[willamette-fcst] Silverton Hills AM Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT THU SEP 19, 2013



BURN ADVISORY:



Recommended times for agricultural burning are from 1:00 p.m. until 5:00 p.m..



WEATHER DISCUSSION:



The upper ridge axis is east of the area today as new strong troughing is developing off the Pacific coast.  The trough's weather will hold off for a day and warm, dry conditions will dominate today as southerly winds develop aloft.



Afternoon mixing heights are likely to rise to above 3000 feet, with high temperatures above the seasonal average and mostly sunny skies. Weak winds are expected.





TODAY'S FORECAST:



Mostly sunny.



Salem's high temperature today will be around 80 (average is 75).

Relative humidity:  Drops near 50% by noon and to near 30% by 5 p.m.

Surface winds: Light and variable this morning; becoming S 5-10 mph by mid-afternoon.

Transport winds: SSE 5 mph by late morning; becoming S 5-10 mph this afternoon.

Mixing height: Rising to 3500 feet by late afternoon.

Salem's sunset tonight: 7:14 p.m.



(Salem Airport data for Wed., Sept. 18th: High 74°F; rainfall 0)

(Maximum Ventilation Index expected today: 25)



EXTENDED DISCUSSION:



Increasing southerly flow aloft is forecast for Friday as the major trough approaches from the west, pushing a cold front and rain showers onshore into the area Friday afternoon and evening.



Expect rainy and cool weather on Saturday, and then mostly cloudy, cool conditions much of Sunday between frontal systems.  A new strong trough is forecast to move onshore from the west late on Sunday, bringing another round of precipitation. For early next week more rain and even cooler temperatures appear likely for the area at this point.



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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