[willamette-fcst] Wed, Sep 1st, 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED SEP 01 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning
       are from 8:00am to 4:00pm.
     Prep burning is not allowed.
     Propane flaming is not allowed.


WEATHER DISCUSSION:

     A weakening Pacific cold front is moving through the region this morning. Valley
     rainfall totals have generally been less than a tenth of an inch, but mountain
     locations have received considerably more rain. Log Creek, near Mt Hood picked
     up over 2.5 inches in the past 24 hours. Closer to home, Horse Creek, in the
     Cascade foothills picked up .19 and Yellowstone Mountain recorded .62 inches.
     Silverton had .04 in the rain guage, Jordan Valley recorded .12 and Turner picked
     up .01 during the past 24 hours.

     Upper air charts show a ridge of high pressure aloft beginning to build off the
     Oregon/Washington coast. This ridge will help to gradually decrease
     precipitation during the day today. However skies should remain mostly cloudy
     and relative humidities quite high for little drying of fields this afternoon.

     The ODF morning surface analysis showed the remains of the weak front trailing
     southwestward from a minor surface low near Astoria. There is a well developed
     thermal trough in California but it does not extend into Oregon. Gradients are
     generally southerly and forecast models show a light southerly transport wind
     today.

     For the mid Willamette Valley expect some light showers through the morning
     hours, then mostly cloudy skies this afternoon. Salem's high temperature today
     will be near 69, minimum relative humidity 59%.

     Sunset tonight: 7:43 pm


THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 63       67      69      65
     Relative Humidity:           81%      70%     59%     68%
     Surface Wind Direction:     190      220     210     350
     Surface Wind Speed:           9        5       6       6
     Transport Wind Direction:   210      200     190     180
     Transport Wind Speed:         5        6       6       5
     Estimated Mixing Height:   1000     2500    2900    1000
     Ventilation Index:            5       15      17       5

EXTENDED DISCUSSION:

     Today marks the final surge of moisture into the Pacific Northwest for the next
     couple of days. High pressure will quickly build offshore tonight and shift
     inland later tomorrow for a quick warm-up. Temperatures tomorrow will jump on
     the order of 15 degrees compared to today. At this point it appears that
     transport wind directions will be unfavorable for burning Thursday and Friday.

     By late Friday the ridge over the region begins to shift east as another upper
     level trough approaches from the Pacific. This should bring cooler weather to
     the mid Willamette valley once again this weekend, although not as cool as the
     past couple of days. There is a potential for some light rain over the weekend,
     but moisture appears quite limited. Longer range models indicate a second trough
     aproaching early next week.

EXTENDED FORECAST:

     Thursday: Morning low clouds or fog, otherwisesunny. 85. N 5-10 mph.
     Friday: Sunny, with a high near 87. N 5-10 g 15 mph.
     Saturday: Partly sunny, with a high near 78.
     Sunday: Partly sunny, with a high near 75.
     Monday: Partly sunny, with a high near 75.
     Tuesday: Partly sunny, with a high near 74.

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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