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<div>SILVERTON HILLS FIELD BURNING FORECAST</div>
<div>OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE</div>
<div>12:00 PM PDT MON SEP 13 2010</div>
<div> </div>
<div>BURN ADVISORY:</div>
<div> </div>
<div> Agricultural burning burning is not recommended.</div>
<div> Prep burning is allowed from 12:30pm to 5:00pm with a 50 acre limit. </div>
<div> Propane flaming is not allowed. </div>
<div> </div>
<div> </div>
<div>WEATHER DISCUSSION:</div>
<div> </div>
<div> Upper air charts show a weak ridge aloft with a minor embedded disturbance off</div>
<div> the southwest Oregon coast. The low clouds in the Willamette Valley have been</div>
<div> quite persistent through the morning hours. Satellite pictures show a narrow</div>
<div> band of low clouds stretching from portland to about Roseburg. The base of the</div>
<div> cloud deck is at about 1300 feet and pilot reports indicate tops are at about</div>
<div> 2100 feet. We still should see the clouds burn off this afternoon. As of 11:00am</div>
<div> this morning areas under the clouds were running 2-5 degrees cooler than</div>
<div> yesterday. Cloud free areas were 2-5 degrees warmer. The clouds are thin enough</div>
<div> that we still expect sunshine to break through this afternoon.</div>
<div> </div>
<div> The ODF late morning surface analysis shows higher pressure to the north and a</div>
<div> thermal trough in the central Valley of California, extending northward into</div>
<div> northern California. This is giving a northerly gradient to the Willamette</div>
<div> Valley that will likely produce north or north-northeast transport winds</div>
<div> throughout the day. This is not favorable for open field burning.</div>
<div> </div>
<div> The morning Salem sounding showed a slightly warmer airmass this morning from</div>
<div> about 2300 feet to about 8500 feet. There is a small inversion between 1800 and</div>
<div> 2500 feet. This inversion will easily mix out with some afternoon sunshine and</div>
<div> mixng heights today will climb to 3500 feet. The morning Salem sounding also</div>
<div> showed those north-northeast transport winds. </div>
<div> </div>
<div> Wind direction will likely be the major factor in preventing open field burning</div>
<div> this afternoon. </div>
<div> </div>
<div> The upper disturbance off the SW Coast will rotate inland and could touch off a</div>
<div> few showers or thunderstorms over the southern and central Oregon Cascades this</div>
<div> afternoon.</div>
<div> </div>
<div> For the mid Willamette Valley expect skies to turn sunny this</div>
<div> afternoon with light northerly winds. Salem's high temperature today will be</div>
<div> near 78. Minimum relative humidity 42%.</div>
<div> </div>
<div> Sunset tonight: 7:21 pm </div>
<div> </div>
<div> </div>
<div>THREE-HOURLY DATA: </div>
<div> </div>
<div> 11am 2pm 5pm 8pm </div>
<div> Temperature: 69 74 78 72 </div>
<div> Relative Humidity: 61% 48% 42% 51% </div>
<div> Surface Wind Direction: 010 010 010 310 </div>
<div> Surface Wind Speed: 6 5 6 5 </div>
<div> Transport Wind Direction: 360 360 360 360 </div>
<div> Transport Wind Speed: 08 12 13 05 </div>
<div> Estimated Mixing Height: 2200 3100 3500 500</div>
<div> Ventilation Index: 18 37 46 2</div>
<div> </div>
<div> </div>
<div> </div>
<div>EXTENDED DISCUSSION:</div>
<div> </div>
<div> An upper level low will develop off the coast of British Columbia later today</div>
<div> and tomorrow and drift southward to off the Oregon Coast. By later Wednesday</div>
<div> this will edge closer to the coastline and could produce some light rain by</div>
<div> Wednesday night. As this moves over the region the chances of rain will increase</div>
<div> and remain elevated through the weekend. </div>
<div> </div>
<div> Models indicate a more westerly transport flow tomorrow. However, they also</div>
<div> indicate gradient stacking to the east and pressure patterns may not turn</div>
<div> favorable until early evening tomorrow. Even though conditions look only</div>
<div> marginal, tomorrow looks to be the day with the best chance for open field</div>
<div> burning this week, with Wednesday also a slight possibility.</div>
<div> </div>
<div> </div>
<div>EXTENDED FORECAST: </div>
<div> </div>
<div> Tuesday: Mostly sunny, with a high near 80. NW 3-6.</div>
<div> Wednesday: Mostly cloudy, 78. Chance of rain Wednesday evening. SW 6.</div>
<div> Thursday: Chance of rain early then partly sunny, 75. </div>
<div> Friday: A chance of rain. Partly sunny, with a high near 73. </div>
<div> Saturday: A chance of rain. Mostly cloudy, with a high near 70. </div>
<div> Sunday: A chance of rain. Mostly cloudy, with a high near 71. </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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