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<div>SILVERTON HILLS FIELD BURNING FORECAST</div>
<div>OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE</div>
<div>9:00 AM PDT WED SEP 22 2010</div>
<div> </div>
<div>BURN ADVISORY:</div>
<div> </div>
<div> Recommended burn times for agricultural burning are from 1:00pm to 5:00pm.</div>
<div> Prep burning is allowed from noon to 5:00pm with a 50 acre limit. </div>
<div> Propane flaming is allowed from noon to 5:00pm. </div>
<div> </div>
<div> </div>
<div>WEATHER DISCUSSION:</div>
<div> </div>
<div> The Willamette Valley is between an upper level trough to the south and an upper</div>
<div> ridge to the north in an area of light winds. Satellite pictures show clouds</div>
<div> covering most of western Oregon from the coast to the Cascade crest, but the</div>
<div> cloud deck is quite high, quite thin, and should partially break up this</div>
<div> afternoon. </div>
<div> </div>
<div> The morning Salem sounding was very similar to yesterday morning's sounding.</div>
<div> There was a minor inversion between about 1900 and 2500 feet. This will easily</div>
<div> mix out by late morning. There is also an unusually persistent inversion aloft</div>
<div> with a base that has been near 7500 feet for the past couple of days. Today that</div>
<div> inversion is between about 7800 feet and 8100 feet. Below this feature winds are</div>
<div> light and favor northerly, above winds are westerly.</div>
<div> </div>
<div> Computer models indicate that lower level winds will shift to more westerly</div>
<div> today. Gradient stacking to the east may be an issue, but smoke trajectory</div>
<div> forecasts from the Air Resources Lab based on last night's NAM run indicate a</div>
<div> favorable easterly direction for smoke movement later this afternoon.</div>
<div> </div>
<div> Gradient stacking to the east will be an issue today. Pressure gradients as of</div>
<div> 8am included: Newport to Salem, 1.0 mb onshore; Salem to Redmond, 0.4 mb</div>
<div> offshore. Computer models indicate an increasing onshore gradient. We will</div>
<div> monitor the pressure pattern to see if it becomes favorable this afternoon.</div>
<div> </div>
<div> For the mid Willamette Valley expect mostly cloudy skies this morning, partly</div>
<div> cloudy skies this afternoon. Temperatures will be a bit below average for this</div>
<div> time of year and winds will switch to light westerly.</div>
<div> </div>
<div> Salem's high temperature today will be near 70.</div>
<div> Relative humidity drops to 50% by 12pm. Minimum relative humidity 41%.</div>
<div> </div>
<div> Sunset tonight: 7:04 pm </div>
<div> </div>
<div> </div>
<div>THREE-HOURLY DATA: </div>
<div> </div>
<div> 11am 2pm 5pm 8pm </div>
<div> Temperature: 64 70 70 61 </div>
<div> Relative Humidity: 60% 44% 41% 60% </div>
<div> Surface Wind Direction: 320 260 260 260 </div>
<div> Surface Wind Speed: 2 4 5 5 </div>
<div> Transport Wind Direction: 270 240 240 290 </div>
<div> Transport Wind Speed: 3 6 6 7 </div>
<div> Estimated Mixing Height: 2400 7800 7800 1000</div>
<div> Ventilation Index: 7 47 47 7</div>
<div> </div>
<div> </div>
<div> </div>
<div>EXTENDED DISCUSSION:</div>
<div> </div>
<div> Computer models continue to bounce back and fourth on whether tomorrow's weak</div>
<div> weather front will produce any rain this far south. Any rainfall would be light,</div>
<div> but there is a significant possibility that all rain from the system will stay</div>
<div> to the north. Models are in agreement on a ridge for Friday and Saturday that</div>
<div> will bring sunshine and warm temperatures to the region. </div>
<div> </div>
<div> Models are also coming into line and showing a strong low in the Gulf of Alaska</div>
<div> late in the weekend and into the first part of next week. But the low will be</div>
<div> far enough off the coast that moisture rotating around it will stay well west of</div>
<div> the Pacific Northwest for fair weather and temperatures near seasonal average</div>
<div> into early next week.</div>
<div> </div>
<div> </div>
<div>EXTENDED FORECAST: </div>
<div> </div>
<div> Thursday: Mostly cloudy, chance of rain 66. SSW 5-10. </div>
<div> Friday: Mostly sunny, with a high near 77. S 5.</div>
<div> Saturday: Mostly sunny, with a high near 80. W 5. </div>
<div> Sunday: Partly sunny, with a high near 73. W 4. </div>
<div> Monday: Partly sunny, with a high near 78. </div>
<div> Tuesday: Partly sunny, with a high near 75. </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>
<div> </div>
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