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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED JUL 18 2012
BURN ADVISORY:
Recommended burn times for agricultural burning are from now until 7:00pm.
Prep burning is allowed from now until 5:00pm with a 50 acre limit.
Propane flaming is allowed from now until 5:00pm.
WEATHER DISCUSSION:
An upper level low along the northern California coast will remain nearly stationary
today, then drift slowly northward tomorrow. Cool air aloft with this system has
destabilized the atmosphere yesterday and the result was scattered showers and
thunderstorms. Today, low level onshore flow has brought marine clouds to most areas
west
of the Cascades and this serves to stabilize the atmosphere counteracting some of the
effects of the cool air aloft. So far Radar shows a few showers over the central coast
range and some showers moving from north-central Oregon towards the northern Cascades.
It
is this second batch of showers that could ride the counter-clockwise flow around the
upper low and eventually make it into the Willamette Valley. Chances are low that any
particular location will see showers today – however, chances are high that there will be
showers somewhere in the valley.
As mentioned, low level flow is onshore and winds up to about 4000 feet were generally
southwesterly on the Salem sounding this morning. Cloud cover should keep temperatures
below average this afternoon with highs reaching the upper 70’s.
While mixing heights are favorable, moisture and a sharp wind shift aloft make for poor
for open field burning conditions.
THIS AFTERNOON'S FORECAST:
Mostly cloudy, a few sunbreaks. Humid. Continued chance for showers or possible
thunderstorms.
Salem's high temperature today will be near 78.
Minimum relative humidity 45%.
Surface winds: Southwest 6-9.
Transport winds: Southwesterly 5-7 but with a shift to east-northeast
near the top of the mixed layer.
Maximum mixing height: 5000 feet.
Sunset tonight: 8:53 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 73 79 79
Relative Humidity: 48% 41% 49%
Surface Wind Direction: 010 020 030
Surface Wind Speed: 7 9 6
Transport Wind Direction: 250 310 310
Transport Wind Speed: 5 5 7
Estimated Mixing Height: 4100 5000 2500
Ventilation Index: 20 25 18
EXTENDED DISCUSSION:
The same general weather conditions are likely to persist through Thursday as the upper
pattern changes very little. Model agreement is good that the closed upper low will open
up as a trough and move towards the northeast early Friday. There may be some enhanced
shower activity overnight late Thursday into Friday morning as the trough moves through,
but then precipitation chances will dwindle quickly later Friday once the trough axis
shifts east of the Cascade crest. Temperatures will vary little from average through the
next 7 days.
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:180712:0141
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