[willamette-fcst] Thu, 02 Sep 10

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT THU SEP 02 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 10:00am to 4:00pm.
     Prep burning is allowed from 2:00pm to 5:00pm with a 50 acre limit.
     Propane flaming is allowed from 2:00pm to 5:00pm.


WEATHER DISCUSSION:

     Temperatures are on track to reach the mid 80's this afternoon.

     The late morning ODF surface analysis shows that a well developed thermal trough
     remains from California's central Valley to near Cape Blanco, then right along
     the Oregon Coast to about Astoria. The thermal trough will shift inland later
     today and be over the Willamette Valley by late afternoon.

     Transport winds will be generally northerly or north-northeasterly this
     afternoon. This is an unfavorable wind direction for open field burning.

     Upper air charts show a ridge from Nevada to northwest Canada and a trough in
     the Gulf of Alaska. Both of these features are moving steadily eastward today
     with the trough axis forecast to be about 400 miles west of Vancouver Island by
     this evening.

     For the mid Willamette Valley today, any cloudiness still left will dissipate
     for nearly full sunshine this afternoon. The sun, and the dry north-northeast
     winds should help dry fields from recent rains. Salem's high temperature today
     will be near 87.  Minimum relative humidity this afternoon will dip to 27%.

     Sunset tonight: 7:41 pm


THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 68       81      87      80
     Relative Humidity:           59%      35%     27%     36%
     Surface Wind Direction:     020      020     020     020
     Surface Wind Speed:           8       12      12       5
     Transport Wind Direction:   020      040     030     360
     Transport Wind Speed:         8        9      11       9
     Estimated Mixing Height:   2000     2900    3400    5000
     Ventilation Index:           16       26      37      45

EXTENDED DISCUSSION:

     The trough in the Gulf of Alaska will help kick the thermal trough east of the
     Cascades on Friday opening up the door for northwesterly transport flow over the
     region. Timing is the key, but at this point it looks like northwesterly winds will
     develop by early to mid-afternoon and this could develop into a good burn
     opportunity. Models indicate mixing heights reaching 3000 feet by mid-afternoon
     as well.

     An active weather pattern remains on tap for the Saturday and into the first
     part of next week. A relative quick moving upper level trough will swing down
     from the Gulf of Alaska and over the Pacific Northwest beginning late Saturday
     and move east of the Rockies by early Monday. This will keep cooler temperatures
     across the region for the Labor Day weekend with periodic showers. Models
     indicate only a short break before another trough swings down from Alaska early
     Tuesday.

EXTENDED FORECAST:

     Friday: Sunny, with a high near 87. Wind becoming west 7-10 mph.
     Saturday: Partly sunny, but a chance of light showers. 77. SSW 5-8 becoming NNW.
     Sunday: Partly sunny, but a chance of light showers. 75.
     Monday: Partly sunny, with a high near 75.
     Tuesday: Mostly cloudy periods of rain, 74.
     Wednesday: Partly sunny, with a high near 73.

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.







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