[willamette-fcst] Tuesday, Aug 245h, 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT TUE AUG 24 2010

BURN ADVISORY:

     **** Fire Marshal Burn Ban Conditions Likely This Afternoon ****

     Recommended burn times for agricultural burning are from 1:00pm to 4:00pm.
     Prep burning is not allowed.
     Propane flaming is not allowed.

WEATHER DISCUSSION:

     In the upper atmosphere a ridge of high pressure covers the Pacific Northwest.
     The axis of the ridge runs from NW Alberta, through northeast Oregon to near Las
     Vegas. At the surface, our late morning ODF surface analysis shows a thermal
     trough from the central valley of California into NW California, then just along
     the Oregon and Washington coast. Winds at Newport, for example, have lost their
     easterly component and are now northerly, indicating that the thermal trough
     axis is likely directly over that location.

     The morning Salem sounding showed dramatic warming at all levels. At about 3500
     feet the temperature jumped from 49F yesterday morning to 71F today. Some
     additional warming will occur this afternoon. The sounding showed a strong,
     subsidence inversion from about 1300 feet to about 2200 feet that will not mix
     out this afternoon. Maximum mixing heights today will likely stay below 2000
     feet.

     The air mass is very dry and relative humidities will drop into the teens later
     today.

     It is likely that Fire Marshal conditions will be met this afternoon.

     We do not expect open field burning this afternoon.

     For the mid-Willamette Valley expect sunny skies and gusty northeast winds.
     Salem's high temperature today will be near 95. Minimum relative humidity this
     afternoon will be near 15%. Sunset tonight: 7:58 pm


THREE-HOURLY DATA:

                                     2pm     5pm     8pm
     Temperature:                     91      97      88
     Relative Humidity:               18%     14%     22%
     Surface Wind Direction:         360     010     340
     Surface Wind Speed:               8      10       7
     Transport Wind Direction:       360     350     340
     Transport Wind Speed:            11      12       5
     Estimated Mixing Height:       1900    1900     500
     Ventilation Index:               21      23       2

EXTENDED DISCUSSION:

     The thermal trough will shift inland tomorrow then east of the Cascades late
     tomorrow. Timing of this shift will determine if there is a burn opportunity.
     Latest computer models continue to suggest that this will be a late afternoon or
     early evening event. In addition, the latest NAM model is only marginal on
     direction showing a mean transport wind of 310 at 7 at 5pm tomorrow.

     Thursday will likely see a deep marine layer covering the valley with the
     possibility of some drizzle or light rain. This will be the first day after a
     marine push - days which generally do not produce good burning conditions.

     A deepening trough, now in the Gulf of Alaska will settle over the area late in
     the week and over the weekend. This should result in onshore flow, along with
     increased cloud cover and much cooler temperatures. Shortwaves will periodically
     swing around the base of the trough, helping to dig the trough further south and
     increase the thickness of the marine layer. Models suggest that the potential
     exists for a few rain showers over the weekend, but the timing on these would
     depend on when the shortwaves pass through.


EXTENDED FORECAST:

     Wednesday: Sunny, with a high near 94. E 5-10 mph becoming NW 5-15 late.
     Thursday: A chance of morning drizzle then partly sunny, 75.  NW 5-10  mph.
     Friday: Partly sunny, with a high near 73.
     Friday Night: Mostly cloudy, with a low around 46.
     Saturday: Partly sunny, with a high near 72.
     Sunday: Partly sunny, with a high near 70.
     Monday: Mostly cloudy, 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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