[willamette-fcst] Silverton Hills Forecast - Tue, Aug 17 2010
Willamette Valley Field Burning Forecast
willamette-fcst at listsmart.osl.state.or.us
Tue Aug 17 08:39:35 PDT 2010
SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE AUG 17 2010
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
One more hot day before a marine push kicks in overnight.
Lots going on weather-wise this morning as we get set to transition out of the
hot weather that has affected the Willamette Valley the past several days.
In the upper atmosphere the ridge of high pressure that had been over the area
is being undercut by a minor disturbance moving down the BC coast. At the same
time an upper level low pressure area is evident off the northern California/SW
Oregon coastline. Our lightning detection network indicates that this low is
already producing thunderstorms offshore.
The morning ODF surface analysis showes a well developed thermal trough from the
central Valley of California, into SW Oregon and then into the southern
Willamette Valley. From there the axis of the trough extends up the east side of
the valley then bends over to near Hermiston and finally extends northward
into eastern Washington. Pressure gradients as of 8am included: Newport to
Salem, 2.5 mb onshore; and Redmond to Salem, 1.5 mb offshore.
The morning Salem sounding showed a strong surface based inversion up to about
about 1000 feet above ground level. In addition there were two inversions aloft,
one from 2000 feet to 2800 feet, and another from 5000 feet to about 6200 feet.
The lower inversions should mix out late this afternoon, but the higher inversion
will likely still be there this afternoon.
As the thermal trough shifts east we should get a favorable west to east
pressure pattern for burning. Models indicate a marine push later today. Various
models put the onset of west winds anytime from about 4pm to about 8pm tonight.
In addition, forecast model soundings indicate that the push will initially be
quite shallow with the low level west winds not deepening to above about 2000
feet until after sunset tonight. Winds above this shear level will be northerly.
Complicating all of this is the upper low off the southwest coast. As it moves
closer, some convection is likely over SW Oregon and the southern Cascades. This
will have to be monitored closely since thunderstorms moving farther
north...either into the Willamette Valley or over the central Cascades...will
have an effect on winds and pressure patterns.
Bottom line: it is a complex situation developing, but there may be a short
window of opportunity late this afternoon/early this evening for some open field
burning. We will keep track of all of this and monitor the winds with pibals
Salem's high temperature today will be near 93. Relative humidity drops to 50%
by 10am. Minimum relative humidity 27%.
Sunset tonight: 8:09 pm
11am 2pm 5pm 8pm
Temperature: 80 91 93 77
Relative Humidity: 45% 31% 27% 45%
Surface Wind Direction: 280 310 270 260
Surface Wind Speed: 3 5 6 6
Transport Wind Direction: 030 360 320 280
Transport Wind Speed: 5 5 8 11
Estimated Mixing Height: 1500 2500 4000 100
Ventilation Index: 8 12 32 1
Temperatures will be much cooler during the coming days. Longer range computer
models show troughiness over the eastern Pacific Ocean and a cooler
southwesterly flow aloft. Tomorrow will be the first day after a marine push
which is generally not a good day to burn. However, westerly winds continue on
Thursday and if mixing heights cooperate there may be a burn opportunity then.
Later this week and over the weekend we should be generally in a morning cloud,
afternoon sunshine pattern.
Wednesday: Mostly sunny, with a high near 80. Afternoon winds NW 4-7.
Thursday: Sunny, with a high near 79. Wind WNW 4-8.
Friday: Sunny, with a high near 76.
Saturday: Partly sunny, with a high near 74.
Sunday: Partly sunny, with a high near 74.
Monday: Mostly sunny, with a high near 80.
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
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