[willamette-fcst] Thu, 09 Sep 10

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM 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 low over north-central Washington this
     morning with a trough extending southwest with its axis just east of the Cascade
     crest. This will all move gradually eastward this afternoon for a slowly
     stabilizing airmass. Radar indicates most showers are over and east of the
     mountains this morning, but there still could be a few scattered showers in the
     foothills through this morning.

     The morning Salem sounding looked very similar to yesterday's. The main
     difference is that it was slightly drier 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 this morning are about 290 at 10. Computer models
     suggest a generally westerly flow continuing in the transport region during the
     day today.

     Impediments to open field burning are mainly the wet fields. The Salem
     airport picked up .12 inches of rain yesterday. Cloud cover and high humidities
     will not be conducive to much drying today.

     Gradient stacking to the east will also be an issue. Pressure gradients as of
     8am included: Newport to Salem, 0.8 mb onshore and Salem to Redmond, 1.9
     onshore. The RUC computer model shows the east-west pressure profile becoming
     favorable this afternoon while the NAM does not. 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:

                                 11am     2pm     5pm     8pm
     Temperature:                 63       67      67      60
     Relative Humidity:           65%      54%     52%     67%
     Surface Wind Direction:     200      220     230     260
     Surface Wind Speed:           5        5       5       5
     Transport Wind Direction:   230      240     260     250
     Transport Wind Speed:        10       13      14       6
     Estimated Mixing Height:   3500     5800    6000     500
     Ventilation Index:           35       75      84       3



EXTENDED DISCUSSION:

     A weak ridge develops late this afternoon and evening over the region. There will be
     minor disturbances moving through the ridge, but at this point it does not look
     like there will be any rain this far south. Temperatures will remain cooler than
     average into the weekend. The NAM model indicates 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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