[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
12:00 PM PDT THU SEP 23, 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from now to 5:00pm.
     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:

A westerly flow aloft was pushing a warm front across the Willamette Valley late this
morning.  Radar and surface reports showed light rain extending southward to about
Corvallis, with a back edge to the rainfall apparent near the coastal range. Rainfall
totals through 11 a.m. were very light in the valley, with just a trace reported at the
Salem Airport and only a couple hundredths of an inch from Aurora north to Portland.  The
main area of rain, associated with the warm, should push east of the Silverton Hills area
by 2 p.m., with some drying likely later this afternoon.  However, much of the region
could pick up a few hundredths of an inch over the hour or two.

The ODA surface analysis showed weak southerly pressure gradients across the Willamette
Valley, but slightly stronger gradients along the coast.  Southerly winds were gusting to
around 20 mph along the north coast, late this morning, and had increased to around 10 mph
in the Willamette Valley.  Southerly winds should increase slightly across the Willamette
Valley this afternoon and are still forecast to turn more southwesterly following the
passage of the weak warm front.  If conditions dry out enough this afternoon, in the wake
of the warm front, transport winds and mixing heights may become marginally favorable for
open burning.

FORECAST FOR THE REMAINDER OF TODAY:

Cloudy skies with light rain likely tapering off after about 2 p.m.  Cloudy skies will
hold maximum temperatures in the low to 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 not likely drop
below 60%.  The ventilation index should climb to near 50 late this afternoon.

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

                                 2pm     5pm     8pm			
     Temperature:                 62      65      62  			
     Relative Humidity:           67%     61%     67%     			
     Surface Wind Direction:     180     190     220 		
     Surface Wind Speed:           7       6       5    			
     Transport Wind Direction:   190     210     220 
     Transport Wind Speed:        15      15      10  
     Estimated Mixing Height:   2500    3500    1500
     Ventilation Index:           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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