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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE SEP 18 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
Upper air charts show a ridge of high pressure in the northeastern Padific and a weak
upper level low centered about 400 miles west of Brookings Oregon. Wind flow at higher
altitudes over the Willamette Valley is southwesterly but light.
Satellite pictures show the low cloud deck has pushed northward. Yesterday at this time
it only reached to about North Bend, today it extends up past Gray's Harbor, Washington.
In addition the low clouds are pushing into some of the coastal valleys. Some mid-level
convective clouds are also visible over Douglas County.
Wildfire smoke has setteled into the Willamette Valley and readings remain elevated from
Portland to Cottage Grove.
The morning Salem sounding showed a strong inversion from the surface to about 3100 feet.
This inversion will not mix out today and the maximum mixing height should remain below
3000 feet again today. Winds below the top of the inversion were generally northerly,
while above the inversion winds were light northwesterly.
The morning ODF surface analysis showed the thermal trough has moved onshore. It now
extends from the central valley of California, northward through the Willamette Valley
then northwestward to off the Washington coast north of Gray's Harbor. Models show the
axis of the trough lingering in the Valley through much of the day, then slowly moving
into the Cascades. The result will be light winds gradually developing a westerly
component this afternoon.
With light winds and a low mixing height, as well as the wildfire smoke already in the
atmosphere today is not a good prospect for open field burning.
TODAY'S FORECAST:
Hazy sunshine.
Salem's high temperature today will be near 88.
Relative humidity drops to 50% by 11am. Minimum relative humidity 26%.
Surface winds: Light and Variable becoming SW to W 4-8 mph late.
Transport winds: W-NW 2-6 mph
Maximum mixing height: 2800 feet.
Sunset tonight: 7:15 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 74 85 88 75
Relative Humidity: 43% 30% 26% 41%
Surface Wind Direction: L/V L/V 240 260
Surface Wind Speed: 3 4 5 4
Transport Wind Direction: 260 340 340 310
Transport Wind Speed: 2 5 6 7
Estimated Mixing Height: 1000 2000 2800 1800
Ventilation Index: 2 10 17 13
EXTENDED DISCUSSION:
The warm upper ridge will remain over the area through the rest of the week. The upper
low off the coast will drift northward off the coast the next couple of days. Models
then have it drifting southward again late in the week, coming onshore over SW Oregon.
This isan unusual track, and later model runs are likely to have a different solution to
the movement of this feature. What this means is low confidence in the outlook for
Friday and the weekend.
The current pattern does not indicate a good burning
opportunity anytime soon. With the added element of very high levels of wildfire smoke
the prospects of open field burning for this week are low.
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:180912:0836
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