[willamette-fcst] Silverton Hills Forecast - Thu, Jul 29 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT THU JUL 29 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is allowed from 2:00pm to 5:00pm with a 50 acre limit. 
     Propane flaming is allowed from 12:00pm to 5:00pm.	
									
										
WEATHER DISCUSSION:

     Once again today upper air charts show a weak trough off the coast of North
     America and an upper level high centered over the four-corners area of the
     southwest United States. This is giving a southwesterly flow aloft over western
     Oregon.  At lower levels, however, northerly winds are the rule.  
 
     The morning Salem sounding showed very stable conditions to about 6000 feet. 
     Several weak inversions were visible in the temperature trace.  Sunshine today
     should push mid-valley temperatures into the low 80's and the warmer surface
     temperatures will push mixing heights to around 4000 feet this afternoon, but
     the stable layer above will cap plume rise.  

     The limitation on open field burning today will be wind direction.  Models
     are indicating a continuing northerly wind for today in the Willamette
     Valley.  The Newport to Redmond onshore gradient as of 8:00am this morning
     was 3.1mb while Portland was showing a 3.5mb northerly gradient.  
     Trajectory forecasts from the Air Resources Laboratory show any smoke
     released from mid-Marion County headed southward toware mid-Lane County. 
     Models, of course, have difficulty timing the onset of a sea-breeze which
     could provide a burning opportunity.   

     
     One wild-card in the weather today is thunderstorm development over and east
     of the Cascades.  Careful monitoring of the pressure pattern and pibal releases
     will again be necessary to discern any potential burn window.
   
     Expect full sunshine today.  Temperatures should be up a few degrees from yesterday.
     Salem's high temperature today will be near 83.  Relative humidity drops to 50% by  
     12pm.  Minimum relative humidity 34%.

     Sunset tonight: 8:37 pm	
							
										
THREE-HOURLY DATA:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 68       77      83      75  			
     Relative Humidity:           55%      42%     34%     46%     			
     Surface Wind Direction:     360      350     340     290 		
     Surface Wind Speed:           4        6       7       7    			
     Transport Wind Direction:   360      360     360     330 
     Transport Wind Speed:         5        6       6       7  
     Estimated Mixing Height:   2100     3100    3800    1000
     Ventilation Index:           10       19      23       7

                               				
										
EXTENDED DISCUSSION:

     The upper level trough off the coast will kick out several minor disturbances
     before finally moving eastward over the weekend.  This will leave the region
     under a dry, northerly flow aloft.  Low level gradients will be onshore and
     there may be some burn opportunities early next week.  
 

EXTENDED FORECAST:	
     
     Friday: Mostly sunny, 82. Winds becoming WNW 5-8.  Possible burn opportunity.
     Saturday: Mostly sunny, with a high near 75. Wind WSW-5-10 . 
     Sunday: Mostly sunny, with a high near 77.  
     Monday: Mostly sunny, with a high near 84.   
     Tuesday: Mostly sunny, 87.  
     Wednesday: Mostly sunny, 83.  
									
										
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.