[willamette-fcst] Silverton Hills Forecast - Thu, Sep 30 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT THU SEP 30, 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is not allowed. 
     Propane flaming is not allowed.		
										
WEATHER DISCUSSION:

The strong upper-level ridge of high pressure over the Pacific Northwest is slowly
shifting eastward.  The ridge axis is forecast extend from northern Nevada, through
eastern Oregon and Washington, into eastern British Columbia by this afternoon.  

The ODA surface analysis showed a strong thermal trough extending northward along the
Oregon coast with light offshore flow across western Oregon.  Satellite imagery showed
some low clouds over mainly the southern Willamette Valley, with far less extensive
coverage than yesterday morning.  Only patchy low clouds were evident in the Silverton
Hills area.  Otherwise, skies are generally sunny across the state this morning.

The Salem sounding this morning showed further warming aloft.  Mixing heights will be
greatly suppressed today, likely topping out at or below 2500 feet this afternoon. 
Low-level winds continue to be northeasterly with weak southwesterly flow aloft.  That
pattern is not conducive to open burning. 

TODAY’S FORECAST:

Patchy morning low clouds, then sunny and unseasonably warm.  After reaching 76 degrees on
Wednesday, more sunshine will help today’s high temperature top out near 85.  The mixing
height will rise to only about 2300 feet this afternoon, before quickly dropping below
1000 feet by 8 p.m.

Surface and transport winds N to NE 5-10 mph, becoming light this evening.

Relative humidity will drop to 50% around 1 p.m. and bottom out near 37% around 5 p.m. 
The ventilation index will climb to only 16 this afternoon.

Silverton area sunset tonight: 6:49 p.m.	
										
THREE-HOURLY DATA:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 69       81      85      76  			
     Relative Humidity:           63%      44%     37%     50%     			
     Surface Wind Direction:     020      360     020     280 		
     Surface Wind Speed:           5        7       7       3    			
     Transport Wind Direction:   030      010     010     290 
     Transport Wind Speed:         8        7       7       4  
     Estimated Mixing Height:   1700     2100    2300     500
     Ventilation Index:           14       15      16       2
     										
EXTENDED DISCUSSION:

The upper-level ridge will continue to shift eastward and weaken over the next few days. 
By tomorrow, increasing southwesterly flow aloft is forecast to shift the thermal trough
east of the Cascades.  That will turn transport winds onshore and cool the air aloft
slightly.  South-southwesterly transport winds and slightly higher mixing heights could
provide adequate ventilation for limited open burning by Friday afternoon.

Low-level transport winds are forecast to become north-northwesterly on Saturday, before
turning strongly onshore Sunday.  A cold front will likely bring some showers into the
region Sunday night and Monday.  The ridge is forecast to rebuild over the region Tuesday
and Wednesday with dry weather and mostly northerly transport winds.

EXTENDED FORECAST:	
     
     Friday: Partly sunny with patchy AM fog.  High near 79. Wind SW 5-10.
     Saturday: Morning clouds, then partly sunny.  High near 75.  
     Sunday: Increasing clouds.  Chance of rain late.  High near 72.  
     Monday: Light rain turning to showers.  High near 67.  
     Tuesday: Partly sunny, with a high near 72.  
     Wednesday: Mostly 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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