[willamette-fcst] Silverton Hills Forecast - Wed, Sep 29 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED SEP 29, 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 1:00pm to 5:00pm.
     Prep burning is not allowed. 
     Propane flaming is not allowed.		
										
WEATHER DISCUSSION:

A strong upper-level ridge of high pressure remains over the Pacific Northwest this
morning.  Satellite imagery shows low clouds continuing to blanket much of the Willamette
Valley, and extending south into Douglas County, at midday.  Skies were sunny across the
remainder of the state.  

Even with some sunshine later this afternoon, warm air aloft will cap mixing heights below
3000 feet.  A surface thermal trough is building into southwestern Oregon, which is
resulting in northeasterly transport winds.  This pattern is not conducive to open
burning. 

TODAY’S FORECAST:

Low clouds will slowly give way to sunshine this afternoon with temperatures climbing to
near 80 degrees.  The mixing height will remain below 2000 feet until surface temperatures
warm into the mid 70s.  The maximum mixing height will likely remain just below 3000 feet.
 Surface and transport winds will be N-NE 8-14 mph this afternoon.

Relative humidity will drop to 50% around 3 p.m. and bottom out near 46% around 5 p.m. 
The ventilation index will climb to near 34 this afternoon.

Silverton area sunset tonight: 6:50 p.m.	
										
THREE-HOURLY DATA:		

                                 2pm     5pm     8pm			
     Temperature:                 72      79      71  			
     Relative Humidity:           57%     46%     57%     			
     Surface Wind Direction:     010     030     030 		
     Surface Wind Speed:           8       9       4    			
     Transport Wind Direction:   010     010     020 
     Transport Wind Speed:        10      12       8  
     Estimated Mixing Height:   2000    2800    1000
     Ventilation Index:           20      34       8
     										
EXTENDED DISCUSSION:

The upper-level ridge is forecast to shift eastward, to over eastern Oregon and western
Idaho, on Thursday.  The surface thermal trough will likely shift into the Willamette
Valley.  Ventilation conditions are expected to be poor with light northerly transport
winds and low mixing heights.  By Friday, the thermal trough is forecast to progress east
of the Cascades with transport winds possibly turning enough onshore to create a burning
opportunity.  Weak onshore flow is forecast for Saturday with a weather system
significantly increasing the onshore flow and possibly bringing some rainfall Sunday.

EXTENDED FORECAST:	
     
     Thursday: Sunny, with a high near 84. Wind: N 5-10 mph. 
     Friday: Patchy morning fog, then mostly sunny.  High near 78.
     Saturday: Mostly sunny, with a high near 77.  
     Sunday: Increasing clouds.  Chance of rain late.  High near 74.  
     Monday: A chance of showers. Mostly cloudy, with a high near 72.  
     Tuesday: 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.

Pete Parsons
ODF Meteorologist








jlwx:290910:1157