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<div>SILVERTON HILLS FIELD BURNING FORECAST</div>
<div>OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE</div>
<div>12:00 PM PDT THU AUG 26 2010</div>
<div> </div>
<div>BURN ADVISORY:</div>
<div> </div>
<div> Recommended burn times for agricultural burning are from 12:00pm to 6:00pm.</div>
<div> Prep burning is allowed from 12:00pm to 4:00pm with a 50 acre limit. </div>
<div> Propane flaming is allowed from 12:00pm to 5:00pm. </div>
<div> </div>
<div> </div>
<div>WEATHER DISCUSSION:</div>
<div> </div>
<div> Upper level ridge has moved off to the east this morning as an eastern Pacific</div>
<div> trough moves into the state today. Upper level trough will continue to move into</div>
<div> the state bringing cooler temperatures aloft. Axis of trough moves to the coast</div>
<div> late this afternoon and then will broaden and settle over the entire state</div>
<div> overnight tonight. Onshore pressure gradients have developed with the main</div>
<div> gradient stacking over the Cascades and eastward. Gradient stacking has</div>
<div> increased to the east with Newport to Salem at 2.9 mb onshore while Salem to</div>
<div> Redmond is 6.6 mb onshore. Wind flow will increase from the W to NW today to 8</div>
<div> - 14 mph. </div>
<div> </div>
<div> The morning Salem sounding showed decent cooling aloft from yesterday's</div>
<div> sounding. Temperature inversion is minor this morning with temperatures staying</div>
<div> about the same from surface to 5000 ft. Thus the air mass started out quite</div>
<div> stable but with cooling aloft and warming at the surface the air mass is</div>
<div> destabilizing well with mixing heights expected to rise to near 5000 ft by 2</div>
<div> p.m.</div>
<div> </div>
<div> As the upper trough is moving in today, and temperatures are cooling aloft,</div>
<div> clouds are developing over the northern part of the state. This will cap heating</div>
<div> to the lower to mid 70s this afternoon. In addition, W to NW wind flow will be</div>
<div> on the increase. However, with gradient stacking to the east, will have to exercise</div>
<div> caution as to how much can be burned today. </div>
<div> </div>
<div> Mid-Willamette Valley temperatures will peak out at about 74 degrees with the</div>
<div> relative humidity dropping to around 35%.</div>
<div> </div>
<div> </div>
<div> Sunset tonight: 7:54 pm </div>
<div> </div>
<div> </div>
<div>THREE-HOURLY DATA: </div>
<div> </div>
<div> 2pm 5pm 8pm </div>
<div> Temperature: 72 73 64 </div>
<div> Relative Humidity: 41% 35% 46% </div>
<div> Surface Wind Direction: 280 290 290 </div>
<div> Surface Wind Speed: 7 12 11 </div>
<div> Transport Wind Direction: 290 290 300 </div>
<div> Transport Wind Speed: 11 14 12 </div>
<div> Estimated Mixing Height: 4800 5000 2500</div>
<div> Ventilation Index: 53 70 30</div>
<div> </div>
<div> </div>
<div> </div>
<div>EXTENDED DISCUSSION:</div>
<div> </div>
<div> Upper level trough will remain over the state throughout the day tomorrow with</div>
<div> cool, marine air over the region. Expect more marine low clouds over the</div>
<div> Willamette Valley tomorrow than today. Mixing will be mostly good. Gradient</div>
<div> stacking will be more over the Coast Range than the Cascades or at least even.</div>
<div> Winds will be NW to N during the morning turning more NW'erly during the</div>
<div> afternoon. This could produce some decent burning conditions in the afternoon.</div>
<div> </div>
<div> Upper level trough deepens Saturday as another disturbance moves through the</div>
<div> flow. This trough will gradually move to the east later Saturday and the</div>
<div> remainder of the weekend. Mixing heights remain good but wind flow may turn a</div>
<div> little more northerly with the most NW'erly flow occurring during the</div>
<div> afternoon. Another trough moves in Monday for increased cloud cover and a chance</div>
<div> of showers which will then be followed by flat ridging for Tuesday.</div>
<div> </div>
<div>EXTENDED FORECAST: </div>
<div> </div>
<div> Friday: Morning low clouds and afternoon clearing, with a high near 71. Calm wind</div>
<div>during the morning becoming NW'erly between 8 and 12 mph during the afternoon. </div>
<div> Friday Night: Partly to mostly cloudy, with a low around 45. NW wind between 5 and 8</div>
<div> mph becoming calm. </div>
<div> Saturday: Mostly cloudy, with a high near 68. Calm wind becoming northwest</div>
<div> between 7 and 10 mph. </div>
<div> Saturday Night: Partly to mostly cloudy, with a low around 47. </div>
<div> Sunday: Morning low clouds and afternoon clearing, with a high near 70. </div>
<div> Sunday Night: Partly cloudy, with a low around 47. </div>
<div> Monday: Mostly cloudy with a chance of showers, with a high near 67. </div>
<div> Monday Night: Mostly cloudy, with a low around 50. </div>
<div> Tuesday: Morning clouds with afternoon clearing, with a high near 71. </div>
<div> Tuesday Night: Clear, with a low around 48. </div>
<div> </div>
<div> </div>
<div> </div>
<div>Notes: </div>
<div> 1. Mixing height, as used here, is the lowest height at which the</div>
<div> potential temperature exceeds the potential temperature at the surface. </div>
<div> As a practical matter it is the approximate height to which a </div>
<div> smoke plume will rise assuming good ignition, dry fuels, and </div>
<div> winds less than about 15mph. </div>
<div> 2. Transport winds are a layer average through the mixing height, </div>
<div> weighted slightly toward the winds at the top of the layer. </div>
<div> 3. Ventilation Index is the height of the mixing layer times </div>
<div> the transport wind speed divided by 1000. </div>
<div> 4. Surface wind direction is the general expected wind direction. </div>
<div> At a specific point surface winds are highly dependent on </div>
<div> local terrain conditions.</div>
<div> </div>
<div>This forecast is provided under an agreement between the Oregon Department of</div>
<div>Agriculture and the Oregon Department of Forestry. For information contact ODA</div>
<div>at 503-986-4701.</div>
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