[willamette-fcst] Tue, 31 Aug 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE AUG 31 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 10:00am to 6:00pm.
     Prep burning is not allowed.
     Propane flaming is not allowed.


WEATHER DISCUSSION:

     Cloudy and wet weather likely this afternoon.

     Upper air charts show a broad trough covering the western US this morning. The
     axis of the trough is along the Canadian Rockies, through central Idaho
     then curving southwestward to southern California. This leaves the Pacific
     Northwest in a generally west-northwesterly flow aloft. This flow is steering a
     Pacific weather system at Washington and northwest Oregon.

     The morning ODF surface analysis shows a warm front just off the Washington
     coast, extending southward to off the Oregon coast. Rain already covers most of
     western Washington and it has been raining since about 6:45 this morning in
     Astoria. The rain will slowly spread southeastward this morning. Best estimate
     is for rain to reach Salem by around 10:00am and then cover most of the
     Willamette Valley by early afternoon.

     Computer models indicate over a quarter of an inch of rain by Wednesday morning.

     The morning Salem sounding showed that the air mass had warmed somewhat in the
     past 24 hours. This will keep mixing heights lower than yesterday when the
     maximum mixing height was about 4000 feet. Look for a maximum today of about
     3500 feet. Low level winds measured this morning were southwesterly, veering to
     westerly by about 6000 feet above ground level. Computer models suggest that the
     southwesterly wind direction will continue through this afternoon. Winds and
     mixing heights today will be favorable for burning, but if the rain comes in on
     schedule then open field burning would not be an option.

     For the mid-Willamette valley expect clouds increasing this morning, with rain
     spreading in from the northwest by early afternoon. Salem's high temperature
     today will be near 69. Minimum relative humidity 55%.

     Sunset tonight: 7:45 pm


THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 61       68      69      65
     Relative Humidity:           72%      57%     59%     70%
     Surface Wind Direction:     180      190     200     230
     Surface Wind Speed:           7        7       6       7
     Transport Wind Direction:   200      220     240     200
     Transport Wind Speed:        12       15      18      11
     Estimated Mixing Height:   2200     2800    3500    1000
     Ventilation Index:           26       42      63      11

EXTENDED DISCUSSION:

     After Wednesday expect drier and warmer conditions as a flat upper level ridge
     builds over the area. Temperatures should climb to near or slightly above
     average for the time of year. Computer models suggest that another upper level
     trough will move over the area sometime early in the weekend. Moisture looks
     limited but temperatures will likely fall back below average for Saturday,
     Sunday and the Labor Day holiday.

EXTENDED FORECAST:

     Wednesday: Mostly cloudy, showers decreasing.  High 74.  Wind S 5-9 becomming WNW.
     Thursday: Mostly sunny, with a high near 85. North wind between 3 and 10 mph.
     Friday: Sunny, with a high near 86.
     Saturday: Mostly sunny, with a high near 78.
     Sunday: Partly sunny, with a high near 75.
     Monday: Partly sunny, with a high near 76.

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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