[willamette-fcst] Silverton Hills Forecast - Thu, Jul 22 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT THU JUL 22 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:

Another upper-level trough, cutting across southern British Columbia, will maintain a dry
northwesterly flow aloft and moderately strong onshore low-level flow across western
Oregon today.  Visible satellite imagery shows marine low clouds blanketing most of the
coast, but only patchy low clouds have penetrated inland into the northern Willamette
Valley.  Skies were sunny over the remainder of the state.

The surface map shows only very minor gradient-stacking across western Oregon this morning
with 1.9 mb onshore from Newport to Salem and 2.5 mb onshore from Salem to Redmond.  Like
yesterday, daytime heating should balance out the onshore flow, but transport winds will
likely remain too northerly for open burning.  There has been some minor cooling aloft
since Wednesday afternoon, so high temperatures will likely back off a couple of degrees
today.  Sunny skies will prevail across the valley today with highs near 80.

TODAY’S FORECAST:

Patchy morning clouds, then sunny but a touch cooler.  After reaching 82 degrees on
Wednesday, Salem's high temperature today will be near 80 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 and transport winds will be NNW at 4-8 mph this morning, increasing to 8-15 mph
this afternoon and early evening. Relative humidity will drop to 50% by noon and to near
38% by late this afternoon.  The maximum ventilation index will be near 50 today.

Salem's sunset tonight: 8:50 pm	
										
THREE-HOURLY DATA:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 68       76      80      72  			
     Relative Humidity:           59%      45%     38%     46%     			
     Surface Wind Direction:     350      340     330     310 		
     Surface Wind Speed:           6        7      11      12    			
     Transport Wind Direction:   320      340     340     345 
     Transport Wind Speed:         4        9      13      14  
     Estimated Mixing Height:   2200     3500    3800    1500
     Ventilation Index:            9       32      49      21			
										
EXTENDED DISCUSSION:

A stronger ridge of high pressure is forecast to build over the region Friday and
Saturday.  Transport winds will turn northerly Friday, and perhaps slightly offshore
Saturday, 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.  That could present open burning opportunities, if the flow aloft does
not become too southerly and bring thunderstorms into the valley.  The flow is forecast to
turn more westerly by the middle of next week, with temperatures cooling back to near
normal.  That transition could also create open burning opportunities.

EXTENDED FORECAST:	
     
Friday: Sunny and turning much warmer.  Wind: N 5-15 mph.  50/88
Saturday: Sunny and very warm.  Wind: NNE 5-15 mph.  56/94  
Sunday: Mostly sunny. Very warm. Chance of t-storms near the Cascades. 58/92  
Monday: Partly cloudy.  Slight chance of showers or t-storms.  58/86  
Tuesday: Partly cloudy.  Slight chance of showers or t-storms.  55/82  
Wednesday: Morning clouds, then mostly sunny.  53/80
Thursday: Patchy morning clouds, then sunny. 52/84

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