[willamette-fcst] Silverton Hills Forecast - Mon, Aug 09 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT MON AUG 09 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:

     A persistant broad upper trough remains over the Pacific Northwest this morning.
     Computer models indicate a disturbance now over British Columbia will drop
     southward during the day today to deepen the trough.  Cooler air aloft witht his
     disturbance will allow mixing heights to approach 5000 feet by this afternoon.  
     
     The morning ODF surface analysis shows high pressure offshore and low
     pressure centered in eastern Washington with a trough extending into
     north-central Oregon.  The California thermal trough appears quite week this
     morning.  Pressure gradients as of 8:00am incluced: Newport to Salem, 1.1 mb
     onshore and Salem to Redmond, 3.5 mb onshore.  Thus there is gradient stacking
     to the east.  However, any clearing in the Willamette Valley will warm
     temperatures and lower pressures evening out the pressure gradients. We will
     monitor pressure gradients through the day.  

     Computer model forecasts indicate a northwesterly transport wind this
     afternoon, likely coming around enough to allow smoke evacuation to the
     east of the valley.  The wind pattern is such that the Santiam River
     Canyon will be vulnerable to smoke, however.

     Models also indicate that winds will become more northerly during the early
     evening hours suggesting that the window of opportunity will close by about
     5:00pm. 
 
     Expect clouds this morning, partial clearing this afternoon. Salem's high
     temperature today will be near 79.  Relative humidity
     drops to 50% by 11am.  Minimum relative humidity 35%.

     Sunset tonight: 8:22 pm.	
							
										
THREE-HOURLY DATA:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 66       76      79      71  			
     Relative Humidity:           58%      39%     35%     46%     			
     Surface Wind Direction:     360      320     310     290 		
     Surface Wind Speed:           6        7       9       8    			
     Transport Wind Direction:   350      320     310     300 
     Transport Wind Speed:         3        5       7      10  
     Estimated Mixing Height:   2900     4100    4900    1000
     Ventilation Index:            9       20      34      10

                               				
										
EXTENDED DISCUSSION:

     The upper level disturbance dropping southward from BC today will carve out an
     upper level low over the Pacific Northwest by tonight.  This low could produce
     some showers on Tuesday, wetting fields and eleminating any burn opportunity. 
     Wednesday, however, the low should drift east and there is potential for some
     burning during the afternoon.  As the low continues to drift eastward a strong
     upper high noses in from the Pacific Ocean and into southern BC giving
     northeasterly flow aloft and much warmer temperatures for the end of the week
     and the weekend.  

EXTENDED FORECAST:	
     
     Tuesday:  Partly sunny but a chance for showers. High 73. Wind NNW 5-8. 
     Wednesday: Partly sunny, high 79. Afternoon winds NW 4-7. Possible burn   
        opportunity.   
     Thursday: Mostly sunny, with a high near 87.   
     Friday: Mostly sunny, with a high near 89.  
     Saturday: Sunny, with a high near 92.  
     Sunday: Sunny, with a high near 95.  
     										
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.