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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED OCT 10 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
An upper level low pressure area is drifting south-southeastward along the California
coast this morning. At the same time a weak upper level high pressure area is just off
the southern British Columbia coastline drifting east. This leaves Oregon in an area of
weak upper level flow.
Surface pressures are also weak. Pressure gradients as of 8:00 am included: Newport to
Salem, 1.6 mb onshore; Salem to Redmond, 0.4 mb offshore; and Portland to Medford, 0.7
mb northerly.
The morning Salem sounding shows a few degrees of warming from the surface to to up above
14,000 feet. This makes for a more stable atmosphere today. The temperature trace
showed inversions from the surface to about 2200 feet, from 2500 feet to 3100 feet and
from 3500 to 4100 feet. These will not completely mix out this afternoon. Maximum
mixing heights will only reach about 2800 feet this afternoon.
With the light winds and low mixing heights the prospects for open field burning this
afternoon are not good.
TODAY'S FORECAST:
Partly sunny during the morning, then hazy afternoon sunshine.
Salem's high temperature today will be near 71.
Relative humidity drops to 50% by 1pm. Minimum relative humidity 39%.
Surface winds: Light northwesterly
Transport winds: Light and variable becoming light northerly.
Maximum mixing height: 2800 feet at around 4pm.
Sunset tonight: 6:34 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 57 68 70 58
Relative Humidity: 64% 45% 41% 62%
Surface Wind Direction: 310 320 320 260
Surface Wind Speed: 1 4 4 2
Transport Wind Direction: 310 340 360 360
Transport Wind Speed: 2 3 5 8
Estimated Mixing Height: 1000 2600 2700 500
Ventilation Index: 2 8 14 4
EXTENDED DISCUSSION:
Thursday will once again see very light transport flow. Models are indicating a westerly
wind by afternoon, but speeds less than 4 mph. The situation will need to be closely
monitored for the possibility of winds exceeding model forecasts and a potential for
burning.
There is still good model consistency on bringing rain back to the region on Friday.
However, each model run has been slightly later with the onset of precipitation. It now
looks like rain will reach the Willamette Valley sometime in the early afternoon. If the
rain holds off just a couple of hours later than that, there is another potential burn
opportunity Friday.
The Friday rain, combined with showers on Saturday and another surge of moisture for
later Sunday and Monday should produce significant weekend rainfall totals and likely
bring an end to the 2012 open field burning season.
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:101012:0833
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