[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT MON JUL 18, 2011



BURN ADVISORY:



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

Prep burning is not allowed.

Propane flaming is allowed from 2:00pm to 5:00pm.



WEATHER DISCUSSION:



Upper low circulating off the northern California coast will continue to give  some showers to the area through this afternoon before weakening and shifting slowly eastward on Tuesday.  Expect a little more sunshine today but still generally cloudy skies.  Temperatures will warm over yesterday's cool 60s temperatures to get into the 70s this afternoon.



Satellite picture and radar show clouds and showers developing and moving up from the south late this morning.  With the likelihood of showers increasing I've lowered expected temperatures and increased humidities this afternoon.

Pressure gradients are slowly increasing onshore with .8 mb onshore from Newport to Salem and 1.2 mb onshore from Salem to Redmond.



TODAY'S FORECAST:



Mostly cloudy with scattered showers this afternoon.



Salem's high temperature today will be near 74.

Relative humidity: Minimum relative humidity 58%.

Surface winds:  NW to N at 5-8 mph.

Transport winds: NW'erly at 6-10 mph during the afternoon and evening.

Maximum mixing height:  4500 ft by late afternoon.

Sunset tonight: 8:53 pm





THREE-HOURLY DATA:

                                  2pm     5pm     8pm

     Temperature:                  71      74      68

     Relative Humidity:            63%     58%     71%

     Surface Wind Direction:      340     310     290

     Surface Wind Speed:            5       7       7

     Transport Wind Direction:    330     320     310

     Transport Wind Speed:          5       7      10

     Estimated Mixing Height:    3500    4500    2000

     Ventilation Index:            18      32      20



EXTENDED DISCUSSION:



Even though today's upper low gradually weakens and moves through on Tuesday, the upper level pattern will remain generally trough-like through the week.  Expect continued cloudy skies and a possible shower, especially during the morning under onshore flow.  Clouds will break up more in the afternoon but temperatures will likely remain 5-10 degrees below normal.  Another impulse moves through the trough pattern early Thursday, increasing the onshore flow and bringing another chance of light showers.



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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