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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED JUL 18 2012
BURN ADVISORY:
Recommended burn times for agricultural burning are from 11:00am to 7:00pm.
Prep burning is allowed from 12:00pm to 5:00pm with a 50 acre limit.
Propane flaming is allowed from 12:00pm to 5:00pm.
WEATHER DISCUSSION:
There has been little change to the overall weather pattern in the past 24 hours. An
upper level low pressure area is centered along the coast right at the Oregon-California
border. Cool air aloft with this system has destabilized the atmosphere and the result
has been scattered showers and thunderstorms. Lightning was reported yesterday in parts
of Linn and Benton counties, as well as portions of Clackamas County. Lightning in the
Willamette Valley is a possibility again today.
Low level flow is onshore and winds up to about 4000 feet were generally southwesterly on
the Salem sounding this morning. Above that level, however, winds had an easterly
component, consistent with counter-clockwise flow around the upper low to the south.
Temperatures today will again be a bit below average for this time of year and it will be
rather humid.
While mixing heights are favorable, moisture and a sharp wind shift aloft make for poor
for open field burning conditions.
TODAY'S FORECAST:
Mostly cloudy, a few sunbreaks. Humid. Continued chance for showers or possible
thunderstorms.
Salem's high temperature today will be near 78.
Minimum relative humidity 45%.
Surface winds: Southwest 6-9.
Transport winds: Southwesterly 5-7 but with a shift to east-northeast
near the top of the mixed layer.
Maximum mixing height: 5000 feet.
Sunset tonight: 8:53 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 69 76 77 68
Relative Humidity: 61% 48% 45% 59%
Surface Wind Direction: 250 250 220 250
Surface Wind Speed: 6 8 9 6
Transport Wind Direction: 250 250 310 310
Transport Wind Speed: 5 5 5 7
Estimated Mixing Height: 3200 4100 5000 2500
Ventilation Index: 16 20 25 18
EXTENDED DISCUSSION:
The same general weather conditions are likely to persist through Thursday as the upper
pattern changes very little. Model agreement is good that the closed upper low will open
up as a trough and move towards the northeast early Friday. There may be some enhanced
shower activity overnight late Thursday into Friday morning as the trough moves through,
but then precipitation chances will dwindle quickly later Friday once the trough axis
shifts east of the Cascade crest. Temperatures will vary little from average through the
next 7 days.
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:180712:0831
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