[willamette-fcst] Silverton Hills Forecast - Mon, Sep 27 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00PM PDT MON SEP 27 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is allowed up until 3:00pm with a 50 acre limit. 
     Propane flaming is allowed up until 5:00pm.	
									
										
WEATHER DISCUSSION:

     Upper air charts are showing a deep low in the northern Gulf of Alaska this,
     while an upper ridge continues to build over the intermountain region. This is
     giving a weak southwesterly flow aloft the region. A weak disturbance in that
     flow supports a Pacific system that earlier brought clouds to parts of NW
     Oregon. Those clouds have now retreated into Washington and the state is nearly
     cloud free. 
     
     The morning Salem sounding showed warming at most levels and a fairly stable
     atmosphere. There was an inversion from the surface to about 2800 feet. This
     should mix out this afternoon. We anticipate a mixing height of 3000 feet by
     about 1pm with a maximum mixing height of about 4000 feet this afternoon. Winds
     on the sounding were light southerly at the surface veering to light
     southwesterly by about 3500 feet. Models are still showing a transition to a
     northerly transport wind later this afternoon. 
     
     Early afternoon pibals may continue to show the favorable SW winds, but forecast
     smoke trajectories from the Air Resources Lab show smoke eventually drifting
     southward toward central Lane County. 
     
     Pressure gradients as of 11am are slightly onshore from the coast to Salem
     (0.3mb) but remain offshore from Redmond to Salem (1.7mb).
     
     Look for sunshine this afternoon. Salem's high temperature today will be near
     84. Minimum relative humidity will be near 51%. Sunset tonight: 6:54 pm

					
										
THREE-HOURLY DATA:		

                                    2pm     5pm     8pm			
     Temperature:                    81      83      74  			
     Relative Humidity:              56%     53%     66%     			
     Surface Wind Direction:        240     330     330 		
     Surface Wind Speed:              4       5       4    			
     Transport Wind Direction:      220     020     350 
     Transport Wind Speed:            2       4       6  
     Estimated Mixing Height:      3100    4000     500
     Ventilation Index:               6      16       3                          
										
EXTENDED DISCUSSION:

     The upper ridge centered over Nevada and Utah will remain over the western US
     much of the week. Computer models show a transport wind over northwest Oregon
     from the north or northeast through Thursday, then going back to southwest by
     Friday. Afternoon mixing heights will likely slowly deterioriate through about
     Thursday as well.

EXTENDED FORECAST:	
     
     Tuesday: Partly sunny, with a high near 77. Calm wind becoming north between 5
     and 8  mph. 
     Wednesday: Mostly sunny, with a high near 76. North northeast wind between 6 and
     9 mph.  
     Thursday: Mostly sunny, with a high near 79.  
     Friday: Mostly sunny, with a high near 79.   
     Saturday: Mostly sunny, with a high near 76.  
     Sunday: 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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