[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
12:00 PM PDT FRI SEP 24, 2010

BURN ADVISORY:

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

Residual low-level moisture, from the warm front that passed through the region on
Thursday, led to the formation of widespread low clouds and patchy fog across northwestern
Oregon this morning.  The remainder of the state had mostly sunny skies.

The ODA surface analysis shows very weak southerly pressure gradients across western
Oregon late this morning with Willamette Valley winds generally around 5 mph or less.

An upper-level ridge of high pressure is building over Oregon.  Skies are slowly clearing
over northwestern Oregon late this morning with further clearing expected this afternoon. 
Warming aloft and some sunshine will help high temperatures recover to near-normal this
afternoon.  Light south-southwesterly transport winds may be too light to effectively
evacuate smoke from the valley and will need to be monitored this afternoon.

FORECAST FOR THE REMAINDER OF TODAY:

Partly sunny this afternoon with temperatures climbing into the mid 70s. The mixing height
should only climb to around 3000 feet between 2 p.m. and 5 p.m. Surface winds will be
light southerly with SSW transport winds at less than 10 mph.  Relative humidity levels
will likely drop to 50% around 3 p.m.  The ventilation index will only climb to about 26
this afternoon.

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

                                 2pm     5pm     8pm			
     Temperature:                 70      75      69  			
     Relative Humidity:           53%     43%     49%     			
     Surface Wind Direction:     210     220     280 		
     Surface Wind Speed:           6       6       5    			
     Transport Wind Direction:   210     220     280 
     Transport Wind Speed:         8       8       4  
     Estimated Mixing Height:   3200    3000    1000
     Ventilation Index:           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, late in the afternoon, to allow for limited open burning.

A very weak weather system is forecast to move onshore Sunday, with light rain likely
along the coast and in the extreme northern Willamette Valley.  There is a good chance of
rain as far south as the Silverton Hills region.  If fields stay dry enough for burning,
there is a slight chance that ventilation conditions may be sufficient for limited open
burning Sunday afternoon.

The ridge is forecast to rebuild over the region Monday, which would allow for the drying
of damp fields.  Another weak system may creat 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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