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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT MON SEP 12 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
The heat wave has broken in the Willamette Valley as a moderate marine push brough marine
air into the area yesterday afternoon. Morning temperatures were running 5 to 10 degrees
cooler this morning at valley locations and at higher ridge-tops in the coast range and
in the Cascade foothills temperatures were more than 20 degrees cooler this morning
versus yesterday at the same time.
Air quality has improved considerably in the Valley. After peaking in the 1.2 to 1.8
range most areas yesterday, readings this morning are mostly in the .3 to .4 range.
In addition to the cooler air, clouds have also entered the Valley. Morning satellite
pictures show shallow marine clouds throughout the Willamette Valley with some breaks
especially in Benton and Yamhill counties.
At the Salem airport cloud bases were reported to be 1400 feet. There were no pilot
reports available this morning but the Salem sounding indicates that the top of the cloud
layer is at about 2400'. Clouds should start to break up late morning.
The Salem weather balloon sounding showed some dramatic cooling since yesterday. At 2400
feet the temperature dropped from 84 degrees to 58. There is still an inversion from
about 1500 feet to 4800 feet, but it is not as strong as yesterday.
Winds on the sounding were generally southerly or southwesterly at low levels. There was
still a layer of easterly flow from about 4000 feet to 6000 feet.
Computer models indicate a westerly component to transport winds today. Mixing heights
will struggle to reach 3000 feet but we should see that threashold sometime after 2pm
this afternoon. Maximum mixing heights will be around 3300 feet late this afternoon.
The first day after a marine push often shows strong gradient stacking to the east and
that is the case today. As of 8am gradients included: Newport to Salem, 0.3 mb onshore;
Salem to Redmond, 2.6 mb onshore; and total Newport to Redmond gradient, 2.9 mb onshore.
If the clouds in the Valley break as scheduled then warming temperatures should drop
pressures and even out the pressure gradients by this afternoon. We will monitor.
TODAY'S FORECAST:
Morning clouds, afternoon sunshine.
Salem's high temperature today will be near 82. (Average is 78)
Relative humidity drops to 50% by 2pm. Minimum relative humidity 44%.
Surface winds: S to SW 3-6 mph this morning, becoming SW 5-8 this afternoon
Transport winds: SW 8 mph this morning, becoming W 6-12 this afternoon.
Maximum mixing height: 3300 feet.
Sunset tonight: 7:28 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 68 77 82 72
Relative Humidity: 65% 50% 44% 59%
Surface Wind Direction: 180 220 230 260
Surface Wind Speed: 5 4 5 5
Transport Wind Direction: 210 240 270 290
Transport Wind Speed: 8 6 6 11
Estimated Mixing Height: 2000 2900 3300 900
Ventilation Index: 16 17 20 10
EXTENDED DISCUSSION:
A weak upper ridge remains to the north and a weak upper low meanders over California
tomorrow. Transport winds are forecast to be quite light and there is no obvious burn
opportunity, but close monitoring may allow some burning.
On Wednesday, transport winds are forecast to be northwesterly and again, close
monitoring may reveal a small window of opportunity.
The remains of a weak Pacific cold front should reach the Pacific Northwest sometime on
Thursday which, if timing is right, provide a burn opportunity for the Silverton Hills
area.
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:120911:0839
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