[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
9:00 AM PDT TUE AUG 10 2010

BURN ADVISORY:

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

     Satellite pictures show clouds covering virtually all of the Willamette Valley
     again this morning.  The clouds do not appear to be quite as thick as yesterday,
     however the top of the moist layer is at about 4100 feet so the clouds will be
     slow to burn off again today.
 
     In the upper atmosphere a trough extends from
     central British Columbia to northwestern California. This will develop into a
     cut-off low aloft that will drift across eastern Washington today.  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.  
  
     The morning Salem sounding showed two inversions in the lower atmosphere, one
     between about 1900 and 2500 feet and another between about 3900 and 4800 ft. 
     The higher inversion will likely not mix out and will be the limit on maximum
     mixing heights today.
 
     Onshore gradients have weakened since yesterday.  The 8:00am Newport to
     Redmond pressure difference was 3.5mb onshore.  2.6mb of that
     was from Salem to Redmond, so gradient stacking to the east continues.
 
     Forecast models indicate that transport winds in the Valley today will remain
     north-northwesterly, never getting more westerly than about 340 degrees.  This
     should be confirmed with pibal soundings this afternoon, but the possibility of
     open field burning today looks low.
 
     Expect cloudy skies this morning and 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:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 65       71      74      69  			
     Relative Humidity:           60%      47%     41%     51%     			
     Surface Wind Direction:     340      280     310     290 		
     Surface Wind Speed:           2        5       8       7    			
     Transport Wind Direction:   340      340     340     320 
     Transport Wind Speed:         3        5       5       3  
     Estimated Mixing Height:   2100     4000    4500    1000
     Ventilation Index:            6       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.