[willamette-fcst] Silverton Hills Forecast - Mon, Aug 16 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT MON AUG 16 2010

BURN ADVISORY:

     ***** Fire Marshal Burn Ban conditions possible this afternoon ***

     Agricultural burning burning is not recommended.
     Prep burning is not allowed. 
     Propane flaming is not allowed.	
									
										
WEATHER DISCUSSION:

     Hot weather continues over the Pacific Northwest today although there are signs
     that conditions are beginning to change. A ridge of high pressure in the upper
     atmosphere extends from off the Oregon coast, northward into the Yukon.
     
     The latest surface analysis shows a thermal trough from the central valley of
     California, through southwest Oregon then into the central Willamette Valley.
     The thermal trough axis then extends east of the Cascades into the Columbia
     basin. The 11am pressure gradients showed 1.6mb onshore Newport to Salem, but
     1.2 mb easterly from Redmond to Salem. 
     
     Temperatures as of late morning were running 2 to 3 degrees warmer than
     yesterday at the same time. The morning Salem sounding showed a strong
     surface-based inversion extending to about 3100 feet above the ground. This will
     just barely break today and the maximum mixing height will only climb to about
     3200 feet. 

     Wind direction through the Valley will be northerly or slightly
     northeasterly.
     
     The Valley will be close to Fire Marshal Burn Ban conditions this afternoon for
     temperature and humidity. (Ninety-five degrees and less than 30 percent humidity
     is the criterion that is reachable this afternoon.) 

     Expect sunny weather today. Salem's high temperature today will be near 95.
     Relative humidity drops to 50% by noon. Minimum relative humidity 22%.
   
     Sunset tonight: 8:11 pm
					
										
THREE-HOURLY DATA:		

                                     2pm     5pm     8pm			
     Temperature:                     87      95      84  			
     Relative Humidity:               33%     23%     35%     			
     Surface Wind Direction:         350     340     300 		
     Surface Wind Speed:               5       6       6    			
     Transport Wind Direction:       010     010     010 
     Transport Wind Speed:             6       8       5  
     Estimated Mixing Height:       6000    7000    1000
     Ventilation Index:               36      56       5

                               				
										
EXTENDED DISCUSSION:

     The upper ridge will slowly weaken and eventually be undercut by an upper level
     low moving eastward from the Pacific Ocean into northern California. At the same
     time a short wave moving across the Gulf of Alaska will suppress the ridge. 

     As the upper air pattern changes surface pressure patterns will change as well
     and a northwesterly or west-northwesterly transport wind should develop tomorrow
     for a possible burn opportunity. Onshore flow should continue through the week
     with temperatures dropping to near or slightly below average for this time of
     year. This will also lead to a morning cloud-afternoon sunshine pattern.

EXTENDED FORECAST:	

     Tomorrow: Sunny, with a high near 92. Light winds early becoming WNW 6-8 during
       the afternoon. Possible burn opportunity. 
     Wednesday: Sunny, with a high near 82.
       Light winds becoming NW 5-8.
     Thursday: Brief morning clouds then sunny, with a
       high near 81. 
     Friday: Brief morning clouds then sunny, with a high near 76.
     Saturday: Partly sunny, with a high near 76. 
     Sunday: Partly sunny, with a high near 77.										
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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