[willamette-fcst] Silverton Hills Forecast - Tue, Aug 10 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00PM PDT TUE AUG 10 2010

BURN ADVISORY:

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

     Satellite pictures still show clouds covering virtually all of the Willamette
     Valley as of late morning. Thinning of the cloud deck that was visible at this
     time yesterday is not evident today. 
     
     A trough in the upper atmosphere extends from central British Columbia to
     northwestern California. This is developing into a cut-off low aloft that will
     drift across eastern Washington this afternoon and tonight. Moisture is limited
     with this feature, however as impulses rotate around the low across western
     Washington and northwest Oregon there could be some light showers. Some very
     light radar echoes are visible in Yamhill and Polk Counties as of late morning.

     The morning Salem sounding showed an inversion between about 3900 and 4800 ft
     which will be the limit on maximum mixing heights today. 
     
     Onshore gradients have increased through the morning hours. The 11:00am Newport
     to Redmond pressure difference was 4.5mb onshore. 3.6mb of that was from Salem
     to Redmond, so gradient stacking to the east continues. 
     
     Forecast models continue to indicate that transport winds in the Valley today
     will remain north-northwesterly, never getting more westerly than about 340
     degrees. 1:00pm pibal soundings are scheduled to confirm the unfavorable wind
     direction. 
     
     The possibility of open field burning today looks low. 
     
     Expect mostly cloudy skies this afternoon. Salem's high temperature today will
     be near 74. Relative humidity drops to 50% by 1pm. Minimum relative humidity
     today will be near 41%.

   
     Sunset tonight: 8:20 pm
										
THREE-HOURLY DATA:		

                                     2pm     5pm     8pm			
     Temperature:                     68      72      67  			
     Relative Humidity:               55%     46%     59%     			
     Surface Wind Direction:         310     310     310 		
     Surface Wind Speed:               6       8       7    			
     Transport Wind Direction:       340     340     320 
     Transport Wind Speed:             5       5       3  
     Estimated Mixing Height:       4000    4500    1000
     Ventilation Index:               20      22       3

                               				
										
EXTENDED DISCUSSION:

     The upper level low will drift across eastern Washington today and into Idaho by
     Thursday.  A ridge of high pressure now in the eastern Pacific will intensify
     and move closer to the Pacific Northwest for dramatic warming later in the week.
     Expect hot weather for the weekend.  Burning opportunities will be limited.  
 

EXTENDED FORECAST:	
     
     Wednesday: Partly sunny, with a high near 80. Calm wind becoming northwest
     between 4 and 7  mph. 
     Thursday: Mostly sunny, with a high near 86. Calm wind becoming north between 6
     and 9  mph. 
     Friday: Sunny, with a high near 92.  
     Friday Night: Mostly clear, with a low around 52.  
     Saturday: Sunny and hot, with a high near 97.   
     Sunday: Sunny, with a high near 95.    
     Monday: Sunny, with a high near 86.  
     
										
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.