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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED SEP 07 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
* Expect Fire Marshal Burn-Ban Conditions this Afternoon for Temperature and Humidity *
WEATHER DISCUSSION:
A strong ridge in the upper atmosphere continues to cover much of the western US today.
This will produce sunny skies and hot weather for most of the state. Satellite pictures
show some low clouds along the northern Oregon coast and there are some scattered middle
level clouds visible from Salem.
The morning Salem sounding showed considerable warming since yesterday between abuot 2500
feet and 10,000 feet. The warmer air aloft has intensified the low level inversion over
the valley. There is an inversion from the surface to about 1000 feet, then another from
about 1800 feet to about 3200 feet.
The sounding showed a brisk north-northeasterly wind in the inversion layer. Above 3500
feet the winds slowly veered to southerly with due south winds aloft at 12,000 feet.
Weak impulses in that southerly flow aloft may trigger thunderstorms, mainly over the
Cascades, this evening. It is not impossible that one or two of those could drift out of
the mountains and into the foothills.
Transport level winds should remain northerly or northeasterly throughout the day.
The unfavorable wind direction, together with Fire Marshal conditions this afternoon will
preclude open field burning today.
TODAY'S FORECAST:
Hazy sunshine and hot weather.
Salem's high temperature today will be near 96.
Relative humidity drops to 50% by 8am. Minimum relative humidity 18%.
Surface winds: N 5-8 mph.
Transport winds: N 5-8 mph, increasing late afternoon to 8-15 mph.
Maximum mixing height: 3300 feet.
Sunset tonight: 7:37 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 83 93 96 86
Relative Humidity: 33% 22% 18% 29%
Surface Wind Direction: 010 360 360 270
Surface Wind Speed: 6 7 7 5
Transport Wind Direction: 360 010 010 020
Transport Wind Speed: 5 6 8 16
Estimated Mixing Height: 2000 2800 3300 2200
Ventilation Index: 10 17 26 35
EXTENDED DISCUSSION:
A weak upper level disturbance now located off the northern California coast will slowly
undercut the upper level ridge during the next couple of days. This will alter the wind
flow aloft over the region bringing northeasterly winds to the upper atmosphere over
Oregon and Washington beginning Friday. This will only serve to enhance the dry weather
pattern and cut off any chance for mountain thunderstorms.
Dry weather and unfavorable transport winds are likely into the weekend.
Longer range computer models are indicating a change to a more favorable westerly
transport wind pattern by Monday.
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:070911:0843
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