[willamette-fcst] Silverton Hills Forecast - Fri, Sep 24 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT FRI SEP 24, 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is allowed from 12:00pm to 3:00pm with a 50 acre limit. 
     Propane flaming is allowed from 12:00pm to 5:00pm.		
										
WEATHER DISCUSSION:

A ridge of high pressure is building over the west coast.  Considerable low-level moisture
is left over from the warm front that passed through the region Thursday.  Mostly cloudy
skies cover northwestern Oregon with patchy fog in the Willamette Valley this morning. 
Skies should begin to clear this afternoon with temperatures warming to near normal. 
Light south-southwesterly transport winds will need to be monitored for the possibility of
limited open burning this afternoon.

TODAY’S FORECAST:

Mostly cloudy this morning with areas of fog.  Partly sunny this afternoon and much
warmer.  After reaching only 61 degrees on Thursday, today’s high will top out near 75. 
The mixing height should climb to around 3000 feet between 2 p.m. and 5 p.m. Surface winds
will be light southerly with transport winds slowly veering to south-southwesterly at less
than 10 mph.  Relative humidity levels will likely drop below 50% around 2 p.m.  The
ventilation index will only climb to about 26 this afternoon.

Silverton area sunset tonight: 7:00pm	
										
THREE-HOURLY DATA:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 62       71      75      71  			
     Relative Humidity:           72%      51%     43%     47%     			
     Surface Wind Direction:     140      210     220     280 		
     Surface Wind Speed:           5        6       6       5    			
     Transport Wind Direction:   180      210     220     280 
     Transport Wind Speed:         5        8       8       4  
     Estimated Mixing Height:   2000     3200    3000    1000
     Ventilation Index:           10       26      24       4
     										
EXTENDED DISCUSSION:

Southwesterly flow aloft will begin to increase Saturday, as the upper-level ridge shifts
east of the state.  An increase in the south-southwesterly transport winds may improve
ventilation enough to allow for limited open burning.  A very weak weather system is
forecast to move onshore Sunday, with a chance of sprinkles or very light rain making it
as far south and east as the Silverton Hills region.  If the area stays dry enough, there
could be an open burning opportunity Sunday afternoon.  The ridge is forecast to rebuild
over the region Monday with another weak system possibly creating an open burning
opportunity next Tuesday.  The ridge is then forecast to strongly build over the state,
with offshore flow possibly pushing temperatures to well above normal by Thursday.

EXTENDED FORECAST:	
     
     Saturday: Mostly sunny and warm. High near 81. Wind SSW 5-10.
     Sunday: Mostly cloudy. Chance of light rain. High near 75. Wind SW 5-10.
     Monday: Areas of morning fog, then mostly sunny. High near 80.  
     Tuesday: Increasing clouds.  Chance of light rain late.  High near 75.  
     Wednesday: Partly sunny.  High near 78.  North winds.
     Thursday: Sunny and very warm.  High near 85.  NE winds.
     Friday: Increasing clouds. 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.

Pete Parsons
ODF Meteorologist








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