[willamette-fcst] Silverton Hills Forecast - Tue, Sep 28 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT TUE SEP 28 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 1:00pm to 5:00pm.
     Prep burning is allowed from 3:00pm to 5:00pm with a 50 acre limit. 
     Propane flaming is allowed from 2:00pm to 5:00pm.	

WEATHER DISCUSSION:

     A broad ridge of high pressure in the upper atmosphere covers much of the
     western United States today. The Pacific Northwest is on the edge of this ridge
     with a southwesterly flow aloft. That southwesterly flow is pushing some
     cloudiness into western Washington and portions of northwest Oregon. Satelllite
     pictures show the clouds beginning to break up in the Willamette Valley with
     moust cloudiness near the Willamette River. 
     
     The morning Salem sounding showed a light westerly flow at the surface but a
     moderate north-northeasterly flow from about 800 feet to 3000 feet. The
     temperature trace showed a quite stable atmosphere over the Valley. Several
     shallow inversions were present from the surface to about 2500 feet. 
     
     Also there was evidence of subsidence, or sinking air motion, aloft which does
     not allow for good smoke ventilation. 
     
     Latest computer models continue toshow a northerly transport wind today. Smoke
     trajectory forecasts from the Air Resources Lab based on the 12Z NAM and the 15Z
     RUC show smoke from Marion County headed south toward Lane County today. 
     
     It is unlikely that open field burning will occur today. 
     
     Remaining clouds should break to reveal mostly sunny skies this afternoon.
     Salem's high temperature today will be near 82. It will remain quite humid
     today with dewpoint temperatures in the low 60's. Relative humidity drops to
     50% by 3pm. Minimum relative humidity 49%. Sunset tonight: 6:52 pm

									
THREE-HOURLY DATA:		
                                     2pm     5pm     8pm			
     Temperature:                     76      79      71  			
     Relative Humidity:               71%     58%     73%     			
     Surface Wind Direction:         010     020     350 		
     Surface Wind Speed:               5       6       5    			
     Transport Wind Direction:       360     360     020 
     Transport Wind Speed:             6       6       5  
     Estimated Mixing Height:       2500    3200     500
     Ventilation Index:               15      19       2
										
EXTENDED DISCUSSION:

     The northerly transport wind pattern will continue tomorrow for unfavorable
     burning conditions. An upper disturbance embedded in the southwesterly flow
     aloft will move by to the north late on Thursday which should shift the winds
     over the Willamette Valley. This will need to be monitored for a possible burn
     opportunity, however latest computer models still indicate the wind shift will
     come too late for burning. Friday remains an opportunity.


EXTENDED FORECAST:	
     
     Wednesday: Mostly sunny, with a high near 81. NNE 5-9 mph
     Thursday: Mostly sunny, with a high near 81. Wind Light/Variable becomming SW 3-6
        mph during the evening.    
     Friday: Mostly sunny, with a high near 77.    
     Saturday: Mostly sunny, with a high near 76.  
     Sunday: Partly sunny, with a high near 74.  
     Sunday Night: Mostly cloudy, with a low around 51.  
     Monday: Partly sunny, with a high near 72.  
										
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.








jlwx:280910:1144