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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE AUG 28 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is allowed from 11:00am to 5:00pm.
WEATHER DISCUSSION:
Morning satellite pictures show sunshine on the west side of the Willamette Valley but
clouds to the east of I-5 extending to the crest of the Cascades. Temperatures are
running within a couple of degrees of where they were yesterday at the same time.
In the upper atmosphere strong high pressure is centered over Colorado and an upper level
trough extends from central British Columbia to off the Oregon coast. This gives the
Pacific Northwest a southwesterly flow aloft. While there is moisture associated with
the offshore trough, it is limited and is not making it eastward into Oregon.
The morning Salem sounding showed a temperature trace very similar to yesterday's up to
about 6000 feet. Above that there was some warming for a more stable atmosphere aloft.
Maximum mixing heights, however, should easily make it above 5000 feet again this
afternoon.
Winds on the sounding were light southeasterly to about 1000 feet above the grouund.
Above that level winds shifted to southwesterly and increased with height.
Pressure gradients are flat this morning. Pressure gradients as of 8am included: Newport
to Salem, 0.8 mb onshore; Salem to Redmond, 0.8 mb onshore; for a total Newport to Redmond
gradient of 1.6 mb onshore. The Salem to Redmond gradient will likely increase this
morning for gradient stacking east, but the pressures should even out sometime this
afternoon as they did yestersay.
If gradient stacking east does not become a problem, this afternoon may be another burn
opportunity similar to Monday.
TODAY'S FORECAST:
Mostly sunny.
Salem's high temperature today will be near 78.
Relative humidity drops to 50% by 11am. Minimum relative humidity 31%.
Surface winds: Southerly 3-5 becoming southwesterly to westerly 5-8 mph.
Transport winds: Southwesterly to westerly 5-10 mph.
Maximum mixing height: 5500 feet.
Sunset tonight: 7:54 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 71 78 78 68
Relative Humidity: 49% 34% 31% 47%
Surface Wind Direction: 170 220 240 270
Surface Wind Speed: 4 6 6 6
Transport Wind Direction: 240 250 280 300
Transport Wind Speed: 4 7 10 13
Estimated Mixing Height: 4900 5500 5500 1200
Ventilation Index: 20 38 55 16
EXTENDED DISCUSSION:
The upper level trough offshore moves through the region overnight tonight. This will
bring more clouds to the valley tomorrow morning and possibly some drizzle. Temperatures
will be cooler but by afternoon skies will return to partly cloudy. Burn opportunities
will be limited by pressure gradient stacking but winds and mixing heights should remain
favorable.
Later in the week a northerly transport wind pattern will likely develop which will limit
burning opportunities.
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:280812:0816
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