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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM 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.
The morning Salem sounding showed a few degrees of warming from the surface to to up
above 14,000 feet. This makes for a more stable atmosphere today. There were a number of
inversions from the surface to above 4000 feet. These will not completely mix out this
afternoon and maximum mixing heights will only reach about 2800 feet this afternoon.
Pressure gradients remain fairly flat. While the Newport to Salem pressure difference was
2.4 mb onshore the Salem to Redmond gradient was just 0.6 mb onshore. Portland to Medford
was showing 1.2 mb northerly.
With the weak pressure difference across the Cascades and low mixing heights the prospects
for open field burning do not look good for this afternoon. Pibals this afternoon will
monitor winds and detect any potential opportunity.
THIS AFTERNOON'S FORECAST:
Hazy 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:
2pm 5pm 8pm
Temperature: 68 70 57
Relative Humidity: 45% 39% 64%
Surface Wind Direction: 350 350 250
Surface Wind Speed: 3 5 2
Transport Wind Direction: 340 360 360
Transport Wind Speed: 3 5 8
Estimated Mixing Height: 2600 2700 500
Ventilation Index: 8 14 4
EXTENDED DISCUSSION:
Thursday will once again see very light transport flow. Latest models are indicating a
west-northwesterly wind by afternoon, but with low wind speeds. 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. The
latest run was no exception. Onset of precipitation looks to be sometime mid afternoon
versus early afternoon on the previous run. 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:101012:1134
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