[willamette-fcst] Silverton Hills Forecast - Tue, Jul 13 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE JUL 13 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 10:00am to 7:00pm.
     Prep burning is allowed from 11:00am to 5:00pm with a 50 acre limit. 
     Propane flaming is allowed from 11:00am to 5:00pm.	
										
WEATHER DISCUSSION:

     A fairly deep upper level low is moving eastward across southwest Canada today.
     A trailing upper level trough will sweep across Washington and Oregon during the
     day.  The associated cooler air mass will keep the mid Willamette Valley cooler 
     than average for this time of year by several degrees.

     The morning Salem sounding showed a shallow, surface based inversion that should 
     be gone by now and another, stronger inversion aloft from about 4500 to 6300 feet.
     This inversion will not break and the maximum mixing height today will be about 
     5300 feet. While the morning sounding showed temperatures in the lower atmosphere
     had cooled another 5 degrees or so since yesterday morning, increased sunshine 
     today should push temperatures a couple of degrees warmer than yesterday by
     afternoon.  Salem's high temperature today will be near 76.  Relative humidity 
     drops to 50% by 11am.  Minimum relative humidity 33%.

     The ODF morning surface analysis showed high pressure nosing into NW Oregon and
     a well developed thermal trough in the central valley of California.  Gradients 
     were rather flat west to east across the valley, but there is a strong north to
     south gradient toward the thermal trough.  This will give an unfavorable 
     northerly wind to the region throughout the day.  Both the RUC and NAM model
     trajectory forecasts show any smoke from the Silverton Hills region headed toward
     central Lane County.  Open field burning opportunities are not likely this   
     afternoon.  

     Sunset tonight: 8:52 pm	
										
THREE-HOURLY DATA:		
                                 11am     2pm     5pm     8pm			
     Temperature:                 64       71      75      73  			
     Relative Humidity:           52%      39%     34%     38%     			
     Surface Wind Direction:     360      340     350     330 		
     Surface Wind Speed:           9        9      10      10    			
     Transport Wind Direction:   360      010     010     350 
     Transport Wind Speed:         5        7       9       4  
     Estimated Mixing Height:   4800     5000    5300    1000
     Ventilation Index:           24       35      48       4
										
EXTENDED DISCUSSION:

     Weak ridging aloft will develop on Wednesday, replaced by a weak trough aloft late
     in the week.  The net effect should be almost nil in the Willamette Valley with
     mostly sunny weather and temperatures a few degrees above average.  Transport winds
     will be generally northerly with occasional shifts to northwesterly in the 
     afternoon but prospects for favorable transport winds are small through the
     weekend.  Long range charts do show more westerly flow in the transport region 
     next Monday.  

EXTENDED FORECAST:	
     
     Wednesday: Sunny and warmer. 84. Calm wind becoming N 8-12 mph. 
     Wednesday Night: Mostly clear, low 52.
     Thursday: Sunny. 86. Wind becoming NW 6-9 mph.  
     Friday: Sunny, with a high near 86.    
     Saturday: Sunny, with a high near 87.    
     Sunday: Sunny, with a high near 85.  
     Monday: Sunny, with a high near 85.  
     										
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.