[willamette-fcst] Willamette Valley Field Burning Forecast - Noon Update

Fri Sep 22 11:47:46 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT FRI SEP 22, 2017

BURN ADVISORY:

Recommended times for agricultural burning are from now until 5 p.m.

Prep burning is not allowed.

WEATHER DISCUSSION:

Upper level ridge is nosing into British Columbia with the upper low gradually moving eastward. Morning low clouds are stubborn to dissipate but should clear early this afternoon. Residual cool air aloft and a moist ground may allow some afternoon clouds to build over the Cascades and provide a risk of some light showers. Moisture is unlikely in the Willamette Valley. Gradients remain weak but favor a northerly flow.

TODAY'S FORECAST:

Clouds dissipating to mostly clear this afternoon.

Salem's high temperature near 66F (average is 75F).

Relative humidity: Will lower to near 50 percent around 4 p.m.

Surface winds: N at 5 - 10 mph.

Transport winds: N at 5 - 10 mph.

Mixing height: Rising to 3000 ft by noon, rising to 4000 - 4500 ft by 3 p.m.

Salem's sunset tonight: 7:09 p.m.

(Salem Airport data for Thursday, Sep. 21: High 65°F; Rainfall: .01")

(Maximum Ventilation Index expected today: 45)

EXTENDED DISCUSSION:

Upper level ridge gradually moves into the state over the weekend but weakens some, allowing some weak onshore flow during afternoon hours. A weak disturbance rides over the ridge Monday and may provide a little more onshore flow but not likely any moisture. Ridge rebuilds on Tuesday and Wednesday and then gives way to a deeper trough approaching early Friday. Wind will likely turn westerly and potentially provide a burn opportunity.

The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text

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 15 mph.

     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 (ft) times

         the transport wind speed (mph) 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.

     5.  In accordance with OAR 837-110-0090, all field burning shall be
         banned when any two of the following criteria are present:
         A. Temperature of 95 degrees Fahrenheit or above
         B. Relative humidity of 30 percent or below
         C. Wind speed of 15 miles per hour or higher

This forecast is provided under an agreement between the Oregon Department

of Agriculture (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.

To add/remove your email address from this list, please go to:

http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst

Nick Yonker

ODF Meteorologist

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