[willamette-fcst] Silverton Hills Forecast - Tue, Aug 17 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT TUE AUG 17 2010

BURN ADVISORY:

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

     A somewhat complicated weather pattern today as we transition out of the hot
     weather that has affected the Willamette Valley the past several days. In the
     upper atmosphere the ridge of high pressure that had been over the area is being
     undercut by a minor disturbance moving down the BC coast. At the same time an
     upper level low pressure area is approaching Oregon from the southwest. Our
     lightning detection network indicates that this low is already producing
     thunderstorms offshore and a narrow band of convection is moving north over the
     Coast Range as of late morning. 

     The ODF surface analysis showes a well developed thermal trough from the central
     Valley of California, into SW Oregon and then into the southern Willamette
     Valley. From there the axis of the trough extends up the east side of the valley
     then bends over to near Hermiston and finally extends northward into eastern
     Washington. Pressures have been falling in Redmond and as of 11am the Newport to
     Salem gradient has risin to 3.6mb onshore, while the Redmond to Salem gradient
     has fallen to just 0.7mb offshore. 
     
     The morning Salem sounding showed several inversion layers. These should all mix
     out up to about 4000 feet this afternoon.

     Temperatures late this morning were running about 5 to 8 degrees warmer than at
     the same time yesterday. 
     
     As the thermal trough continues to shift east we should get a favorable west to
     east pressure pattern for burning. Models indicate a marine push later today.
     Latest model runs are still not converging on a timing estimate for a shift to
     westerly winds. It still looks like sometime late this afternoon. 

     Complicating all of this is the upper low off the southwest coast. As it moves
     closer, some convection is likely over SW Oregon and the southern Cascades. This
     will have to be monitored closely since thunderstorms moving farther
     north...either into the Willamette Valley or over the central Cascades...will
     have an effect on winds and pressure patterns. 
     
     It still looks like a reasonable chance for at least some burning late today,
     but pibals and careful monitoring of weather conditions will determine when.
     Growers should be ready to move quickly if a burn window opens up. 

     Salem's high temperature today will be near 93. Relative humidity drops to 50%
     by 10am. Minimum relative humidity 26%. Sunset tonight: 8:09 pm

								
THREE-HOURLY DATA:		

                                     2pm     5pm     8pm			
     Temperature:                     91      93      76  			
     Relative Humidity:               30%     26%     45%     			
     Surface Wind Direction:         330     290     260 		
     Surface Wind Speed:               6       8       7    			
     Transport Wind Direction:       360     320     280 
     Transport Wind Speed:             5       8      11  
     Estimated Mixing Height:       2500    4000     100
     Ventilation Index:               12      32       1
          				
										
EXTENDED DISCUSSION:

     Temperatures will be much cooler during the coming days. Longer range computer
     models show troughiness over the eastern Pacific Ocean and a cooler
     southwesterly flow aloft. Tomorrow will be the first day after a marine push
     which is generally not a good day to burn. However, westerly winds continue on
     Thursday and if mixing heights cooperate there may be a burn opportunity then.
     
     Later this week and over the weekend we should be generally in a morning cloud,
     afternoon sunshine pattern. 


EXTENDED FORECAST:	
     
     Wednesday: Mostly sunny, with a high near 80. Afternoon winds NW 4-7.
     Thursday: Sunny, with a high near 79. Wind WNW 4-8.   
     Friday: Sunny, with a high near 76.  
     Saturday: Partly sunny, with a high near 74.   
     Sunday: Partly sunny, with a high near 74.    
     Monday: Mostly sunny, with a high near 80.  
     
										
										
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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