[willamette-fcst] Thu, 02 September 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM 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 should top out 10-12 degrees warmer today than yesterday.

     The morning Salem sounding showed the warming nicely. It was warmer
     than yesterday morning at all elevations above about 1700 feet. At 5000 feet the
     temperature had jumped from 46 degrees yesterday to 62F this morning. Two
     distinct inversions showed up on the sounding, one from 1300 to 1800 feet and
     another from 2500 to 3400 feet. Models show additional warming aloft for today.
     None the less, with mostly sunny skies today surface heating should push surface
     temperatures into the upper 80's which will help push mixing heights to
     above 3000 feet this afternoon.

     The morning ODF surface analysis shows a well developed thermal trough extending
     from California's central Valley to near Cape Blanco, then right along the
     Oregon Coast to about Astoria. The thermal trough will shift inland today and be
     over the Willamette Valley by late afternoon. Weak surface high pressure is
     located just west of northwest Washington. With higher pressure to the north and
     lower pressure with the thermal trough moving into the Willamette Valley
     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 California 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, expect any clouds this morning to quickly
     dissipate and 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. Relative humidity drops to 50%
     by 11am. Minimum relative humidity 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 westerly transport flow over the
     region. Timing is the key, but at this point it looks like westerly winds will
     develop by early afternoon and this could develop into a good burn opportunity.

     An active weather pattern is 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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