[willamette-fcst] Silverton Hills Forecast - Tue, Jul 27 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE JUL 27 2010

BURN ADVISORY:

     Agricultural burning is not recommended.
     Prep burning is not allowed. 
     Propane flaming is allowed from 2:00 p.m. until 5:00 p.m.		
										
WEATHER DISCUSSION:

A weak upper-level trough, just offshore, will bring light south-southwesterly flow aloft
to western Oregon today.  Weak onshore flow at the surface brought marine clouds into the
coastal range gaps this morning, with some low clouds making up the Columbia River into
the extreme north valley.  Otherwise skies were sunny over the Willamette Valley at
mid-morning, but temperatures were generally about 5 degrees cooler than 24 hours ago.

The surface map shows weak but even onshore flow across western Oregon, so
gradient-stacking should not be an issue today.  Weather conditions and transport winds
will need to be monitored today for the slight possibility of open burning later this
afternoon.  A strong southerly component to the upper-level winds will act to suppress
mixing heights. In addition, onshore flow may be too weak to permit open burning. 
Afternoon thunderstorm development is likely again over central Oregon, which would also
act to inhibit smoke evacuation over the Cascades.
 
TODAY’S FORECAST:

Sunny but not quite as warm.  After reaching 91 degrees on Monday, Salem's high
temperature today will be near 88 degrees.  The mixing height will not climb to 3000 feet
until around 2 p.m. and will likely top out just below 4000 feet late this afternoon. 
Surface and transport winds will be NW 3-8 this morning and NW 5-10 this aftenroon.
Relative humidity will drop to 50% by noon and bottom out near 30% late this afternoon. 
The ventilation index will only climb to about 21.

Salem's sunset tonight: 8:44 pm	
										
THREE-HOURLY DATA:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 71       81      86      77  			
     Relative Humidity:           51%      37%     30%     41%     			
     Surface Wind Direction:     330      330     330     280 		
     Surface Wind Speed:           3        6       7       7    			
     Transport Wind Direction:   300      330     330     300 
     Transport Wind Speed:         4        5       6       7  
     Estimated Mixing Height:   2000     3000    3500    1500
     Ventilation Index:            8       15      21      10			
										
EXTENDED DISCUSSION:

A weak upper-level trough will slightly increase the southwesterly flow aloft over Oregon
Wednesday and Thursday.  That will further increase the onshore flow at the surface and
cool high temperatures a couple of more degrees each day.  The onshore flow may still be
too weak to allow for open burning but will need to be monitored closely.  The upper-level
trough is forecast to finally move onshore Friday, which may generate enough onshore flow
t bring morning marine clouds into the Willamette Valley.  That will cool temperatures
back to near normal and could also create an open burning opportunity.

The upper-level trough is forecast to move across Oregon over the weekend with the flow
aloft turning northwesterly by Sunday.  That will continue to bring onshore flow and
near-normal temperatures to the Willamette Valley.  Another weak upper-level trough may
move into the region early next week, with little change in the overall weather
conditions.

EXTENDED FORECAST:	
     
Wednesday: Mostly sunny.  A touch cooler.  Wind: NW 5-10 mph.  52/85
Thursday: Mostly sunny. A little cooler. Wind: NW 5-10 mph.  52/83
Friday: Morning clouds, then sunny.  Wind:  NW 5-15 mph.  50/82
Saturday: Morning clouds, then sunny.  50/80  
Sunday: Morning clouds, then sunny. 50/83  
Monday: Morning clouds, then sunny. 52/83  
Tuesday: Morning clouds, then sunny. 52/82  

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