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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE SEP 04 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from 12:00pm to 3:00pm with a 50 acre limit.
Propane flaming is allowed from 12:00pm to 5:00pm.
WEATHER DISCUSSION:
Morning satellite pictures show clear skies over the Willamette Valley. Some ocean
stratus has moved into coastal valleys and covers much of the Oregon coastline north of
Cape Blanco but most of this should burn off quickly.
In the upper atmosphere, the region is in a dry north-northwesterly flow aloft around a
broad upper level low centered in northern Manatoba. In addition, a weak upper low is
centered about 450 miles west of San Francisco.
The morning ODF surface pressure analysis shows a thermal trough extending from the
central valley of California to along the southern Oregon Coast. Pressure gradients as of
8am included: Newport to Salem, 0.9 mb onshore; Salem to Redmond, 0.5 mb offshore; and
Portland to Eugene, 0.5 mb northerly.
Surface winds have been light and variable this morning but with a tendency toward
northerly. This northerly wind should increase in velocity today and by afternoon winds
will be mostly in the 10-15 mph range. Some areas will actually see winds becoming
north-northeast this afternoon.
Bottom line: Today does not look like an open field burning day.
TODAY'S FORECAST:
Sunny and warm.
Salem's high temperature today will be near 85.
Relative humidity drops to 50% by 12pm. Minimum relative humidity 29%.
Surface winds: Northerly increasing to 10 to 15 mph.
Transport winds: Northerly around 15 mph increasing this evening
Mixing height reaches 3000 feet by about 1 pm, and tops out around 4500 feet.
Sunset tonight: 7:42 pm.
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 68 79 85 76
Relative Humidity: 57% 38% 29% 42%
Surface Wind Direction: 360 360 010 350
Surface Wind Speed: 5 7-10 10-15 10-15
Transport Wind Direction: 010 016 016 016
Transport Wind Speed: 12 14 16 21
Estimated Mixing Height: 2100 3500 4500 2200
Ventilation Index: 25 49 72 46
EXTENDED DISCUSSION:
Tomorrow looks line a near repeat of today with respect to transport winds. The upper
level low off the California coast opens up and drifts closer to the Pacific Northwest on
Thursday. This complicates the winds aloft pattern for western Oregon, but models are
indicating a good chance of some offshore or northeasterly flow for unfavorable burning
conditions. Dry weather will continue Thursday and Friday and likely through the
weekend.
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:040912:0834
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