[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 WED. JUL 31, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from 2:00 p.m. to 4:00 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



Upper troughing continues just off the coast, with south-southwesterly winds aloft over the area. Mostly cloudy skies are in place with extensive mid-level moisture as well as low clouds. Some light shower activity is present off to the south and east, and moving north. Pressure gradients are still weak as this time and surface winds are nearly calm.



Mostly cloudy skies with some scattered light showers and possible thundershowers are expected in the area this afternoon. Transport winds will be northerly but generally weak through early afternoon, with some onshore flow likely developing late this afternoon as gradients become weakly favorable and the trough begins to move inland. Mixed layer depth will be limited with the lack of sunshine today and southerly flow in the mid-levels. Winds will be erratic in the vicinity of showers.



(Salem Airport data for Tuesday, July 30th: High 84°F; Rainfall .00")

(Maximum Ventilation Index expected today:  25)

TODAY'S FORECAST:



Expect mostly cloudy skies with weak winds.



Salem's high temperature today will be around 80.

Relative humidity will be 35-40% late this afternoon.

Surface winds: Light.

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

Maximum mixing height:  2500 ft.

Sunset tonight: 8:39 p.m.



EXTENDED DISCUSSION:



Upper troughing is expected move eastward over the area on Thursday, with cooler conditions, extensive cloudiness and scattered showers. General onshore flow is expected but limited surface heating will likely keep the mixed layer depth down. Mostly cloudy conditions and cool surface temperatures are also expected Friday as the upper trough persists, with a continued chance of showers.



On Saturday a warming, drying trend should start but low clouds early 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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