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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE SEP 06 2011
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 broad ridge of high pressure over the western United States this
morning. The morning surface analysis shows a typical summertime pattern with a trough
centered in eastern Washington and a thermally induced trough from central California
extending into SW Oregon.
Gradients across western Oregon are fairly flat. As of 8am Newport to Salem was 0.5 mb
onshore while Redmond to Salem was 1.3 mb offshore. (i.e. Lowest pressures were in the
Willamette Valley.) Meanwhile Portland to Medford was 1.4mb northerly.
Wildfires have caused elevated smoke levels throughout much of the Pacific Northwest.
With wildfires continuing and a north-northeasterly transport wind today smoke levels in
the Willamette Valley are likely to increase.
The morning Salem sounding shows a strong, surface-based, inversion extending to about
2200 feet. This should mix out with a temperature of about 83 degrees, and maximum
mixing heights will top out just above 4000 feet. Winds on the sounding were northeast
to north up to about 7000 feet then swithced to west or southwest above that level.
Temperature and winds will be close, but likely below, Fire Marshall conditions today
although relative humidity criteria will likely be met. (Two of the three conditions
must be met to trigger a Fire Marshal burn-ban.)
Bottom line: Northerly transport winds today will likely preclude any burning
opportunity.
TODAY'S FORECAST:
Sunny and hot.
Salem's high temperature today will be near 91.
Relative humidity drops to 50% by 10am. Minimum relative humidity 22%.
Surface winds: N 5-7 mph.
Transport winds: N to NE 2-6 through early afternoon, then N 10-14 during the afternoon.
Maximum mixing height: 4100 feet.
Sunset tonight: 7:39 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 74 86 91 82
Relative Humidity: 44% 28% 22% 32%
Surface Wind Direction: 010 360 010 350
Surface Wind Speed: 5 7 7 5
Transport Wind Direction: 020 020 360 360
Transport Wind Speed: 2 6 12 12
Estimated Mixing Height: 1400 3100 4100 2200
Ventilation Index: 3 19 49 26
EXTENDED DISCUSSION:
Computer models show the broad upper level ridge continuing through the week. It will
likely reach maximum intensity on Thursday, but hot temperatures will continue into the
weekend and likely remain above average even into next Monday. Forecast charts also show
a generally north or northeast transport wind through the week.
Thunderstorms are possible mainly over the Cascades tomorrow and Thursday. Except for the
possibility of one of those drifting into the valley the prospects for rain this week are
nil.
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:060911:0836
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