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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 FRI AUG 13, 2010<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>…Fire Marshal Conditions are possible this afternoon
and likely both Saturday and Sunday afternoons…<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> Recommended burn times for agricultural burning are
from noon to 6:30pm<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 is building just offshore with a dry
north-northwesterly flow aloft over western Oregon. The surface chart shows a
thermal trough building northward, from southwestern Oregon, into the
Willamette Valley this morning. Satellite imagery shows low clouds along most
of the immediate coastline, where the flow is still weakly onshore. However,
skies are clear just inland and over the entire Willamette Valley.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Gradients are already offshore this morning, from Redmond to
Salem, and expected to increase this afternoon. Down-sloping northeasterly
winds and a full day of August sunshine will warm valley temperatures well into
the 90s, with very low humidity levels. Northeasterly transport winds are not
conducive to good valley ventilation, so the prospect for any burning today is
poor. The focus of today’s discussion is on next week’s weather.
More on that in a moment…<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>Sunny and warmer. After reaching 88 degrees on Thursday,
Salem's high temperature today will be near 93 degrees. The mixing height will
climb to 3000 feet about noon and likely top out near 5000 feet around 5 p.m.
No evening sea breeze is expected this evening, with persistent dry offshore
flow, so surface temperatures will be slow to cool. That will likely keep the
mixing height above 1500 feet until about sunset. Light northerly surface
winds, this morning, will turn north-northeasterly and increase to about 10 mph
this afternoon. NE transport winds will increase to about 15 mph this
afternoon. Relative humidity will drop to 50% by 11 a.m. and to near 21% around
5 p.m. The ventilation index will climb to 80 this afternoon.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Silverton area sunset tonight: 8:16 pm <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: 76 87 93
84 <o:p></o:p></p>
<p class=MsoNormal> Relative Humidity: 45% 29% 21% 29%
<o:p></o:p></p>
<p class=MsoNormal> Surface Wind Direction: 010 010 010
010 <o:p></o:p></p>
<p class=MsoNormal> Surface Wind Speed: 7 8 12
8 <o:p></o:p></p>
<p class=MsoNormal> Transport Wind Direction: 020 030 040
030 <o:p></o:p></p>
<p class=MsoNormal> Transport Wind Speed: 7 14 16
12 <o:p></o:p></p>
<p class=MsoNormal> Estimated Mixing Height: 2200 3800 5000
1500<o:p></o:p></p>
<p class=MsoNormal> Ventilation Index: 15 53 80
18<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 strong upper-level ridge of high pressure will build over
the Pacific Northwest Saturday through Monday. Sunny skies and dry offshore
flow will bring near-record warmth to the Willamette Valley. A weak upper-level
trough is forecast to undercut the ridge and move across northern California on
Sunday, possibly bringing some thunderstorms to the southern Oregon Cascades.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The ridge is forecast to peak over the region on Monday,
with a stronger upper-level trough approaching the southern Oregon Coast by
Tuesday. That system may circulate thundershower activity further north, along
the Cascades, and should initiate a cooling trend. That may create an open
burning opportunity, as early as Tuesday afternoon, with transport winds
forecast to turn back onshore.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The long-range models are beginning to come into agreement
that the upper-level trough will come onshore Wednesday and/or Thursday.
Increasing onshore flow and continued cooling may create an enhanced
opportunity open burning opportunity one or both of those days. Temperatures
will likely cool back to normal by Thursday. Strong onshore flow is forecast
on Friday with considerable marine clouds penetrating into the Willamette
Valley. Gradient-stacking may be too great for open burning by that time. <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>Saturday: Sunny with near-record warmth. NE wind 5-10 mph.
58/97<o:p></o:p></p>
<p class=MsoNormal>Sunday: Sunny with near-record warmth. Light wind. 59/98<o:p></o:p></p>
<p class=MsoNormal>Monday: Sunny with near-record warmth. Light wind. 61/99<o:p></o:p></p>
<p class=MsoNormal>Tuesday: Sunny. A little cooler. Wind possibly becoming W
5-15 mph. 61/94<o:p></o:p></p>
<p class=MsoNormal>Wednesday: Partly cloudy and cooler. Wind W 5-15 mph. 57/87<o:p></o:p></p>
<p class=MsoNormal>Thursday: Partly cloudy. Slight chance of showers. 54/80<o:p></o:p></p>
<p class=MsoNormal>Friday: Morning clouds with afternoon clearing. 51/77<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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