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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT MON AUG 29 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from now until 2:00pm.
Propane flaming is allowed from now until 5:00pm.
WEATHER DISCUSSION:
Upper air charts show a ridge extending from Texas/New Mexico through Alberta, Canada and
a weak trough over SE Alaska. A small disturbance that brought thunder and lightning to
central Oregon is moving out of NE Oregon.
The Alaska trough will deepen and stretch southward today and be over the region through
mid week.
Low level flow is onshore and satellite pictures still show a nearly solid marine cloud
layer over the Willamette Valley. A pilot report from over Salem around 10am this
morning
indicated the base of the cloud deck was at 1300 feet and the tops were at 3800 feet. A
later pirep from near Corvallis showed a thinner layer of clouds. In any case the clouds
are thick enough that any break up would not be until late this afternoon, if at all.
Computer models show that the southwesterly low level winds shown on this morning's Salem
sounding will veer to more northerly this afternoon.
Pressure gradients are not favorable for burning with gradient stacking to the east. As
of 11am Newport to Salem was 0.6 mb onshore; Salem to Redmond, 3.6 mb onshore; and the
total Newport to Redmond gradient was 4.2 mb onshore.
THIS AFTERNOON'S FORECAST:
Cloudy, just partial afternoon clearing.
Salem's high temperature today will be near 75.
Relative humidity drops to 50% by 1pm. Minimum relative humidity 43%.
Surface winds: W-WNW 5-7 mph.
Transport winds: NW 5-9 increasing to NW 9-15 late afternoon.
Maximum mixing height: 3300 feet.
Sunset tonight: 7:55 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 70 75 68
Relative Humidity: 57% 46% 61%
Surface Wind Direction: 260 290 290
Surface Wind Speed: 5 6 6
Transport Wind Direction: 320 330 330
Transport Wind Speed: 5 9 15
Estimated Mixing Height: 3000 3300 1200
Ventilation Index: 15 30 18
EXTENDED DISCUSSION:
As mentioned, the trough now over southeast Alaska will elongate and settle in over the
Pacific Northwest and remain through mid week. This will set up a generally northerly
transport wind pattern and any burn opportunities would be associated with afternoon sea
breezes and would need to be assessed on a day-to-day basis.
At this point no rain is expected through the week, although there is a slight chance of
drizzle from morning clouds.
As the trough moves east, upper level winds turn more northerly, reinforcing the
northerly pattern at transport wind levels. Thus the outlook for later in the week is
not optimistic 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:290811:1153
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