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<p class=MsoNormal>SILVERTON HILLS FIELD BURNING FORECAST<o:p></o:p></p>
<p class=MsoNormal>OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE<o:p></o:p></p>
<p class=MsoNormal>12:00 PM PDT WED SEP 15, 2010<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>BURN ADVISORY:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal> Agricultural burning burning is not recommended.<o:p></o:p></p>
<p class=MsoNormal> Prep burning is allowed from now to 4:00pm. <o:p></o:p></p>
<p class=MsoNormal> Propane flaming is allowed from now to 2:00pm. <o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal>WEATHER DISCUSSION:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>A strengthening southwesterly flow aloft is slowly pushing a
weak autumn-like cold front onshore today. Satellite imagery shows middle and
high clouds spreading across most of Washington and Oregon at midday, with
clouds lowering and thickening along the coastal strip. Radar is showing light
rain slowly advancing to near the coastline, but there were still no reports of
rain, inland over Oregon, as of late this morning. There were some sprinkles
making it into northwestern Washington.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The surface map continued to show weak pressure gradients
across western Oregon late this morning. Southerly winds were beginning to
increase from about Silverton southward across the Willamette Valley. Some
sunbreaks have helped temperatures climb to near 70 degrees.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Clouds should increase this afternoon along with
south-southwesterly winds. Mixing heights will be slow to rise today, due to
warm air aloft and the filtering of sunshine by the clouds. There could be a
brief window of opportunity for open burning this afternoon, if the approaching
light rain holds off until this evening and transport winds take on enough of a
westerly component. It is also possible that surface winds may become too
strong for good plume development later in the afternoon. Monitoring of the
transport winds, via pibals, will begin at noon.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>TODAY’S FORECAST:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Mostly cloudy. A chance of rain by late afternoon with rain
likely this evening. Increasing clouds should hold high temperature in the mid
70s this afternoon. The mixing height will likely only climb to between 3000
and 4000 feet in the mid-afternoon, then drop below 3000 feet by 5 p.m.
Relative humidity will drop to near 50% this afternoon with the ventilation
index climbing to around 42.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Silverton area sunset tonight: 7:17pm <o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal>THREE-HOURLY DATA: <o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal> 2pm 5pm 8pm <o:p></o:p></p>
<p class=MsoNormal> Temperature: 74 74 65 <o:p></o:p></p>
<p class=MsoNormal> Relative Humidity: 51% 51% 73% <o:p></o:p></p>
<p class=MsoNormal> Surface Wind Direction: 190 210 190 <o:p></o:p></p>
<p class=MsoNormal> Surface Wind Speed: 10 12 7 <o:p></o:p></p>
<p class=MsoNormal> Transport Wind Direction: 200 230 200 <o:p></o:p></p>
<p class=MsoNormal> Transport Wind Speed: 12 15 10 <o:p></o:p></p>
<p class=MsoNormal> Estimated Mixing Height: 3500 2500 1000<o:p></o:p></p>
<p class=MsoNormal> Ventilation Index: 42 38 10<o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal>EXTENDED DISCUSSION:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>A transitory ridge should be just strong enough to provide
mostly dry weather on Thursday under weak southwesterly flow aloft. If fields
are dry enough, there is a possibility that transport winds will turn enough
onshore to allow for open burning Thursday afternoon. Another weather system
is slated to come onshore by Friday afternoon. That could also present an
opening burning opportunity, Friday afternoon, if the rain holds off long
enough. Wet weather is forecast this weekend…perhaps lasting into
Monday. Long-range computer models are showing a potentially favorable
pattern for burning, next Tuesday and Wednesday, if conditions dry out enough.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>EXTENDED FORECAST: <o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal> Thursday: Mostly cloudy. Chance of rain. High near
75. <o:p></o:p></p>
<p class=MsoNormal> Friday: Rain likely by the afternoon. High near 72. <o:p></o:p></p>
<p class=MsoNormal> Saturday: Rain likely. High near 70. <o:p></o:p></p>
<p class=MsoNormal> Sunday: Mostly cloudy. Chance of rain. High near 70.
<o:p></o:p></p>
<p class=MsoNormal> Monday: A chance of showers. High near 73. <o:p></o:p></p>
<p class=MsoNormal> Tuesday: Partly sunny. Slight chance of showers. High
near 76.<o:p></o:p></p>
<p class=MsoNormal> Wednesday: Partly sunny. Slight chance of showers.
High near 72.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Notes:<o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal> 1. Mixing height, as used here, is the lowest height
at which the<o:p></o:p></p>
<p class=MsoNormal> potential temperature exceeds the potential
temperature at the<o:p></o:p></p>
<p class=MsoNormal> surface. <o:p></o:p></p>
<p class=MsoNormal> As a practical matter it is the approximate height
to which a <o:p></o:p></p>
<p class=MsoNormal> smoke plume will rise assuming good ignition, dry
fuels, and <o:p></o:p></p>
<p class=MsoNormal> winds less than about 15mph. <o:p></o:p></p>
<p class=MsoNormal> 2. Transport winds are a layer average through the
mixing height, <o:p></o:p></p>
<p class=MsoNormal> weighted slightly toward the winds at the top of
the layer. <o:p></o:p></p>
<p class=MsoNormal> 3. Ventilation Index is the height of the mixing layer
times <o:p></o:p></p>
<p class=MsoNormal> the transport wind speed divided by 1000. <o:p></o:p></p>
<p class=MsoNormal> 4. Surface wind direction is the general expected wind
direction. <o:p></o:p></p>
<p class=MsoNormal> At a specific point surface winds are highly
dependent on local<o:p></o:p></p>
<p class=MsoNormal> terrain conditions.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>This forecast is provided under an agreement between the
Oregon Department of<o:p></o:p></p>
<p class=MsoNormal>Agriculture and the Oregon Department of Forestry. For
information contact ODA at 503-986-4701.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Pete Parsons<o:p></o:p></p>
<p class=MsoNormal>ODF Meteorologist<o:p></o:p></p>
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