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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE SEP 13 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from 12:00pm to 2:00pm with a 50 acre limit.
Propane flaming is not allowed.
WEATHER DISCUSSION:
Upper air charts this morning show a weak ridge extending from Washington up through
British Columbia and into Canada's Yukon and Northwest Territories. At the same time a
weak upper low is located over southern California. Oregon is in an area of weak and
variable flow aloft between these two systems.
At the surface a thermal trough extends from central California into SW Oregon and a low
is centered in eastern Washington with a trough extending south-southwest into central
Oregon.
Satellite pictures show low marine clouds have again filled in in the Willamette Valley.
The base of this cloud deck is at about 1100 feet above ground, while the tops of the
layer are at about 2300 feet. There has been some light drizzle from these clouds in the
northern Willamette Valley early this morning. Similarly to yesterday, this cloud layer
should begin to break up around noontime.
The morning Salem sounding showed a thermal profile nearly identical to yesterday. An
inversion extends from 1500 feet to 4900 feet. This will not mix out today and cap
mixing heights at just under 3000 feet this afternoon.
The sounding showed some variability to low layer winds but with a tendency toward
southwesterly. The winds became a definite westerly above 4000 feet.
Pressure gradients as of 8am included: Newport to Salem, 0.3 mb onshore; Salem to
Redmond, 2.3 mb onshore; Newport to Redmond, 2.6 mb onshore.
Computer models suggest transport winds will become light northerly during the day today,
thus unfavorable for open field burning.
TODAY'S FORECAST:
Morning clouds, then mostly sunny but hazy.
Salem's high temperature today will be near 78.
Relative humidity drops to 50% by 2pm. Minimum relative humidity 45%.
Surface winds: N 4-6 mph.
Transport winds: Light and Variable, becoming northerly 4-7 mph.
Maximum mixing height: 2900 feet.
Sunset tonight: 7:26 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 65 74 78 69
Relative Humidity: 70% 51% 45% 59%
Surface Wind Direction: 360 360 350 300
Surface Wind Speed: 4 5 6 5
Transport Wind Direction: Var 010 350 340
Transport Wind Speed: 2 4 7 9
Estimated Mixing Height: 1900 2800 2800 500
Ventilation Index: 4 11 20 4
EXTENDED DISCUSSION:
Computer models show the upper ridge to the north drifting eastward and flattening while
the upper low to the south drifts east into Arizona. This allows a more westerly flow
aloft to develop over the Pacific Northwest. At lower levels, in the smoke transport
region, wind flow will be north-northwesterly. There is a slight possibility that an
afternoon sea breeze could develop for a burn opportunity, but this will have to be
monitored closely.
Thursday, the remains of a weak front will move across the region. It appears at this
time that this will not be accompanied by rain. If the timing is right this could be a
burn opportunity, but models indicate that this may come through too early in the day.
Again, this will be monitored closely.
Longer range models show a more significant Pacific cold front moving in early Saturday.
This could produce a burn opportunity ahead of the front on Friday. Expect some light
rain during the day on Saturday.
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:130911:0846
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