[willamette-fcst] Tue, 07 Sep 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE SEP 07 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 9:00am to 5:30pm.
     Prep burning is allowed from 2:00pm to 5:00pm with a 50 acre limit.
     Propane flaming is allowed from 2:00pm to 5:00pm.

WEATHER DISCUSSION:

     Upper air charts show a northwesterly flow over the Pacific Northwest this
     morning. Embedded in that flow is an upper disturbance located off the coast of
     central British Columbia. This will drop into the region tomorrow and carve out
     an upper level trough that will linger until about Thursday.

     Satellite pictures show a band of clouds centered on a line from south-central
     Washington to SW Oregon. To the northwest of that cloud band is an area of
     showers.

     Rainfall totals so far this morning have ammounted to just a couple of
     hundredths to a little less than a tenth of an inch so far.

     The morning sounding showed some cooling aloft and a fairly unstable atmosphere.
     Cloud cover will hold temperatures about 10 degrees below average for this time
     of year however, and the cool surface temperatures will limit mixing heights
     somewhat. Maximum Mixing heights will be around 3900 feet this afternoon.

     Pressure gradients, and consequently transport winds, will be quite light today.
     While the Newport to Salem gradient was 0.7mb onshore and the Salem to Redmond
     gradient was 0.4 onshore at 8:00am this morning, models indicate gradient
     stacking to the east will increase this afternoon.

     For the mid Willamette Valley expect mostly cloudy skies today with scattered
     light showers this afternoon. Salem's high temperature today will be near 69
     and the minimum relative humidity will drop to about 53%

     Sunset tonight: 7:32 pm

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 64       68      69      62
     Relative Humidity:           70%      57%     53%     67%
     Surface Wind Direction:     170      200     240     290
     Surface Wind Speed:           4        4       5       4
     Transport Wind Direction:   300      250     280     290
     Transport Wind Speed:         5        5       5       3
     Estimated Mixing Height:   2800     3500    3900     500
     Ventilation Index:           14       18      20       2

EXTENDED DISCUSSION:

     Models indicate onshore flow continuing but gradient stacking to the east for at
     least the next two days. The upper level trough will move to the east Friday and
     temporary ridging will set up over the region for more sunshine. Another trough
     moves in over the weekend for cooler temperatures. The details of the trough are
     not clear at this point but there is a potential for at least some rain at some
     point over the weekend.

EXTENDED FORECAST:

     Wednesday: Mostly cloudy, a few showers.  Wind SW 6 mph.  Chance of precip 30%.
     Thursday: Partly sunny, with a high near 69. Calm wind becoming west southwest
     around 6 mph.
     Friday: Mostly sunny, with a high near 71.
     Saturday: Partly sunny, with a high near 71.
     Sunday: Mostly cloudy, with a high near 70.
     Monday: 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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