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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE SEP 11 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
After a brief downturn in the weather the past couple of days, conditions will return to
more typical of last summer today.
At the surface high pressure has been building overnight in western Washington and NW
Oregon, while the thermal trough continues over northern California extending northward
into SW Washington. The result is a strong northerly pressure gradietn. The Portland to
Medford pressure difference was 5.3 mb as of 8am this morning. This is giving northerly
winds to the area.
The morning Salem sounding showed north or northeast winds, with speeds around 15-20 mph,
from just above the surface up to about 8000 feet.
As the thermal builds northward off the Oregon coast today and tomorrow transport level
flow will veer to more northeasterly to easterly later today and tomorrow.
The Salem sounding also showed considerable warming from about 5000 feet to 8000 feet for
a more stable atmoshere today. Mixing heights will reach 3000 feet by noon and top out
at around 4800 feet.
While mixing heights will not be a problem today, the unfavorable wind direction and
strong wind speeds will likely preclude any open field burning opportunities today.
TODAY'S FORECAST:
Sunny but breezy. Temperatures near average for mid September.
Salem's high temperature today will be near 75.
Relative humidity drops to 50% by 12pm. Minimum relative humidity 26%.
Surface winds: Northerly 7-12 mph with higher gusts.
Transport winds: North to northeast 15-25.
Maximum mixing height: 4800 feet.
Sunset tonight: 7:28 pm.
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 60 70 75 67
Relative Humidity: 55% 35% 26% 39%
Surface Wind Direction: 360 360 010 360
Surface Wind Speed: 7 11 12 8
Transport Wind Direction: 020 030 030 020
Transport Wind Speed: 17 19 22 25
Estimated Mixing Height: 2900 4000 4800 2000
Ventilation Index: 49 76 106 50
EXTENDED DISCUSSION:
A flat upper ridge progresses west to east over the region through Thursday. At the
surface the thermal trough builds northward off the coast for offshore flow. The thermal
trough shifts inland and eventually east of the Cascades and low level flow switches to
onshore.
The big question is timing of this sequence. Models are indicating that the shift to
onshore flow will occur sometime later Thursday. If this shift occurs soon enough there
could be a burn opportunity Thursday afternoon. If the shift occurs overnight Thursday
night Friday would be the first day after a marine push and likely would not be a good
burn opportunity. If the switch-over to an onshore pattern occurs during the day on
Friday there could be an opportunity during the day on Friday.
The main message is this: There may be an opportunity Thursday or Friday and everyone
should be ready to jump on it if it develops.
The weekend outlook is for onshore flow, but no dramatic opportunities are evident at
this point.
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:110912:0845
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