[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT TUE JUL 12, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are 9:00 a.m. to 7:00 p.m.



WEATHER DISCUSSION:



An unseasonably strong upper-level trough is centered about 300 miles off the northern Oregon Coast this morning.  An associated cold front was bringing cloudy skies and areas of light rain to the region.  Mid-morning satellite imagery showed solid cloud-cover extending from well offshore eastward, across the Cascades, and into Central Oregon.  The northern Willamette Valley has already received around one-tenth of an inch of rain.



The upper-level trough is not forecast to move much today, so light rain should continue, at times, under cloudy skies.  Temperatures will be about 15 degrees below normal.



TODAY'S FORECAST:



Cloudy with light rain at times.  Additional rainfall totals around could exceed one-tenth (.10") of an inch.



Surface winds:  SSW 5 mph this morning; SW 5-8 mph this afternoon

Transport winds: SSW 6-10 mph this morning; SW 10 mph this afternoon

Salem's forecast high temperature:  66 degrees

Maximum mixing height:  4500 feet

Relative humidity:  Remains above 65% all day

Salem sunset tonight: 8:58pm



THREE-HOURLY DATA:



                                 11am     2pm     5pm     8pm

     Temperature:                 62       64      66      62

     Relative Humidity:           78%      76%     68%     78%

     Surface Wind Direction:     200      210     230     260

     Surface Wind Speed:           5        5       6       7

     Transport Wind Direction:   200      220     250     260

     Transport Wind Speed:         7        8      10      10

     Estimated Mixing Height:   2700     4000    4500    3800

     Ventilation Index:           19       32      45      38



EXTENDED DISCUSSION:



The upper-level trough is forecast to moves over the state on Wednesday, before weakening Thursday and Friday.  Look for showers to continue on Wednesday with some drying and warming on Thursday.  By Friday, temperatures should recover to within about 5 degrees of normal with some afternoon sunshine.



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.



Pete Parsons

ODF Meteorologist



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