[willamette-fcst] Wed, 22 Sep 10

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM 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:

     An upper level trough moving mainly through California is slowly pulling east
     with weak high pressure aloft beginning to build over the eastern Pacific Ocean.
     Satellite pictures as of late morning show a mixture of clouds and sun over the
     Willamette Valley with thicker cloud cover over the western Cascade foothills.
     Temperatures are running very close to where they were yesterday at the same
     time.

     The morning Salem sounding was very similar to the sounding yesterday morning.
     There was 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 light light but favored
     northerly, above winds were westerly.

     The pressure gradient profile is marginally favorable at 11am. As of 11:00 am
     gradients included: Newport to Salem, 1.2 mb onshore; Salem to Redmond, 0.2 mb
     onshore Computer models indicate an increasing onshore gradient. We will monitor
     the pressure pattern to see if it remains favorable this afternoon.

     Smoke trajectory forecasts from the Air Resources Lab based on this morning's
     NAM run indicate a favorable easterly direction for smoke movement this
     afternoon.

     A pibal sounding is scheduled for 1:00 pm this afternoon to confirm wind direction.

     For the mid Willamette Valley expect partly
     cloudy skies this afternoon. Temperatures will be a bit below average for this
     time of year and winds will be 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:

                                     2pm     5pm     8pm
     Temperature:                     68      68      60
     Relative Humidity:               47%     45%     62%
     Surface Wind Direction:         260     270     260
     Surface Wind Speed:               4       6       5
     Transport Wind Direction:       240     240     290
     Transport Wind Speed:             6       6       7
     Estimated Mixing Height:       7800    7800    1000
     Ventilation Index:               47      47       7


EXTENDED DISCUSSION:

     The morning runs of both the NAM and GFS showed no rain for Salem with the weak
     cold front that will move through the area tomorrow. Models have been
     flip-flopping on this for the past couple of days. If any rain does fall it will
     likely be light.

     Models continue 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, slight 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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