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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT MON SEP 17 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
An upper level ridge is building today over the Pacific Northwest. Low level flow
is offshore.
The ODF surface pressure analysis shows the thermal trough extending
north-northwestward from the central valley of California into SW Oregon. The trough
then extends northward along the coastline all the way up into Washington.
As of 11am there was a slight onshore gradient between Newport and Salem (0.4mb) but a
3.9mb offshore or easterly gradient from Redmond to Salem.
The morning Salem sounding showed a strong inversion from about 1300 to 2100 feet
that won't mix out until around 3pm. Maximum mixing heights today will reach
around 3400 feet.
The Salem sounding also showed light north through east winds at all levels up to 12,000
feet.
Latest rajectory models continue to show smoke movement today from any field burning in
the Silverton Hills would be to the southwest, an unfavorable direction.
WEATHER DISCUSSION:
THIS AFTERNOON'S FORECAST:
Salem's high temperature today will be near 91.
Relative humidity drops to 50% by 11am. Minimum relative humidity 18%.
Surface winds:
Transport winds:
Maximum mixing height:
Sunset tonight: 7:18 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 84 89 78
Relative Humidity: 28% 19% 31%
Surface Wind Direction: 340 010 300
Surface Wind Speed: 7 7 5
Transport Wind Direction: 030 030 030
Transport Wind Speed: 7 7 7
Estimated Mixing Height: 2900 3400 1200
Ventilation Index: 20 24 8
EXTENDED DISCUSSION:
Models keep a ridge aloft over western North America through at least Sunday. The thermal
trough will be near the coast or over the Willamette Valley today and tomorrow,
then shift it into the Cascades. Latest models show the trough into centeral Oregon on
Wednesday, which under ordinary circumstances might indicate a burn opportuntiy. With
wildfire smoke being generated by the Pole Creek fire, however, it would be in-advisable
to risk burning unless mixing heights were extremely high and/or a trailing wet cold
front was due in to cleanse the air. Neither of those conditions is likely.
An upper level low, now off the California coast, complicates the weather picture.
Current models move this low northward and slowly weaken it. This will thicken the
marine layer and slowly cool the air mass. Thus today looks like the warmest day of the
week, but temperatures will remain significantly above average into next weekend.
The remains of the upper low should kick eastward sometime over the weekend, likely about
Sunday. Whether or not this will produce any kind of burn opportunity depends on just
how the pattern evolves. Until then, the prospects of open field burning remain very
low.
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:170912:1150
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