[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT TUE JUL 9, 2013



BURN ADVISORY:



Recommended burn times for agricultural burning from noon until 7:00 p.m.

Prep burning is not allowed.



WEATHER DISCUSSION:



Late-morning visible satellite imagery shows marine clouds banked up along the immediate Oregon coast with clear skies across the remainder of the state.  With weaker onshore flow today, temperatures in the Willamette Valley are running about 10 degrees warmer than 24 hours ago.  Highs this afternoon should top out near 90 degrees.  Transport winds will be mostly northerly.



TODAY'S FORECAST:



Sunny and warmer.



Salem's high temperature today will be near 90 degrees (average is 81).

Relative humidity:  Drops to near 30% by 5 p.m.

Surface winds:  N 3-8 mph; becoming NNW 7-15 mph later this afternoon.

Transport winds: N 5-10 mph; becoming N 10-15 mph later this afternoon.

Mixing height: Rising to near 4000 feet late this afternoon.

Salem's sunset tonight: 8:59 p.m.



(Maximum Ventilation index expected today: 60)



EXTENDED DISCUSSION:



An approaching upper-level trough will increase the southwesterly flow aloft on Wednesday.  Increasing onshore flow may bring some morning marine clouds into the Willamette Valley.  Otherwise, expect sunshine but with cooler temperatures.  Transport winds are forecast to turn northwesterly in the afternoon with cooling aloft improving mixing.



A weak upper-level trough will move over Oregon Thursday and Friday.  No showers are expected, but strong onshore flow should blanket the Willamette Valley with morning low clouds.  Some morning drizzle is possible, especially on Thursday, with partly cloudy skies in the afternoons.  High temperatures will cool back into the mid 70s. Drying and warming is expect this weekend.



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.



Pete Parsons

ODF Meteorologist
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