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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>9:00 AM PDT SAT SEP 25, 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 not allowed. <o:p></o:p></p>
<p class=MsoNormal> Propane flaming is not allowed. <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>An upper-level ridge has shifted east, to over Idaho, with
increasing southwesterly flow aloft over Oregon this morning. Satellite
imagery shows only patchy fog and low clouds in the Willamette Valley, which
should quickly give way to sunny skies later this morning.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The Salem sounding showed a surface-based temperature
inversion that extended upward to about 1800 feet. That will not likely get
mixed out until temperatures climb into the mid 70s early this afternoon. The
air aloft is quite warm, so look for valley temperatures to climb at least into
the low 80s by late this afternoon.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The ODA surface analysis showed weak southeasterly pressure
gradients this morning over western Oregon with valley winds generally 5 mph or
less. As a weakening offshore cold front moves closer to the coastline today,
southerly surface and transport winds are forecast to turn more southwesterly,
which may provide adequate ventilation for limited open burning later this
afternoon.<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>Patchy morning fog, then sunny and warmer. After reaching
76 degrees on Friday, today’s high will top out near 83. The mixing
height will rise to 3000 feet around 2 p.m., top out near 4000 feet around 5
p.m., then quickly drop to 1500 feet by 8 p.m.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Surface winds S 3-8 mph this morning, becoming SW 5-10 mph
this afternoon.<o:p></o:p></p>
<p class=MsoNormal>Transport winds S 5 mph this morning, veering to SW 12-18
mph this afternoon.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Relative humidity will drop to 50% around noon and bottom
out near 35% around 5 p.m. The ventilation index will climb to near 70 late
this afternoon.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Silverton area sunset tonight: 6:58 p.m. <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> 11am 2pm 5pm
8pm <o:p></o:p></p>
<p class=MsoNormal> Temperature: 70 80 82
71 <o:p></o:p></p>
<p class=MsoNormal> Relative Humidity: 55% 38% 34%
53% <o:p></o:p></p>
<p class=MsoNormal> Surface Wind Direction: 170 220 230
250 <o:p></o:p></p>
<p class=MsoNormal> Surface Wind Speed: 7 5 6
6 <o:p></o:p></p>
<p class=MsoNormal> Transport Wind Direction: 180 200 230
240 <o:p></o:p></p>
<p class=MsoNormal> Transport Wind Speed: 5 12 18
15 <o:p></o:p></p>
<p class=MsoNormal> Estimated Mixing Height: 1500 2800 4000
1500<o:p></o:p></p>
<p class=MsoNormal> Ventilation Index: 8 34 72
22<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 weak cold front is forecast to move onshore Sunday
morning, with light rain likely along the coast and in the northern Willamette
Valley. There is a good chance of rain as far south as the Silverton Hills
region. <o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The upper-level ridge is forecast to rebuild over the region
Monday, with sunshine and warm temperatures helping to dry any damp fields.
Another weak system may turn transport winds onshore and create an open burning
opportunity next Tuesday. The ridge is then forecast to strongly build over
the state for the second half of next week, with above normal temperatures.
The next burning opportunity appears to be next Friday.<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> Sunday: Mostly cloudy. Chance of light rain. High near
74. Wind SW 5-12.<o:p></o:p></p>
<p class=MsoNormal> Monday: Areas of morning fog…mostly sunny. High
near 80. Light wind. <o:p></o:p></p>
<p class=MsoNormal> Tuesday: Increasing clouds. Chance of light rain
late. High near 75. <o:p></o:p></p>
<p class=MsoNormal> Wednesday: Partly sunny. High near 78.<o:p></o:p></p>
<p class=MsoNormal> Thursday: Mostly sunny and warm. High near 80.<o:p></o:p></p>
<p class=MsoNormal> Friday: Increasing clouds. High near 78. <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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