[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 WED SEP 18, 2013



BURN ADVISORY:



Recommended times for agricultural burning are from 11 a.m. until 4:30 p.m..



WEATHER DISCUSSION:



Northwesterly flow aloft is in place behind yesterday's disturbance, which is east of Oregon today.  Mostly cloudy skies cover the Silverton Hills this morning, but a clearing trend is expected through the late morning.



Afternoon mixing heights are likely to rise to above 5000 feet, with temperatures just below seasonal average and partly cloudy skies. Weak winds are expected.





TODAY'S FORECAST:



Partly cloudy.



Salem's high temperature today will be near 72 degrees (average is 75).

Relative humidity:  Drops below 60% 1 p.m. and to near 50% by 5 p.m.

Surface winds: Light and variable this morning; becoming N 5 mph this afternoon.

Transport winds: NW 5-10 mph this morning; becoming NE 5-10 mph this afternoon.

Mixing height: Rising to 4000 feet by early afternoon and to above 5000 feet by 5 p.m..

Salem's sunset tonight: 7:16 p.m.



(Salem Airport data for Tuesday, Sept. 17th: High 72°F; Rainfall: Trace)

(Maximum Ventilation Index expected today: 40)



EXTENDED DISCUSSION:



Upper ridging will dominate Thursday with surface warming to near the seasonal average and dry conditions.  Increasing southwesterly flow aloft is forecast for Friday as a major trough approaches from the west, likely pushing rain onshore Friday afternoon and night.



Expect rainy and cool weather on Saturday and mostly cloudy, cool conditions Sunday.  A new strong trough is forecast to move onshore from the west Sunday night, and for early next week more rain and even cooler temperatures appear likely 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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