[willamette-fcst] September 3

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT FRI SEP 03 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is allowed from 12:30pm to 5:00pm with a 50 acre limit.
     Propane flaming is allowed from 12:30pm to 5:00pm.


WEATHER DISCUSSION:

     Upper level ridge has moved off to the east with the upper trough moving in this
     morning. Surface pressure pattern shows the thermal trough in the Willamette
     Valley this morning. The Newport to Salem gradient is .9 mb onshore while the
     Salem to Redmond gradient is 3.6 mb offshore. As the upper level trough
     continues marching in today, onshore pressure gradients will increase as the
     surface thermal trough shifts east of the Cascades late this morning to early
     afternoon. This will set up a decent marine push this afternoon.

     The morning sounding from Salem showed a strong surface and subsidence inversion
     to nearly 4000 ft, which demonstrates the remaining strength of the exiting upper
     level ridge. Winds up to 1500 ft are generally light and variable but show a
     distinct light SW'erly flow above.

     Weather conditions for today will again be mostly sunny with the high
     temperature a little cooler than yesterday's high of 90. Expect the temperature
     to top out around 85 a little earlier this afternoon as the cooler marine air
     starts pushing into the region. Mixing heights will remain fairly low this
     morning but as cooler air moves in aloft this afternoon combined with surface
     heating, mixing will rapidly improve during the early afternoon. Relative
     humidity will drop below 50 percent by 11 a.m. with the minimum humidity nearly
     32 percent. SW'erly flow aloft will continue and increase today with westerly
     surface winds starting to show up in the valley by early to mid afternoon. All
     indications point to a good burning opportunity today.


     Sunset tonight: 7:40 pm


THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 75       82      84      73
     Relative Humidity:           48%      37%     33%     49%
     Surface Wind Direction:     230      270     270     290
     Surface Wind Speed:           4        5      10       8
     Transport Wind Direction:   220      240     260     290
     Transport Wind Speed:         7       10      12      10
     Estimated Mixing Height:   1800     4200    5000    1500
     Ventilation Index:           13       42      60      15



EXTENDED DISCUSSION:

     Upper level trough moves into the state on Saturday ushering in much cooler
     conditions. Expect mostly cloudy skies with breezy onshore, NW'erly winds
     during the afternoon. Mixing will be good but the more gradient stacking will be
     to the east.

     Trough moves off to the east Sunday with a minor ridge building in. This will be
     very temporary as another trough forms in the eastern Pacific Monday and moves
     in Monday night. This trough will likely be a little stronger and bring some
     light rain to the region on Tuesday. Trough exits to the east Wednesday.
     Overall, expect a considerably cooler, fall-like air mass over the region over
     the weekend and into next week.

EXTENDED FORECAST:

     Saturday: Partly sunny, with a high near 72. Northwest wind between 5 and
     10 mph.
     Saturday Night: A 30 percent chance of showers after 11pm. Mostly cloudy, with a
     low around 46. North northwest wind between 5 and 8 mph becoming calm.
     Sunday: A 30 percent chance of showers before 11am. Partly sunny, with a high
     near 68. Southwest wind 5 to 7 mph becoming west northwest.
     Sunday Night: Mostly cloudy, with a low around 47.
     Monday: Mostly cloudy, with a high near 72.
     Monday Night: Mostly cloudy, with a low around 48.
     Tuesday: Mostly cloudy with occasional light rain, with a high near 67.
     Tuesday Night: Mostly cloudy with occasional light rain, with a low around 48.
     Wednesday: Mostly cloudy early then partly sunny, with a high near 70.
     Wednesday Night: Mostly cloudy, with a low around 50.
     Thursday: Partly sunny, with a high near 72.













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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