[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT MON JUL 25 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are from 12:00pm to 7:00pm.

Prep burning is not allowed.

Propane flaming is allowed from 12:00pm to 5:00pm.



WEATHER DISCUSSION:



An upper-level trough will move through the region today.  Expect a considerably cooler day today with temperatures around 15 degrees lower than yesterday.  Marine cloud cover will slowly break up to partly to mostly cloudy skies this afternoon.  Air mass will be generally unstable today with considerably cooler air aloft from the upper trough.



Current satellite picture shows still considerable cloud cover over the Willamette Valley but thinning and breaking up of the clouds is beginning.  Pressure gradients are still stacked toward the Cascades with 5.7 mb difference between Salem to Redmond while only 1.2 mb difference between Newport and Salem.  This should slowly equalize this afternoon and evening but probably remain stacked toward the Cascades.



TODAY'S FORECAST:



Salem's high temperature today will be near 74.

Minimum relative humidity 51%.

Surface winds:  SW 3-6 mph during the morning becoming SW to WNW at 5-9 mph this afternoon.

Transport winds: SW at 4-7 mph during the morning becoming W to NW at 6-10 mph during the afternoon.

Mixing height:  Rising to 4000 ft by noon and up to 5000 ft during the afternoon.

Sunset tonight: 8:47 pm





THREE-HOURLY DATA:

                                 2pm     5pm     8pm

     Temperature:                 71      74      68

     Relative Humidity:           57%     51%     63%

     Surface Wind Direction:     230     280     290

     Surface Wind Speed:           5       7       8

     Transport Wind Direction:   280     300     320

     Transport Wind Speed:         6       8      10

     Estimated Mixing Height:   4500    5000    2500

     Ventilation Index:           27      40      25



EXTENDED DISCUSSION:



Upper level trough will move by to the east of the state tonight as it weakens.  Expect gradual warming.  A minor ridge tries to move in on Wednesday but flattens as it does, leaving the region under westerly flow aloft.  Temperatures gradually return to seasonal normals.  Wind flow turns northerly through the remainder of the week with slight onshore flow during the later afternoon.  However, it appears it won't be enough to make for good burning conditions.



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.



Nick Yonker

ODF Meteorologist



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