[willamette-fcst] Mon, Aug 23d 2010
Willamette Valley Field Burning Forecast
willamette-fcst at listsmart.osl.state.or.us
Mon Aug 23 11:56:53 PDT 2010
SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT MON AUG 23 2010
BURN ADVISORY:
Recommended burn times for agricultural burning are from Noon to 6:00pm.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
Upper air charts show a cool trough to the east of the state and moving east
away from the region. An upper ridge of high pressure is building off the coast.
The late morning ODF surface analysis shows a thermal trough from central
California extending northwestward to just off the SW Oregon coast. Weak surface
high pressure covers the Puget Sound region.
As of 11am there was a .4mb offshore gradient Redmond to Newport and the
Portland to Medford gradient had increased to 5.3mb.
Satellite pictures show clear skies over the entire state. The morning Salem
sounding showed the atmosphere had warmed considerable above about 4000 feet
since yesterday, and models are showing additional warming throughout the day
today. This will stabilize the atmosphere and suppress mixing heights. The
maximum mixing height today in the mid-Willamette Valley will likely not reach
3000 feet.
The morning sounding showed northeasterly transport winds and computer models
continue to show that pattern through the afternoon and evening. Late morning
temperatures were running fairly close to where they were yesterday at the same
time, but high temperatures today should be 6-8 degrees warmer than yesterday.
With the low mixing heights and northeasterly transport winds today, there will
likely be no open field burning this afternoon. Salem's high temperature today
will be near 83. Minimum relative humidity today: 24%. Sunset tonight: 7:59 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 74 81 75
Relative Humidity: 36% 25% 33%
Surface Wind Direction: 360 010 020
Surface Wind Speed: 10 13 7
Transport Wind Direction: 010 010 010
Transport Wind Speed: 11 12 5
Estimated Mixing Height: 2700 2800 500
Ventilation Index: 30 34 2
EXTENDED DISCUSSION:
The upper level ridge off the coast this morning will strengthen and move over
the Oregon and Washington tomorrow for hot weather and gusty, dry offshore
winds. Temperatures will be around 10-12 degrees warmer than today. Burning is
not likely Tuesday.
The next opportunity for open field burning may be Wednesday as the upper ridge
axis moves east and a marine push ends the minor warm spell. Timing is the key.
Both the GFS and the RUC models show this as a late afternoon event which would
allow for some burning. However this event is far enough in the future that we
have little confidence in the exact timing of the onset of westerly, burnable
winds.
Thursday will be the first day after a marine push, which usually means low
mixing heights and gradient stacking (subsidence aloft) to the east and thus not
likely a good burn day. Friday will be partly sunny and the GFS model shows an
unfavorable northerly transport flow over the region.
EXTENDED FORECAST:
Tuesday: Sunny, with a high near 94. Wind ENE 5-15, gusts to 20.
Wednesday: Sunny, with a high near 91. E 10mph becoming NNW, possibly WNW late.
Thursday: Partly sunny but chance of sprilkles. Hihg 78.
(First day after a marine push)
Friday: Partly sunny, 72.
Saturday: Mostly cloudy, 72.
Sunday: Mostly cloudy, chance of showers. High 68.
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.
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