[willamette-fcst] Silverton Hills Forecast - Thu, Sep 23 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT THU SEP 23, 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 11:00am to 5:00pm.
     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 westerly flow aloft was pushing a warm front onshore at mid-morning.  Clouds extended
inland, over the Willamette Valley, to the crest of the Cascades.  Light rain was falling
along the coast, from Astoria to Newport, with radar showing patches of sprinkles making
it over the coastal range and into the Willamette Valley.

The ODA surface analysis showed weak southeasterly pressure gradients across the
Willamette Valley, in response to the approaching warm front.  As the warm front comes
onshore, surface and transport winds should turn more southerly by early this afternoon. 
The warm front is forecast to weaken and lift north of the region by late in the day. 
Surface and transport winds are forecast to continue veering to south-southwesterly by
this evening.  If conditions remain dry enough, then marginally favorable conditions for
open burning could materialize late this afternoon.

TODAY’S FORECAST:

Cloudy with sprinkles or very light rain at times, mainly north of Salem.  After reaching
71 degrees on Wednesday, cloudy skies will hold maximum temperatures in the mid 60s this
afternoon.  The mixing height should climb above 3000 feet by late this afternoon with
southerly surface and transport winds slowly veering to south-southwesterly.  Relative
humidity levels will likely drop below 60%, by around 2 p.m., if the bulk of the light
rain falls mostly north of the district.  The ventilation index should climb to near 50
late this afternoon.

Silverton area sunset tonight: 7:02pm	
										
THREE-HOURLY DATA:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 60       66      66      62  			
     Relative Humidity:           67%      54%     56%     67%     			
     Surface Wind Direction:     170      180     190     220 		
     Surface Wind Speed:           8        7       6       5    			
     Transport Wind Direction:   180      190     210     220 
     Transport Wind Speed:        12       15      15      10  
     Estimated Mixing Height:   1800     2500    3500    1500
     Ventilation Index:           22       38      52      15
     										
EXTENDED DISCUSSION:

Long-range models are continuing to show a ridge of high pressure building over the west
coast beginning Friday.  Skies should begin to clear Friday afternoon with temperatures
warming to near normal.  Winds will become very light, which ventilation conditions
deteriorating. A very weak weather system is forecast to move mainly north of the state
Sunday.  That will increase the southwesterly transport flow over the weekend and improve
ventilation conditions. The ridge is then forecast to rebuild over the region for
continued dry and warm conditions through at least early next week.

EXTENDED FORECAST:	
     
     Friday: Patchy am fog, then mostly sunny and warmer.  High near 78.
     Saturday: Sunny and warm.  High near 83.  
     Sunday: Partly sunny. High near 76.  
     Monday: Mostly sunny.  High near 82.  
     Tuesday: Mostly sunny.  High near 81.  
     Wednesday: Partly sunny.  High near 78.

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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