[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 THU SEP 5, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



The upper level low is making its way northeast into southwestern OR today. Residual moisture from last night's rain will not dry out today as the upper low is causing cloudy skies, and widespread showers will develop later.



(Salem Airport data for Wed., Sept. 4th: High 77°F; Rainfall T)

(Maximum Ventilation Index expected today:  25)

TODAY'S FORECAST:



Mostly cloudy with weak surface winds and numerous showers and thunderstorms developing this afternoon. Wetting rains are likely over the Silverton Hills.



Salem's high temperature today will be in the low 70s.

Relative humidity drops to 55-60% this afternoon.

Surface winds: Light and variable, becoming W-NW 5-10 mph by mid-afternoon.

Transport winds: Light and variable, becoming NW 10-15 during the afternoon.

Maximum mixing height: 2500 ft.

Sunset tonight: 7:41 p.m.



EXTENDED DISCUSSION:



The low will continue to move off to the northeast on Friday, leaving cool temperatures, light showers at least through the morning, and wet fields.  Expect cloudy skies Friday morning with some clearing possible during the afternoon.  Wind flow will be mostly W to NW.  Very little field drying is expected during the afternoon.



Cloudy skies are also expected early on Saturday, but clearing should take over during the afternoon with mostly N winds. The drying trend will continue on Sunday accompanied by warming.



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 times

         the transport wind speed 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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