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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE JUL 24 2012
BURN ADVISORY:
Recommended burn times for agricultural burning are from 11:00am to 7:00pm.
Prep burning is allowed from 12:00pm to 2:00pm with a 50 acre limit.
Propane flaming is allowed from 12:00pm to 2:00pm.
WEATHER DISCUSSION:
The Pacific Northwest is still somewhat under the influence of an upper level low now
drifting northeastward across southern Alberta Canada. But that influence will become
less and less during the day today.
The ODF morning surface pressure analysis shows high pressure nosing into western
Washington and a thermal trough extending from the central valley of California to along
the southern Oregon coast. Low level flow is onshore and low clouds have formed through
most of the Willamette Valley. The base of the cloud layer is at about 1600 feet above
ground level and the top of the cloud layer looks to be around 4000 feet. The clouds
should begin to break up around 11am.
The morning Salem sounding showed considerable warming in the 3-8000 foot range. At about
4000 feet the temperature yesterday was 41F but today it had increased to 60F. This has
produced a strong inversion from about 2800 feet to 4000 feet. As the clouds break and
temperatures warm up the lower portion of this inversion will erode, but the maximum
mixing height today is likely to be only about 3600 feet. Winds on the sounding were
northerly or north-northeasterly up to about 8000 feet.
The main limiting factor on open field burning today is wind direction. Indications are
that a low level sea-breeze may develop around sunset it is not likely that the
north-northeasterly transport wind will change today.
TODAY'S FORECAST:
Morning clouds, afternoon sunshine. Temperatures in the low 80's.
Salem's high temperature today will be near 82. (Yesterday's high was 75.)
Relative humidity drops to 50% by 1pm. Minimum relative humidity 28%.
Surface winds: Northerly 6-10 mph.
Transport winds: Northerly 8-15 mph.
3000 foot mixing height reached: 11:00am.
Maximum mixing height: 3600 feet.
Sunset tonight: 8:47 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 69 78 82 71
Relative Humidity: 49% 34% 28% 46%
Surface Wind Direction: 360 010 010 340
Surface Wind Speed: 6 6 8 6
Transport Wind Direction: 360 010 010 010
Transport Wind Speed: 9 10 11 13
Estimated Mixing Height: 3000 3200 3600 1500
Ventilation Index: 27 32 40 20
EXTENDED DISCUSSION:
The Alberta upper low will continue to move away and the Pacific Northwest will be under
the influence of a dry southwesterly flow aloft for the rest of the week. This pattern
will likely keep low level flow onshore for morning clouds. Conditions look dry, however
and temperatures should be fairly close to average the next few days. Weather patterns
look more favorable for a westerly component to transport flow (and better open field
burning prospects) possibly as soon as tomorrow afternoon.
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:240712:0838
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