[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 FRI AUG 30, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



Upper-level ridging is in place today, stabilizing the atmosphere with warmer temperatures at all levels and drier conditions taking over. However, recent rains have left fields too wet for burning and this has continued so far today. As of 11:00 a.m. surface relative humidity measurements are still well above 60% over the Silverton Hills. Gradients are very weak.



The stabilization and upper ridging means that local wind patterns are now dominant today over the area. Northerly surface and transport winds are expected this afternoon, with gradients generally weakly favorable. Winds should switch to northwesterly late in the day and conditions will be monitored closely. Surface humidities are expected to decrease significantly by late afternoon.



TODAY'S FORECAST:



Mostly sunny skies this afternoon.



Salem's high temperature today will be near 82 degrees.

Relative humidity:  50-55% by 2 p.m. and 40-45% by 5 p.m..

Surface winds: Becoming NE 5-10 mph, then NW 5-10 late afternoon.

Transport winds: N 5-10 mph, becoming NNW 10 mph late this afternoon.

Mixing height: Rising to 4000-4500 feet by 5 p.m..

Salem's sunset tonight: 7:52 p.m.



(Salem Airport data for Thurs., Aug. 29th: High 80°F and 0.17" rainfall)

(Maximum Ventilation Index expected today: 45)



EXTENDED DISCUSSION:



After some morning cloudiness Saturday, mostly sunny skies are expected for the region through Sunday as upper ridging continues to strengthen. Surface maximum temperatures are expected to be above average both days.



Early next week clouds and moisture will increase as a new trough approaches from the west. Cooler conditions with a chance of showers are expected on Tuesday, with an onshore wind pattern. At this point, an extensive rain event is not expected on 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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