[willamette-fcst] Field Burning Forecast for Silverton Hills

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT FRI SEP 07 2012



BURN ADVISORY:



Agricultural burning burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



Upper level ridge continues to build over the state today with a minor low pressure area to the southwest in the eastern Pacific.  Expect mostly sunny skies.



Expect mixing to slowly improve this afternoon with the heating of the day.  Maximum mixing rises to about 4000 ft.  Transport winds will be light N to NE, and then turn NW'erly this evening.



Surface pressure pattern shows the thermal trough along the western side of the Willamette Valley extending to the northern Oregon coast.  Current pressure gradients are .8 mb onshore from Newport to Salem and 4.5 mb offshore from Salem to Redmond.  The thermal trough will remain in the Willamette Valley throughout the day with increasing onshore flow over the Coast Range with offshore flow remaining but weakening over the Cascades.



TODAY'S FORECAST:



Sunny



Salem's high temperature today will be near 93.

Relative humidity: Minimum relative humidity 22%.

Surface winds:  Mostly northerly at 4-7 mph, turning NW'erly during the evening.

Transport winds: N to NE at 5-8 mph, turning NW'erly during the evening.

Maximum mixing height:  near 4000 ft.

Sunset tonight: 7:36 pm



THREE-HOURLY DATA:

                                      2pm     5pm     8pm

     Temperature:                      85      93      81

     Relative Humidity:                30%     22%     35%

     Surface Wind Direction:          360     360     320

     Surface Wind Speed:                7       7       5

     Transport Wind Direction:        020     030     310

     Transport Wind Speed:              7       6       6

     Estimated Mixing Height:        2700    4000    1500

     Ventilation Index:                18      23       9



EXTENDED DISCUSSION:



Upper level ridge will shift eastward on Saturday as the upper low in the eastern Pacific develops into a trough and starts moving inland over the weekend.  This will bring some cooling over the region tomorrow and a chance of thunderstorms over the Cascades.  Pressure pattern turns onshore.  Skies, however, should remain mostly clear in the Willamette Valley.



The trough will strengthen and move inland on Sunday bringing further cooling as marine low clouds move into the Valley.  Another trough will drop down from the northwest Sunday evening and may produce some light rain over the region early Monday.  This trough will quickly move to the east late Monday and Tuesday with another upper ridge moving in for the middle of next 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 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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