[willamette-fcst] Silverton Hills Forecast - Wed, Jul 21 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED JUL 21 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 12:00pm to
     7:00pm.
     Prep burning is allowed from 12:00pm to 2:00pm with a 50 acre limit. 
     Propane flaming is allowed from 12:00pm to 5:00pm.		
										
WEATHER DISCUSSION:

A weak upper-level trough is moving across Washington and Oregon this morning and
maintaining the onshore flow into the Willamette Valley.  Visible satellite imagery shows
marine low clouds blanketing the coast and most of the Willamette Valley with some breaks
in the clouds over the coast range.  The marine clouds should give way to sunny skies this
afternoon.  In the wake of the weak trough, a warming northwesterly flow aloft will help
temperatures recover into the low 80s.

The surface map shows some minor gradient-stacking across western Oregon this morning with
1.2 mb onshore from Newport to Salem and 3.6 mb onshore from Salem to Redmond.  Daytime
heating may be sufficient to balance out the onshore flow but transport winds will likely
remain too northerly for open burning.

TODAY’S FORECAST:

Morning clouds…becoming sunny and a little warmer this afternoon.  After reaching 78
degrees on Tuesday, Salem's high temperature today will be near 83 degrees.  The mixing
height will climb to near 3000 feet in the early afternoon with a maximum near 3800 feet
around 5 p.m.  Surface winds will be north at 5-10 mph this morning, becoming NW 5-12 mph
this afternoon and evening.  Transport winds will be NNW at 7 mph this morning, becoming
NNW at 10 mph this afternoon and NW at 10 mph this evening. Relative humidity will drop to
50% by noon and to near 35% by late this afternoon.

Salem's sunset tonight: 8:51 pm	
										
THREE-HOURLY DATA:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 67       77      83      77  			
     Relative Humidity:           56%      42%     34%     43%     			
     Surface Wind Direction:     360      350     340     300 		
     Surface Wind Speed:           5        6       8      10    			
     Transport Wind Direction:   350      350     350     330 
     Transport Wind Speed:         7       10      10      10  
     Estimated Mixing Height:   2200     3200    3800    1500
     Ventilation Index:           15       32      38      15				
										
EXTENDED DISCUSSION:

Another weak upper-level trough will maintain a typical summertime onshore flow pattern
Thursday.  Depending on the timing of the trough, there could be an open burning
opportunity Thursday afternoon, if any fields are ready.  A stronger ridge of high
pressure is forecast to build over the region Friday and Saturday.  Transport winds will
turn northerly and perhaps slightly offshore with temperatures climbing to well above
normal.  The upper-level ridge is forecast to slide east of the region by Sunday with
increasing south-southwesterly flow aloft introducing a chance of showers or afternoon
thunderstorms to much of the state…mainly south and east of the Willamette Valley.

Increasing southwesterly flow aloft is forecast to eventually turn transport winds onshore
early next week, which could present an open burning opportunity.  However, if the flow
aloft becomes too southerly, showers and thunderstorms may also may their way northward
into the Willamette Valley.

EXTENDED FORECAST:	
     
Thursday: Morning clouds with afternoon clearing.  Wind: NNW 5-15 mph.  53/80
Friday: Sunny and turning much warmer.  Wind: N 5-15 mph.  50/90
Saturday: Sunny and very warm.  56/95  
Sunday: Mostly sunny. Very warm. Chance of t-storms near the Cascades. 58/92  
Monday: Partly cloudy.  Chance of showers or t-storms.  58/86  
Tuesday: Partly cloudy.  Chance of showers or t-storms.  55/83  
Wednesday: Partly cloudy.  Chance of showers or t-storms.  54/82  
										
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