[willamette-fcst] Silverton Hills Midday Forecast.doc

Fri Oct 12 11:56:22 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT FRI OCT 12, 2012

...Final Forecast for the 2012 Season...

BURN ADVISORY:

Recommended burn times for agricultural burning are now until 4:30 p.m.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

The upper-level ridge that has been over the Pacific Northwest since July has flattened in response to a strengthening westerly jet stream.  The first in a series of progressively stronger weather systems pushed areas of light rain onto the northern and central coast this morning, with rain progressing inland across most of the Willamette Valley by midday. Hillsboro and McMinnville have both recorded close to one-quarter of an inch, with much lighter totals in Corvallis and Salem.

Late-morning satellite imagery showed clouds covering virtually all of Washington and western Oregon.  Radar showed bands of light rain extending southward to about Eugene with some light rain pushing eastward into the northern Cascades.  Bands of light rain should continue to rotate onshore today and tonight, with another tenth to one-quarter of an inch of rain possible. Light southerly winds have helped to improve air quality across the Willamette Valley.

TODAY'S FORECAST:

Light rain at times and much cooler.

Salem's high temperature today will be near 56 degrees (normal is 66).

Relative humidity:  Remaining above 75% all day.

Surface winds:  S 5-15 mph.

Transport winds: SW 15 mph; becoming SW 20-25 mph this afternoon.

Mixing height: Rising to near 3500 feet this afternoon.

Salem's sunset tonight: 6:31 p.m.

THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 55      56      53

     Relative Humidity:           77%     80%     89%

     Surface Wind Dir/Speed:     S 10    S 10     S 7

     Transport Wind Dir/Speed:  SW 20   SW 22   SW 25

     Mixing Height:              3000    3500    3000

     Ventilation Index:           60      77      75

EXTENDED DISCUSSION:

The rain should taper off of Saturday and the first half of Sunday, as the jet stream pushes north to over Washington.  Valley temperatures should warm back into the 60s.  Sunday will likely be the last chance to burn any remaining fields, if they can dry out enough. The jet stream will drop south again late Sunday, with a couple of potent storms forecast to bring soaking rains to western Oregon Sunday night and Monday.  That should effectively end field-burning for the season.

The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text

Notes:

     1.  Mixing height, as used here, is the lowest height at which the

         potential temperature exceeds the 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.

Pete Parsons

ODF Meteorologist
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