[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT TUES JUL 26 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from now to 2:00pm with a 50 acre limit.

Propane flaming is allowed from now to 5:00pm.



WEATHER DISCUSSION:



Upper level trough from yesterday has weakened and moved off to the east.  The current upper level pattern is a weak trough with generally zonal flow overhead.  Expect a tranquil day with skies partly to mostly cloudy under residual marine air.  Expect generally light northerly flow.



Satellite picture shows clouds slowly breaking up and dissipating.  Clear skies east of the Cascades are causing pressures to lower and thus creating gradient stacking over the Cascades.  Salem to Redmond is 2.4 mb onshore while Newport to Salem is .6 mb onshore.



TODAY'S FORECAST:



Salem's high temperature today will be near 75.

Relative humidity drops to 50% by 2pm.  Minimum relative humidity 41%.

Surface winds:  NW to N at 5-10 mph.

Transport winds: Northerly at 8-12 mph.

Mixing height:  Rising above 5000 ft by 2 pm.

Sunset tonight: 8:46 pm.





THREE-HOURLY DATA:

                                 2pm     5pm     8pm

     Temperature:                 72      75      70

     Relative Humidity:           49%     41%     51%

     Surface Wind Direction:     340     340     320

     Surface Wind Speed:           7      10      11

     Transport Wind Direction:   350     350     360

     Transport Wind Speed:         9      10      12

     Estimated Mixing Height:   5000   5000+    4500

     Ventilation Index:           45      50      54



EXTENDED DISCUSSION:



Little change in the weather pattern expected for the remainder of the week.  Look for weak westerly flow with a minor ridge nosing in late Wednesday and Thursday.  Wind flow will remain generally northerly with a little more offshore flow likely Thursday and Friday.  Temperatures will gradually warm through the week.



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 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.



Nick Yonker

ODF Meteorologist



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