[willamette-fcst] Silverton Hills Forecast - Wed, Aug 18 2010
Willamette Valley Field Burning Forecast
willamette-fcst at listsmart.osl.state.or.us
Wed Aug 18 11:42:07 PDT 2010
SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED AUG 18 2010
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from 2:00pm to 5:00pm with a 50 acre limit.
Propane flaming is allowed from 2:00pm to 5:00pm.
WEATHER DISCUSSION:
Yesterday's impressive marine push has cooled temperatures considerably in the
Willamette Valley. Temperatures as of 11am were running 16 to 24 degrees cooler
than yesterday at the same time.
Clouds still cover most of the Valley, but satellite pictures show some breaks
and we will see more sunshine this afternoon.
The morning Salem sounding showed cooler temperatures at all levels from the
surface to above 14,000 feet. There is a significant stable layer from about
1800 feet to about 6200 feet. Much of that stable layer will be eroded from the
bottom by daytime heating, but subsiding air aloft will hold smoke from
evacuating over the Cascades.
There is considerable gradient stacking to the east today with the Newport to
Salem pressure difference 0.9mb onshore and the Salem to Redmond gradient has
risin to 3.2mb onshore as of 11am. The gradient stacking is associated with the
subsidence mentioned above. The gradient stacking issue should continue all day.
That continued gradient stacking associated with the downward vertical air
movement (or subsidence) over the Cascades will preclude open field burning
today.
Expect slow clearing during the early afternoon, with mostly sunny skies by
later this afternoon. Salem's high temperature today will be near 79. Relative
humidity drops to 50% by noon. Minimum relative humidity 38%. Winds will be
westerly generally 5-8 mph. Sunset tonight: 8:08 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 71 78 71
Relative Humidity: 53% 40% 53%
Surface Wind Direction: 290 300 280
Surface Wind Speed: 5 6 7
Transport Wind Direction: 280 300 280
Transport Wind Speed: 5 6 5
Estimated Mixing Height: 2900 3900 800
Ventilation Index: 14 23 4
EXTENDED DISCUSSION:
Longer range upper air forecast charts show a generally west to east flow across
the eastern Pacific, gradually sagging off the coast for an upper level trough
that will not move through until after the weekend. This is a pattern that will
produce late night and morning marine clouds in the Valley with afternoon sun
and temperatures generally around average for this time of year. Westerly winds
tomorrow could produce a burn opportunity if the gradient stacking and
subsidence eases.
EXTENDED FORECAST:
Thursday: Mostly sunny, with a high near 79. Wind WNW 3-8 mph.
Friday: Sunny, with a high near 79. NNW wind around 7 mph.
Saturday: Partly sunny, with a high near 74.
Sunday: Partly sunny, with a high near 74.
Monday: Mostly sunny, with a high near 80.
Tuesday: Mostly sunny, with a high near 83.
Notes:
1. Mixing height, as used here, is the lowest height at which the
potential temperature exceeds the 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.
jlwx:180810:1142
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