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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE AUG 30 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from 11:00am to 2:00pm with a 50 acre limit.
Propane flaming is allowed from 11:00am to 5:00pm.
WEATHER DISCUSSION:
An upper level trough is deepening over the Pacific Northwest this morning. Satellite
pictures show a band of clouds stretching from NE Washington to near The Dalles, then
southwestward to Newport. Some spotty light rain has fallen as this band progressed
southward through the Valley. Amounts, however, have been limited to just a couple of
hundredths of an inch.
Satellite pictures also show marine clouds covering the entire Willamette Valley again
today. The marine layer is quite deep and the Salem sounding indicates cloud bases
around 3000 feet and tops about 5000 feet. Thus the clouds will once more be quite slow
to burn off.
The morning Salem sounding showed considerable cooling from about 2500 feet to about 5000
feet. At 5500 feet the temperature dropped from 61 degrees yesterday to 41 degrees
today. This has set up a strong inversion aloft between 5500 feet and 6000 feet.
However, below the inversion the atmosphere is not particularly stable and smoke
dispersion conditions should be good.
Winds on the morning sounding were light north-northeasterly to about 4000 feet. Above
that level they backed to northwesterly, then west-northwesterly.
Pressure gradients as of 8am included: Newport to Salem, 0.8 mb onshore; Salem to
Redmond, 3.1 mb onshore and Newport to Redmond, 3.9 mb onshore. Meanwhile Eugene to
Medford showed 2.5 mb northerly.
Computer models show transport level winds remaining generally north-northwesterly
through the day today. This is supported by the morning surface pressure analysis which
shows a well developed thremal through extending from central California into SW Oregon.
At this point it appears that unfavorable gradient stacking and unfavorable transport
wind directions will limit open field burning opportunities for today.
TODAY'S FORECAST:
Salem's high temperature today will be near 73.
Relative humidity drops to 50% by 12pm. Minimum relative humidity 38%.
Surface winds: N 6-9 mph gradually becoming NNW 8-11 this afternoon.
Transport winds: N 5-9, increasing to N 12-18 this afternoon
Maximum mixing height: 5600 feet.
Sunset tonight: 7:52 pm.
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 66 72 72 63
Relative Humidity: 58% 41% 39% 54%
Surface Wind Direction: 350 340 330 300
Surface Wind Speed: 7 8 10 9
Transport Wind Direction: 360 350 340 340
Transport Wind Speed: 6 9 15 18
Estimated Mixing Height: 3000 5500 5500 2500
Ventilation Index: 18 50 82 45
EXTENDED DISCUSSION:
The upper level trough axis moves to the east of the area during the day tomorrow after
one last weak short wave impulse moves through. This will leave the Willamette Valley
under a dry northwesterly flow aloft. This should produce more sunshine and warmer
afternoon temperatures. It should also give a better chance for afternoon sea breezes to
develop and, depending on timing, could produce some burn opportunities. This will need
to be assessed on a day-to-day basis.
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:300811:0834
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