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<div>SILVERTON HILLS FIELD BURNING FORECAST</div>
<div>OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE</div>
<div>12:00 PM PDT WED AUG 25 2010</div>
<div> </div>
<div>BURN ADVISORY:</div>
<div> </div>
<div> Agricultural burning is not recommended.</div>
<div> Prep burning is not allowed. </div>
<div> Propane flaming is not allowed. </div>
<div> </div>
<div> </div>
<div>WEATHER DISCUSSION:</div>
<div> </div>
<div> Hot weather again today, but a cooling trend is developing...</div>
<div> </div>
<div> The late morning ODF surface analysis shows a thermal trough from the central</div>
<div> valley of California northward through the Willamette Valley. The trough is</div>
<div> beginning to show signs of laying over in the north end of the Valley prior to a</div>
<div> jump to east of the Cascades. Light west winds have developed through the western</div>
<div> Columbia River Gorge. In addition, temperatures are running mostly a degree or</div>
<div> two cooler in the north end of the valley versus yesterday at the same time. </div>
<div> </div>
<div> The cooling has already kicked in for coastal areas with low clouds and fog</div>
<div> banked up against the coastline and some locations as much as 18 degrees cooler</div>
<div> than yesterday.</div>
<div> </div>
<div> Pressure gradients have increased from the Newport to Salem. As of 11:00am the</div>
<div> difference was 1.3mb onshore. The Redmond to Salem offshore gradient has</div>
<div> decreased to 1.9mb. If present pressure trends continue the thermal trough will</div>
<div> be east of the Cascades by about 6pm tonight. </div>
<div> </div>
<div> The morning Salem sounding showed a series of inversions from the surface to</div>
<div> about 2800 feet, with the temperature at that top of the inversion 18 degrees</div>
<div> warmer than at the bottom. With the strong inversion it will be difficult to get</div>
<div> very high mixing heights. The maximum mixing height will be around 3100 feet at</div>
<div> about 5pm this afternoon.</div>
<div> </div>
<div> Computer models indicate that while the thermal trough will move east of the</div>
<div> valley late this afternoon finding a westerly component to the winds will be</div>
<div> difficult. Latest computer models agree on a transport wind in the 320-330. Much</div>
<div> more of a westerly component is necessary for optimum burning conditions. </div>
<div> </div>
<div> Pibals through the afternoon will determine if the models are correct or if</div>
<div> a burning opportunity develops ahead of our transition to cooler</div>
<div> weather. </div>
<div> </div>
<div> For the mid Willamette Valley expect sunny and hot weather this afternoon.</div>
<div> Salem's high temperature will be near 94. Relative humidity will drop to 20%.</div>
<div> </div>
<div> Sunset tonight: 7:56 pm</div>
<div> </div>
<div>THREE-HOURLY DATA: </div>
<div> </div>
<div> 2pm 5pm 8pm </div>
<div> Temperature: 91 97 83 </div>
<div> Relative Humidity: 22% 18% 33% </div>
<div> Surface Wind Direction: 360 340 270 </div>
<div> Surface Wind Speed: 7 7 7 </div>
<div> Transport Wind Direction: 340 340 290 </div>
<div> Transport Wind Speed: 5 5 5 </div>
<div> Estimated Mixing Height: 2200 3100 500</div>
<div> Ventilation Index: 11 16 2</div>
<div> </div>
<div>EXTENDED DISCUSSION:</div>
<div> </div>
<div> An upper level low in the Gulf of Alaska will drop southeastward and settle in</div>
<div> over the Pacific Northwest through the weekend. This means cooler than average</div>
<div> temperatures and some unsettled weather. There is a cold front extending</div>
<div> southwestward from southern British Columbia today that will sweep across the</div>
<div> region overnight. This is fairly weak, but it will deepen the marine layer</div>
<div> overnight and may produce some drizzle tomorrow morning. </div>
<div> </div>
<div> Low level flow will be onshore through the extended period. Disturbances</div>
<div> rotating through the bottom of the trough will periodically deepen the marine</div>
<div> layer over the valley for possible drizzle or showers. Timing of these</div>
<div> disturbances is in doubt, but right now it looks like Friday will be mainly dry,</div>
<div> with a better chance of precipitation later Saturday and Sunday.</div>
<div> </div>
<div> </div>
<div>EXTENDED FORECAST: </div>
<div> </div>
<div> Thursday: Mostly cloudy, a little morning drizzle. High 79. Winds NW 7-10 mph.</div>
<div> Calm wind becoming north northwest between 7 and 10 mph. </div>
<div> Friday: Partly sunny, 72. NW 5-8.</div>
<div> Saturday: Mostly cloudy, chance of showers. High 70. </div>
<div> Sunday: Mostly cloudy, chance of showers, with a high near 70. </div>
<div> Monday: Mostly cloudy, with a high near 73. </div>
<div> Tuesday: Partly sunny, with a high near 74. </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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