trough has weakened.
The morning sounding from Salem shows some warming from about 3000 feet to 5000 feet.
This has resulted in a weak inversion from about 3200 feet to about 5200 feet. This will
not completely mix out this afternoon, but with plenty of sunshine and high temperatures
in the upper 70's the maximum mixing height should climb to around 4800 feet this
afternoon.
There is still gradient stacking to the east, but with gradients not as strong today, and
with the weak disturbance moving by to the north, there is a much better chance of
overcoming this than there has been the past couple of days. Pressure gradients as of 8am
included: Newport to Salem, 0.3 mb onshore; Salem to Redmond, 2.7 mb onshore; and
Newport to Redmond, 3.0 mb onshore.
In addition the "Medford Draw" is not as strong as it's been the past few days.
Computer models indicate a generally northerly flow pattern with transport winds slowly
coming around to more northwesterly, or marginally favorable, direction later this
afternoon.
If everything comes together there may be a small window of opportunity for open field
burning late this afternoon.
TODAY'S FORECAST:
Salem's high temperature today will be near 79.
Relative humidity drops to 50% by 11am. Minimum relative humidity 31%.
Surface winds: N 4 mph slowly backing to NW 5-7 by late afternoon.
Transport winds: NNW 3-5 mph slowly backing to NW 6-9 by late afternoon.
Maximum mixing height: 4800 feet.
Sunset tonight: 8:26 pm.
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 69 77 79 72
Relative Humidity: 51% 36% 31% 44%
Surface Wind Direction: 350 320 320 300
Surface Wind Speed: 4 6 7 8
Transport Wind Direction: 340 330 320 300
Transport Wind Speed: 3 5 6 9
Estimated Mixing Height: 3000 4600 4800 1000
Ventilation Index: 9 23 35 9
EXTENDED DISCUSSION:
An upper level low developing over Alaska today will deepen and drop into the Gulf of
Alaska the next few days. By late Friday this should shift upper level winds to more
southwesterly and should give more of a westerly component to transport level flow.
Longer range charts still indicate a possibility of cooler weather and a chance for some
precipitation beginning Monday of next 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:100811:0832
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