[willamette-fcst] WED, 22 Sep 10

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED SEP 22 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 1:00pm to 5:00pm.
     Prep burning is allowed from noon to 5:00pm with a 50 acre limit.
     Propane flaming is allowed from noon to 5:00pm.


WEATHER DISCUSSION:

     The Willamette Valley is between an upper level trough to the south and an upper
     ridge to the north in an area of light winds. Satellite pictures show clouds
     covering most of western Oregon from the coast to the Cascade crest, but the
     cloud deck is quite high, quite thin, and should partially break up this
     afternoon.

     The morning Salem sounding was very similar to yesterday morning's sounding.
     There was a minor inversion between about 1900 and 2500 feet. This will easily
     mix out by late morning. There is also an unusually persistent inversion aloft
     with a base that has been near 7500 feet for the past couple of days. Today that
     inversion is between about 7800 feet and 8100 feet. Below this feature winds are
     light and favor northerly, above winds are westerly.

     Computer models indicate that lower level winds will shift to more westerly
     today. Gradient stacking to the east may be an issue, but smoke trajectory
     forecasts from the Air Resources Lab based on last night's NAM run indicate a
     favorable easterly direction for smoke movement later this afternoon.

      Gradient stacking to the east will be an issue today. Pressure gradients as of
     8am included: Newport to Salem, 1.0 mb onshore; Salem to Redmond, 0.4 mb
     offshore. Computer models indicate an increasing onshore gradient. We will
     monitor the pressure pattern to see if it becomes favorable this afternoon.

     For the mid Willamette Valley expect mostly cloudy skies this morning, partly
     cloudy skies this afternoon. Temperatures will be a bit below average for this
     time of year and winds will switch to light westerly.

     Salem's high temperature today will be near 70.
     Relative humidity drops to 50% by 12pm.  Minimum relative humidity 41%.

     Sunset tonight: 7:04 pm


THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 64       70      70      61
     Relative Humidity:           60%      44%     41%     60%
     Surface Wind Direction:     320      260     260     260
     Surface Wind Speed:           2        4       5       5
     Transport Wind Direction:   270      240     240     290
     Transport Wind Speed:         3        6       6       7
     Estimated Mixing Height:   2400     7800    7800    1000
     Ventilation Index:            7       47      47       7



EXTENDED DISCUSSION:

     Computer models continue to bounce back and fourth on whether tomorrow's weak
     weather front will produce any rain this far south. Any rainfall would be light,
     but there is a significant possibility that all rain from the system will stay
     to the north. Models are in agreement on a ridge for Friday and Saturday that
     will bring sunshine and warm temperatures to the region.

     Models are also coming into line and showing a strong low in the Gulf of Alaska
     late in the weekend and into the first part of next week. But the low will be
     far enough off the coast that moisture rotating around it will stay well west of
     the Pacific Northwest for fair weather and temperatures near seasonal average
     into early next week.


EXTENDED FORECAST:

     Thursday:  Mostly cloudy, chance of rain 66. SSW 5-10.
     Friday:    Mostly sunny, with a high near 77. S 5.
     Saturday:  Mostly sunny, with a high near 80. W 5.
     Sunday:    Partly sunny, with a high near 73. W 4.
     Monday:    Partly sunny, with a high near 78.
     Tuesday:   Partly sunny, with a high near 75.


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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