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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE SEP 20 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from 12:00pm to 2:00pm with a 50 acre limit.
Propane flaming is allowed from 12:00pm to 5:00pm.
WEATHER DISCUSSION:
Upper air charts show a sharp ridge extending from northern California through the
Pacific Northwest and northward to Canads's Northwest Territories. A very deep upper
level low is centered over Kodiak Island in the Gulf of Alaska.
The upper ridge will shunt any storm systems well to the north and keep skies sunny
today.
The morning surface pressure analysis shows high pressure over British Columbia and
Alberta and a thermal trough extending from central California, into southwest Oregon
then northward to just off the central Oregon coast.
This is giving some northeasterly low level flow to the Willamette Valley. The morning
Salem sounding showed low level north-northeast winds. The the winds veered to ENE by
2000 feet, then veered to ESE up to 3200 feet. By 4000 feet the winds were SSW and above
6000 feet a generally westerly flow prevailed.
The sounding also showed significant warming above 1500 feet for a more stable atmosphere
today. Mixing heights should top out around 3200 feet this afternoon.
Pressure gradients as of 8am included: Newport to Salem, 1.2 mb offshore; Salem to
Redmond, 1.6 mb offshore; and Redmond to Newport, 2.8 mb offshore.
The offshore flow should go neutral this afternoon with transport winds becoming
northerly, not favorable for open field burning.
TODAY'S FORECAST:
Mostly sunny.
Salem's high temperature today will be near 78.
Relative humidity drops to 50% by 12pm. Minimum relative humidity 35%.
Surface winds: Northerly 3-5 mph.
Transport winds: Northerly 2-5 mph, becoming northwesterly 5-9 this evening.
Maximum mixing height: 3200 feet.
Sunset tonight: 7:13 pm.
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 65 74 78 70
Relative Humidity: 63% 43% 35% 49%
Surface Wind Direction: 010 340 360 280
Surface Wind Speed: 3 4 5 4
Transport Wind Direction: 360 350 340 330
Transport Wind Speed: 4 2 5 9
Estimated Mixing Height: 1400 3000 3200 500
Ventilation Index: 6 6 16 4
EXTENDED DISCUSSION:
The upper level ridge moves to the east over the next few days and the Gulf of Alaska low
swings several impulses inland to the north of Oregon. The upper level flow over the
region goes southwesterly and winds at transport levels should have a significant
westerly component for potential burn opportunities Wednesday, Thursday and Friday.
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:200911:0838
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