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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT MON SEP 24 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
Hazy sunshine continues over the region today. Smoke from fires in the central
Washington Cascades has spread over the Willamette Valley. So far this seems to be
mainly aloft as nephelometer readings are not particularly high except in Portland.
Pressure gradients actually look favorable as of 11:00am with 1.9mb Newport to Salem, and
1.2mb Salem to Redmond.
However, latest computer models show a northerly transport flow in the valley this
afternoon. The sounding in Salem early this morning actually showed light northeasterly
flow at low levels. As of late morning winds in the valley were light and variable.
The smoke aloft has held temperatures lower than what was forecast earlier and mixing
heights will not be as high as a result. The revised mixing height forecast is 2900
feet.
THIS AFTERNOON'S FORECAST:
Hazy sunshine.
Salem's high temperature today will be near 74.
Relative humidity drops to 50% by 2pm. Minimum relative humidity 40%.
Surface winds: Northerly 3-6 mph becoming northwesterly 3-6 this evening.
Transport winds: Northerly to north-northeasterly 5-10 mph.
Becoming northerly 10-15 this evening.
Maximum mixing height: 2900 feet.
Sunset tonight: 7:04 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 69 74 65
Relative Humidity: 48% 40% 61%
Surface Wind Direction: 350 350 310
Surface Wind Speed: 4 5 6
Transport Wind Direction: 360 360 360
Transport Wind Speed: 7 10 15
Estimated Mixing Height: 2500 2900 1400
Ventilation Index: 23 34 21
EXTENDED DISCUSSION:
A weak upper level disturbance moves through British Columbia overnight tonight and early
tomorrow. A cold front associated with this feature is offshore this morning, and will
push through the area overnight. It will be very weak however. Another upper ridge will
build behind during the day tomorrow keeping transport wind flow mostly northerly for
Tuesday.
A weak, flat upper ridge will then remain over the Pacific Northwest Wednesday for
another seasonal, pleasant day. Dry weather will continue Thursday with another weak
front sometime on Friday. This could present a burn opportunity, but it is too soon to
tell about timing and strength.
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:240912:1154
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