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<div>SILVERTON HILLS FIELD BURNING FORECAST</div>
<div>OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE</div>
<div>12:00 PM PDT TUE AUG 24 2010</div>
<div> </div>
<div>BURN ADVISORY:</div>
<div> </div>
<div> **** Fire Marshal Burn Ban Conditions Likely This Afternoon ****</div>
<div> </div>
<div> Recommended burn times for agricultural burning are from 1:00pm to 4:00pm.</div>
<div> Prep burning is not allowed. </div>
<div> Propane flaming is not allowed. </div>
<div> </div>
<div>WEATHER DISCUSSION:</div>
<div> </div>
<div> In the upper atmosphere a ridge of high pressure covers the Pacific Northwest.</div>
<div> The axis of the ridge runs from NW Alberta, through northeast Oregon to near Las</div>
<div> Vegas. At the surface, our late morning ODF surface analysis shows a thermal</div>
<div> trough from the central valley of California into NW California, then just along</div>
<div> the Oregon and Washington coast. Winds at Newport, for example, have lost their</div>
<div> easterly component and are now northerly, indicating that the thermal trough</div>
<div> axis is likely directly over that location. </div>
<div> </div>
<div> The morning Salem sounding showed dramatic warming at all levels. At about 3500</div>
<div> feet the temperature jumped from 49F yesterday morning to 71F today. Some</div>
<div> additional warming will occur this afternoon. The sounding showed a strong,</div>
<div> subsidence inversion from about 1300 feet to about 2200 feet that will not mix</div>
<div> out this afternoon. Maximum mixing heights today will likely stay below 2000</div>
<div> feet.</div>
<div> </div>
<div> The air mass is very dry and relative humidities will drop into the teens later</div>
<div> today. </div>
<div> </div>
<div> It is likely that Fire Marshal conditions will be met this afternoon. </div>
<div> </div>
<div> We do not expect open field burning this afternoon. </div>
<div> </div>
<div> For the mid-Willamette Valley expect sunny skies and gusty northeast winds.</div>
<div> Salem's high temperature today will be near 95. Minimum relative humidity this</div>
<div> afternoon will be near 15%. Sunset tonight: 7:58 pm</div>
<div> </div>
<div> </div>
<div>THREE-HOURLY DATA: </div>
<div> </div>
<div> 2pm 5pm 8pm </div>
<div> Temperature: 91 97 88 </div>
<div> Relative Humidity: 18% 14% 22% </div>
<div> Surface Wind Direction: 360 010 340 </div>
<div> Surface Wind Speed: 8 10 7 </div>
<div> Transport Wind Direction: 360 350 340 </div>
<div> Transport Wind Speed: 11 12 5 </div>
<div> Estimated Mixing Height: 1900 1900 500</div>
<div> Ventilation Index: 21 23 2</div>
<div> </div>
<div>EXTENDED DISCUSSION:</div>
<div> </div>
<div> The thermal trough will shift inland tomorrow then east of the Cascades late</div>
<div> tomorrow. Timing of this shift will determine if there is a burn opportunity.</div>
<div> Latest computer models continue to suggest that this will be a late afternoon or</div>
<div> early evening event. In addition, the latest NAM model is only marginal on</div>
<div> direction showing a mean transport wind of 310@7 at 5pm tomorrow.</div>
<div> </div>
<div> Thursday will likely see a deep marine layer covering the valley with the</div>
<div> possibility of some drizzle or light rain. This will be the first day after a</div>
<div> marine push - days which generally do not produce good burning conditions.</div>
<div> </div>
<div> A deepening trough, now in the Gulf of Alaska will settle over the area late in</div>
<div> the week and over the weekend. This should result in onshore flow, along with</div>
<div> increased cloud cover and much cooler temperatures. Shortwaves will periodically</div>
<div> swing around the base of the trough, helping to dig the trough further south and</div>
<div> increase the thickness of the marine layer. Models suggest that the potential</div>
<div> exists for a few rain showers over the weekend, but the timing on these would</div>
<div> depend on when the shortwaves pass through.</div>
<div> </div>
<div> </div>
<div>EXTENDED FORECAST: </div>
<div> </div>
<div> Wednesday: Sunny, with a high near 94. E 5-10 mph becoming NW 5-15 late.</div>
<div> Thursday: A chance of morning drizzle then partly sunny, 75. NW 5-10 mph. </div>
<div> Friday: Partly sunny, with a high near 73. </div>
<div> Friday Night: Mostly cloudy, with a low around 46. </div>
<div> Saturday: Partly sunny, with a high near 72. </div>
<div> Sunday: Partly sunny, with a high near 70. </div>
<div> Monday: Mostly cloudy, with a high near 73. </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>
<div> </div>
<div> </div>
<div> </div>
<div> </div>
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