[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
9:00 AM PDT WED SEP 29, 2010

…Corrected wording in Today’s forecast…

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 covering the western valleys of Oregon and
along much of the coast.  Otherwise, skies are generally sunny across the state.  The
morning low clouds should clear from the Willamette Valley this afternoon with
temperatures climbing into the low 80s.

The Salem sounding this morning showed northeasterly winds below about 3000 feet with weak
westerly flow aloft.  Warm air aloft will cap mixing heights near or below 3000 feet this
afternoon.  A surface thermal trough is forecast to slowly build into southwestern Oregon
today, which will maintain northeasterly transport winds.  That pattern is not conducive
to open burning. 

TODAY’S FORECAST:

Morning low clouds, then mostly sunny and warm.  After reaching 83 degrees on Tuesday,
today’s high will top out near 81.  The mixing height will rise to only about 2800 feet
this afternoon, then drop to around 1000 feet by 8 p.m.

Surface winds N to NE 5-10 mph.
Transport winds NE near 10 mph.

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:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 67       76      81      72  			
     Relative Humidity:           73%      56%     46%     61%     			
     Surface Wind Direction:     010      010     030     050 		
     Surface Wind Speed:           7        8       9       4    			
     Transport Wind Direction:   030      020     010     020 
     Transport Wind Speed:        10       10      12       8  
     Estimated Mixing Height:   1800     2800    2800    1000
     Ventilation Index:           18       28      34       8
     										
EXTENDED DISCUSSION:

The upper-level ridge is forecast to shift eastward, to over Idaho, on Thursday, with
slowly increasing southwesterly flow aloft.  The surface thermal trough will likely shift
inland, by the afternoon, with transport winds becoming northerly.  It is unlikely that
winds will turn enough onshore to allow for open burning Thursday afternoon.  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.  The first significant
weather system is not forecast to come onshore until 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








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