[willamette-fcst] Silverton Hills Forecast - Wed, Aug 18 2010

Willamette Valley Field Burning Forecast willamette-fcst at listsmart.osl.state.or.us
Wed Aug 18 11:42:07 PDT 2010


SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED AUG 18 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 2:00pm to 5:00pm.	
									
										
WEATHER DISCUSSION:

     Yesterday's impressive marine push has cooled temperatures considerably in the
     Willamette Valley. Temperatures as of 11am were running 16 to 24 degrees cooler
     than yesterday at the same time. 
     
     Clouds still cover most of the Valley, but satellite pictures show some breaks
     and we will see more sunshine this afternoon. 
     
     The morning Salem sounding showed cooler temperatures at all levels from the
     surface to above 14,000 feet. There is a significant stable layer from about
     1800 feet to about 6200 feet. Much of that stable layer will be eroded from the
     bottom by daytime heating, but subsiding air aloft will hold smoke from
     evacuating over the Cascades. 
     
     There is considerable gradient stacking to the east today with the Newport to
     Salem pressure difference 0.9mb onshore and the Salem to Redmond gradient has
     risin to 3.2mb onshore as of 11am. The gradient stacking is associated with the
     subsidence mentioned above. The gradient stacking issue should continue all day.
  
     That continued gradient stacking associated with the downward vertical air
     movement (or subsidence) over the Cascades will preclude open field burning
     today. 

     Expect slow clearing during the early afternoon, with mostly sunny skies by
     later this afternoon. Salem's high temperature today will be near 79. Relative
     humidity drops to 50% by noon. Minimum relative humidity 38%. Winds will be
     westerly generally 5-8 mph. Sunset tonight: 8:08 pm

								
THREE-HOURLY DATA:		

                                     2pm     5pm     8pm			
     Temperature:                     71      78      71  			
     Relative Humidity:               53%     40%     53%     			
     Surface Wind Direction:         290     300     280 		
     Surface Wind Speed:               5       6       7    			
     Transport Wind Direction:       280     300     280 
     Transport Wind Speed:             5       6       5  
     Estimated Mixing Height:       2900    3900     800
     Ventilation Index:               14      23       4
										
EXTENDED DISCUSSION:

     Longer range upper air forecast charts show a generally west to east flow across
     the eastern Pacific, gradually sagging off the coast for an upper level trough
     that will not move through until after the weekend. This is a pattern that will
     produce late night and morning marine clouds in the Valley with afternoon sun
     and temperatures generally around average for this time of year. Westerly winds
     tomorrow could produce a burn opportunity if the gradient stacking and
     subsidence eases.

EXTENDED FORECAST:	
     
     Thursday: Mostly sunny, with a high near 79. Wind WNW 3-8 mph. 
     Friday: Sunny, with a high near 79. NNW wind around 7 mph. 
     Saturday: Partly sunny, with a high near 74.  
     Sunday: Partly sunny, with a high near 74.  
     Monday: Mostly sunny, with a high near 80.   
     Tuesday: Mostly sunny, with a high near 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.








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