[willamette-fcst] Thu, 09 Sep 10

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT THU SEP 09 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 10:00am to 5:30pm.
     Prep burning is allowed from 1:00pm to 5:00pm with a 50 acre limit.
     Propane flaming is allowed from 1:00pm to 5:00pm.


WEATHER DISCUSSION:

     Upper air charts show an upper level trough beginning to pull out of the Pacific
     Northwest and a weak upper level ridge developing to the northwest.

     The morning Salem sounding was very similar to yesterday's. The main difference
     was that it indicated slightly drier air between 1500 and 4000 feet. It is still
     conditionally unstable and in spite of the upper trough moving east models are
     showing very little warming aloft for significant stabilization. Consequently
     mixing heights will be quite high today, peaking near 6000 feet.

     Transport wind directions on the morning sounding were about 290 at 10. Computer
     models suggest a generally westerly flow continuing in the transport region
     during the day today.

     With rain the past few days and mostly cloudy skies this morning and high
     humidities, dry fields will be an issue for open field burning this afternoon.
     If any growers feel their fields are dry enough to burn today they need to make
     that known to field burning staff.

     Gradient stacking to the east will also be an issue this afternoon. Pressure
     gradients as of 11am included: Newport to Salem, 1.2 mb onshore; Salem to
     Redmond, 2.6 mb onshore. The the latest RUC computer model shows the east-west
     pressure profile becoming marginally favorable this afternoon while the NAM is
     marginally unfavorable. We will monitor this through the day.

     For the mid Willamett Valley today expect mostly cloudy skies. There may be a
     few showers, especially toward the Cascade foothills this morning. Winds will be
     from the south-southwest during the morning becoming more west-southwesterly
     during the afternoon. Salem's high temperature today will be near 68. Minimum
     relative humidity will be near 51%.

     Sunset tonight: 7:28 pm


THREE-HOURLY DATA:

                                    2pm     5pm     8pm
     Temperature:                    67      67      60
     Relative Humidity:              54%     52%     67%
     Surface Wind Direction:        220     230     260
     Surface Wind Speed:              5       5       5
     Transport Wind Direction:      240     260     250
     Transport Wind Speed:           13      14       6
     Estimated Mixing Height:      5800    6000     500
     Ventilation Index:              75      84       3



EXTENDED DISCUSSION:

     Latest computer models still show a ridge developing late this afternoon and
     evening over the region. There will be minor disturbances moving through the
     ridge, but at this point it still does not look like there will be any rain this
     far south. Temperatures will remain cooler than average into the weekend. The
     latest NAM model continues to show gradient stacking to the east both tomorrow
     and Saturday.

     Longer range charts have turned more optimistic with ridging building the first
     part of next week for drier weather and temperatures returning to near average
     for mid September.

EXTENDED FORECAST:

     Tomorrow: Partly sunny, 72. Wind becoming NNW 5-8 mph.
     Saturday: Partly sunny, 74. Wind becoming NW 6 mph.
     Sunday: Mostly cloudy, with a high near 74.
     Monday: Partly sunny, with a high near 80.
     Tuesday: Partly sunny, with a high near 79.
     Wednesday: Partly sunny, with a high near 77.


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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