[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 FRI SEP 20, 2013



BURN ADVISORY:



Recommended times for agricultural burning are from 11:00 a.m. until 5:00 p.m..



WEATHER DISCUSSION:



The strong trough and cold front just west of us is currently moving onshore, bringing extensive middle and upper-level clouds with it and rain off the coast this morning. Deep south-southwesterly flow is developing ahead of the trough. Rain showers are expected to develop this afternoon over the Silverton Hills with breezy south winds. Afternoon mixing heights are likely to rise to moderately high levels as the cold air aloft moves in. High temperatures will be lower today as the trough approaches and frontal passage occurs later today.





TODAY'S FORECAST:



Mostly cloudy and cooler with showers likely by mid-afternoon.



Salem's high temperature today will be in the low 70s (average is 75).

Relative humidity:  Drops near 60% by noon and then increasing as showers move in this afternoon.

Surface winds: Becoming SSW 5-10 mph by late morning and 5-15 mph for the afternoon.

Transport winds: SSW 15 mph by late morning and strengthening this afternoon.

Mixing height: Rising to 3-4000 feet by mid-afternoon.

Salem's sunset tonight: 7:13 p.m.



(Salem Airport data for Thurs., Sept. 19th: High 80°F; rainfall 0)

(Maximum Ventilation Index expected today: 65)



EXTENDED DISCUSSION:



Expect cloudy, rainy and cool weather on Saturday through Monday as strong onshore flow continues and the upper trough is reinforced late in the weekend into early next week. At this point looks like Tuesday will be cool with a continued chance of rain; precipitation chances are expected to begin to diminish by mid-week.



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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