[willamette-fcst] Wed, Aug 25th, 2010

Willamette Valley Field Burning Forecast willamette-fcst at listsmart.osl.state.or.us
Wed Aug 25 11:51:33 PDT 2010


SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED AUG 25 2010

BURN ADVISORY:

     Agricultural burning is not recommended.
     Prep burning is not allowed.
     Propane flaming is not allowed.


WEATHER DISCUSSION:

     Hot weather again today, but a cooling trend is developing...

     The late morning ODF surface analysis shows a thermal trough from the central
     valley of California northward through the Willamette Valley.  The trough is
     beginning to show signs of laying over in the north end of the Valley prior to a
     jump to east of the Cascades.  Light west winds have developed through the western
     Columbia River Gorge. In addition, temperatures are running mostly a degree or
     two cooler in the north end of the valley versus yesterday at the same time.

     The cooling has already kicked in for coastal areas with low clouds and fog
     banked up against the coastline and some locations as much as 18 degrees cooler
     than yesterday.

     Pressure gradients have increased from the Newport to Salem. As of 11:00am the
     difference was 1.3mb onshore.  The Redmond to Salem offshore gradient has
     decreased to 1.9mb. If present pressure trends continue the thermal trough will
     be east of the Cascades by about 6pm tonight.

     The morning Salem sounding showed a series of inversions from the surface to
     about 2800 feet, with the temperature at that top of the inversion 18 degrees
     warmer than at the bottom.  With the strong inversion it will be difficult to get
     very high mixing heights.  The maximum mixing height will be around 3100 feet at
     about 5pm this afternoon.

     Computer models indicate that while the thermal trough will move east of the
     valley late this afternoon finding a westerly component to the winds will be
     difficult. Latest computer models agree on a transport wind in the 320-330. Much
     more of a westerly component is necessary for optimum burning conditions.

     Pibals through the afternoon will determine if the models are correct or if
     a burning opportunity develops ahead of our transition to cooler
     weather.

     For the mid Willamette Valley expect sunny and hot weather this afternoon.
     Salem's high temperature will be near 94. Relative humidity will drop to 20%.

     Sunset tonight: 7:56 pm

THREE-HOURLY DATA:

                                     2pm     5pm     8pm
     Temperature:                     91      97      83
     Relative Humidity:               22%     18%     33%
     Surface Wind Direction:         360     340     270
     Surface Wind Speed:               7       7       7
     Transport Wind Direction:       340     340     290
     Transport Wind Speed:             5       5       5
     Estimated Mixing Height:       2200    3100     500
     Ventilation Index:               11      16       2

EXTENDED DISCUSSION:

     An upper level low in the Gulf of Alaska will drop southeastward and settle in
     over the Pacific Northwest through the weekend. This means cooler than average
     temperatures and some unsettled weather. There is a cold front extending
     southwestward from southern British Columbia today that will sweep across the
     region overnight. This is fairly weak, but it will deepen the marine layer
     overnight and may produce some drizzle tomorrow morning.

     Low level flow will be onshore through the extended period. Disturbances
     rotating through the bottom of the trough will periodically deepen the marine
     layer over the valley for possible drizzle or showers. Timing of these
     disturbances is in doubt, but right now it looks like Friday will be mainly dry,
     with a better chance of precipitation later Saturday and Sunday.


EXTENDED FORECAST:

     Thursday: Mostly cloudy, a little morning drizzle.  High 79.  Winds NW 7-10 mph.
     Calm wind becoming north northwest between 7 and 10  mph.
     Friday: Partly sunny, 72. NW 5-8.
     Saturday: Mostly cloudy, chance of showers. High 70.
     Sunday: Mostly cloudy, chance of showers, with a high near 70.
     Monday: Mostly cloudy, with a high near 73.
     Tuesday: Partly sunny, with a high near 74.

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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