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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE OCT 02 2012
BURN ADVISORY:
Agricultural burning burning is allowed.
Recommended burn times 12:00pm to 5:00pm north of Woodburn. Woodburn south
8:00am to 5:00pm.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
In the upper atmosphere the broad ridging covering the Gulf of Alaska and the western
portion of North America is being suppressed by a short wave moving through British
Columbia. This, in turn, supports a dry cold front that moved through locally overnight.
This brought some spotty clouds to the region, but also cooled temperatures considerably.
Today's high temperatures will be down about 10 degrees from yesterday.
The cooling showed up nicely on this morning's Salem sounding. At 4000 feet yesterday's
temperature trace showed 63 degrees while today's temperature at that elevation had
dropped to 52. With the cooler air aloft the atmosphere is less stable and mixing heights
will reach about 3500 feet this afternoon - versus only 2500 feet yesterday.
The sounding also measured brisk northerly winds up to about 4500 feet. Above that winds
backed to west-northwesterly.
Computer models are showing northerly transport winds all day today. The lone exception
is the less detailed GFS model which shows winds coming around to about 340 degrees late
this afternoon. The GFS has had a consistent bias in this regard but has consistently
failed to verify as well. The NAM based smoke trajectory model takes smoke from the
Silverton hills south and slightly east directly into the heart of the Willamette Valley.
The morning surface pressure analysis shows the thermal trough over SW Oregon with high
pressure building into western Washington.
Pressure gradients as of 8am included: Newport to Salem, 1.9 mb onshore; Salem to
Redmond, 2.9 mb onshore. Thus there is gradient stacking to the east which will likely
not ease this afternoon. Portland to Medford was 3.6 mb northerly.
TODAY'S FORECAST:
Mostly sunny, breezy and cooler.
Salem's high temperature today will be near 72.
Relative humidity drops to 50% by 1pm. Minimum relative humidity 33%.
Surface winds: Northerly 6-12 mph. Higher gusts.
Transport winds: Northerly 10-20 mph.
Maximum mixing height: 3500 feet.
Sunset tonight: 6:49 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 63 70 72 58
Relative Humidity: 56% 41% 33% 51%
Surface Wind Direction: 350 360 020 330
Surface Wind Speed: 9 9 11 4
Transport Wind Direction: 020 020 010 020
Transport Wind Speed: 11 15 14 18
Estimated Mixing Height: 1900 3200 3500 1700
Ventilation Index: 21 48 49 31
EXTENDED DISCUSSION:
All computer models (including the GFS) are showing a strong east wind event coming up for
the latter portion of the week. Easterly winds will slacken but continue over the weekend
as well. This, of course, is not a favorable wind direction for open field burning.
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:021012:0847
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