[willamette-fcst] Silverton Hills Midday Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT THURS AUG 29, 2013



BURN ADVISORY:



Recommended times for agricultural burning are now through 6 p.m..

Prep burning is not allowed.



WEATHER DISCUSSION:



The strong upper-level trough continues to be entrenched just off Oregon's Pacific coastline today, and this is creating unsettled, showery conditions over the area and that will persist throughout the day. Additional wetting rains have covered large portions of the northern Willamette Valley and the Silverton Hills this morning. Surface pressure gradients are weak at the moment and are not expected to get better than a weakly-favorable level today



Wind profiles will be generally southwesterly through the day over the area with strong winds above 1000-2000 feet. Surface winds, generally weak SE now, will likely shift to more SW-W as we progress through the afternoon. Heating at the surface and mixing heights is limited compared to yesterday, with some periods of partly cloudy skies possible but continued scattered showers and a possible thunderstorm. Favorable conditions for burning are not expected today.





TODAY'S FORECAST:



Mostly cloudy and cooler with scattered showers through the day and possible thunderstorm activity this afternoon and evening. General rainfall amounts will be light, with local areas of moderate amounts.



Salem's high temperature today will be near 75 degrees.

Relative humidity:  60-70% by 2 p.m. and about 55% by 5 p.m..

Surface winds: Becoming SW 5-10 mph early this afternoon and W 5-15 mph late this afternoon.

Transport winds: Becoming SW 20-25 mph this afternoon.

Mixing height: Rising to 3000-3500 feet by 5 p.m..

Salem's sunset tonight: 7:54 p.m.



(Salem Airport data for Wed., Aug. 28th: High 82°F and 0.04" rainfall)

(Maximum Ventilation Index expected today: 65)



EXTENDED DISCUSSION:



The trough flattens and a quieter pattern is in the works for Friday along with drier low-level conditions. More sunshine will help temperatures recover to near 80 late in the day with winds aloft weakening and partly cloudy skies.  Potential burn conditions in the afternoon are expected, if fields can dry sufficiently during the late morning and early afternoon. Dry and warm conditions should persist through the weekend. An approaching trough may bring some moisture back into the region by late Monday or Tuesday.



The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text



Notes:



     1.  Mixing height, as used here, is the lowest height at which the

         potential temperature exceeds the potential temperature at the

         surface.  As a practical matter it is the approximate height to

         which a smoke plume will rise assuming good ignition, dry fuels, and

         winds less than about 15mph.



     2.  Transport winds are a layer average through the mixing height,

         weighted slightly toward the winds at the top of the layer.



     3.  Ventilation Index is the height of the mixing layer (ft) times

         the transport wind speed (mph) divided by 1000.



     4.  Surface wind direction is the general expected wind direction.

         At a specific point surface winds are highly dependent on local

         terrain conditions.



This forecast is provided under an agreement between the Oregon Department of

Agriculture and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.



Doug Wesley

ODF Meteorologist

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