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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT MON OCT 08 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
The persistent blocking pattern in the upper atmosphere that has been responsible for the
prolonged dry spell is finally showing signs of breaking down. Upper level forecast
charts show a weak low aloft slowly moving eastward toward California while an upper
level high pressure area in the Gulf of Alaska weakens and shifts west. Low level flow
has become weakly onshore and low ocean clouds have surged northward along the coast.
The marine layer is shallow and so while coastal valleys will see morning clouds no
clouds have made it to the Willamette Valley. Thus skies should be mostly sunny. While
temperatures will be down a notch from yesterday, afternoon highs should still be a few
degrees above average for this time of year.
The morning Salem sounding showed a surface based inversion to about 2500 feet. This
should mix out late this afternoon for a maximum mixing height near 3300 feet. Winds on
the sounding were light but were mostly southwest through northwest up to about 10,000
feet.
As of 8:00am this mornign there was still a negative, or offshore, gradient between Salem
and Redmond, but this should go onshore this afternoon.
Forecast models indicate a light westerly component to transport winds today. The
ventilation index will low as a result, but with a favorable wind direction for the first
time in many days it may be possible for some limited open field burning this afternoon.
TODAY'S FORECAST:
Sunny
Salem's high temperature today will be near 74.
Relative humidity drops to 50% by 11am. Minimum relative humidity 29%.
Surface winds: Light and variable becoming light SW to W by afternoon.
Transport winds: Light southerly becoming SW to NW 5-7 late afternoon.
Maximum mixing height: 3300 feet.
Sunset tonight: 6:38 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 59 70 74 59
Relative Humidity: 47% 32% 29% 53%
Surface Wind Direction: 180 260 200 210
Surface Wind Speed: 3 4 5 2
Transport Wind Direction: 195 210 310 310
Transport Wind Speed: 4 4 5 8
Estimated Mixing Height: 900 2200 3300 800
Ventilation Index: 4 9 16 6
EXTENDED DISCUSSION:
The surface pressure pattern suggest more northerly low level winds tomorrow for
unfavorable burning conditions, then back to a light westerly flow for Wednesday.
Westerly winds increase a bit for Thursday ahead of a Pacific cold front - the first of
the fall season - on Friday. The Friday front should bring some wetting rains as well as
open the door to additional Pacific weather systems that will finally bring significant
rains back to the Pacific Northwest.
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:081012:0821
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