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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED SEP 19 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
Another sunny and warm late summer day in store for the Willamette Valley.
Low level flow turned weakly onshore and temperatures cooled through the Valley last
night. Low clouds are again covering the coast, with some cloudiness working its way
into
the southern Willamette Valley and up the Columbia River and into the Portland area.
The morning ODF surface pressure analysis showed the thermal trough extending from the
central valley of California into SW Oregon, then to the central Oregon Cascades and into
south-central Washington. At 8am this morning the Newport to Salem pressure gradient was
1.8 mb onshore, while Salem to Redmond was 0.5 mb offshore.
While temperatures had cooled on this morning's Salem sounding the lower atmosphere was
still very stable. There were several inversion layers from the surface to about 5400
feet. These will not completely mix out today and the maximum mixing height this
afternoon will be just 3000 feet. Winds on the sounding were light westerly below about
1000 feet but easterly above.
With an influx of marine air, air quality has improved in the Willamette Valley.
Nephelometer readings are mostly near about 0.5 bscat versus 1.0-2.0 readings yesterday
afternoon. There is still smoke aloft, however. Sisters is still getting hit hard by
smoke from the Pole Creek fire and neph readings there were in the upper 80's this
morning.
While wind directions are marginally favorable today, the combination of low mixing
heights, little or no westerly gradient across the Cascades, and high levels of smoke
already in the airshed make today a poor candidate for open field burning.
TODAY'S FORECAST:
Sunny
Salem's high temperature today will be near 82. (Average: 75)
Relative humidity drops to 50% by 1pm. Minimum relative humidity 34%.
Surface winds: NW 5-9 mph.
Transport winds: NW 3-5 mph.
Maximum mixing height:
Sunset tonight: 7:13 pm.
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 66 77 82 71
Relative Humidity: 56% 40% 34% 49%
Surface Wind Direction: 320 320 320 320
Surface Wind Speed: 5 5 8 8
Transport Wind Direction: 330 350 340 300
Transport Wind Speed: 3 5 5 14
Estimated Mixing Height: 1500 2500 3000 1100
Ventilation Index: 4 12 15 15
EXTENDED DISCUSSION:
The string of sunny warm days the Pacific Northwest has been enjoying is coutesy of a
ridge of high pressure aloft that has persisted over the eastern Pacific Ocean. Embedded
in this ridge is an upper level low now centered off the northern California/southern
Oregon coast. This upper low has been getting slightly closer to the coast and is the
reason for the deepening marine clouds that have covered the coast and begun to filter
inland the past two days.
Computer models continue to show the low drifting north through Friday night, then
turning
southeast and moving onshore over southwest Oregon. This would indicate increasing
morning marine clouds but afternoon sunshine the tomorrow and Friday. There will also
could be a chance for a shower as the upper low kicks through. However, there are no
obvious indications of any burn opportunities through the end of the week.
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:190912:0837
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