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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT MON AUG 27 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is allowed from 11:00am to 5:00pm.
WEATHER DISCUSSION:
Cloud cover has been persistent this morning, holding temperatures down and humidities up
a bit from where we thought they would be in the earlier forecast. However, satellite
pictures show plenty of breaks in the clouds and it still looks like temperatures should
reach around 77 degrees today.
Latest trajectory models show smoke from the Silverton hills would move northeastward
this afternoon.
A problem may be gradient stacking and the subsiding air/down mixing that it implies.
Weak gradient stacking east has developed during the morning hours. Pressure gradients
as of 11am showed Newport to Salem, 0.4 mb onshore and Salem to Redmond, 1.7 mb onshore.
However, water-vapor satellite pictures do not show a dry-slot which indicates
broad-scale subsidence has not developed as yet.
We will monitor gradients through the afternoon. Additional sunshine in the valley
should help improve the gradient stacking.
THIS AFTERNOON'S FORECAST:
Turning partly sunny.
Salem's high temperature today will be near 77.
Relative humidity drops to 50% by 2pm and reaches 36% by late afternoon.
Surface winds: Becoming SW to W 5-7 mph in the afternoon.
Transport winds: SW to W 8-10 mph.
Mixing height peaks around 6000 feet today.
Sunset tonight: 7:56 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 71 75 69
Relative Humidity: 51% 40% 51%
Surface Wind Direction: 280 290 290
Surface Wind Speed: 6 7 7
Transport Wind Direction: 250 270 270
Transport Wind Speed: 9 8 7
Estimated Mixing Height: 5000 6000 1000
Ventilation Index: 45 48 7
EXTENDED DISCUSSION:
An offshore upper low will continue over the northeast Pacific Ocean into early Tuesday
with little push to bring any precipitation onshore. Models indicate that a short wave
will swing around the base of the low and cause it to swing onshore and lift out later
Tuesday and into Wednesday. It looks like Tuessday night and Wednesday morning will be
the main time frame for a better chance of showers.
Models indicate troughiness remaining over the eastern Pacific and Pacific northwest
through the week. An impulse swings through the base of this trough late Thursday for
another slight chance of showers. Beginning Friday and through the weekend temperatures
warm and dry weather is likely.
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:270812:1143
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