[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 THURS. AUG. 1, 2013



BURN ADVISORY:



Recommended burn times for agricultural burning are 1:00 p.m. to 6:00 p.m.

Prep burning is not allowed.



WEATHER DISCUSSION:



The upper trough is centered right on the coastline today, with persistent widespread low cloud cover over the area and areas of drizzle. Showers are also persisting on the eastern side of the Cascade Range, as part of the trough energy is ejecting northeastward. Surface winds are light and variable, and surface pressure gradients are not favorable and not expected to improve significantly during the day.



Mostly cloudy skies are expected for the area this afternoon with some areas of light drizzle. The low cloud is not expected to burn off, so surface temperatures are much cooler than yesterday. Scattered light showers this afternoon are possible. Transport winds will be mostly westerly at speeds of 5 to 15 mph. Surface winds will also likely become westerly this afternoon. Mixed layer depth will be limited to some degree with the lack of sunshine.



(Salem Airport data for Wednesday, July 31st: High 80°F; Rainfall .00")

(Maximum Ventilation Index expected today:  35)

TODAY'S FORECAST:



Expect mostly cloudy skies with low-level winds becoming westerly.



Salem's high temperature today will be around 70.

Relative humidity will be 50-60% late this afternoon.

Surface winds: W 5-10 mph.

Transport winds: W to NW at 5-15 mph.

Maximum mixing height:  3500 ft.

Sunset tonight: 8:38 p.m.



EXTENDED DISCUSSION:



Upper troughing is expected to persist over the area on Friday, with mostly cloudy and cool conditions continuing for much of the day, and a chance of light showers. Some decrease in low clouds is possible late in the day. General onshore flow is expected, but the lack of a favorable surface gradient pattern could limit the burning potential.



On Saturday a gradual warming, drying trend should start, with the upper trough weakening, but low clouds early in the day over the area are likely.



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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