[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT THU OCT 11, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



Fog continued to blanket much of the Willamette Valley late this morning, but visibilities were beginning to improve.  Some break-up of the fog had begun, especially in the south valley.  Late-morning temperatures were only in the mid to upper 40s in foggy areas, but partly sunny skies had already boosted Eugene into the low 50s.  The fog layer should continue to break up, with mostly sunny skies likely by about 2 p.m. in the Silverton Hills.



There may be a late-afternoon burning opportunity today, very similar to yesterday, if the valley can get enough sunshine.  The air aloft remains quite warm, so surface temperatures will need to reach the low 60s to achieve 2000-foot mixing heights. Pressure gradients are very weakly onshore with light and variable winds across the valley.  Transport winds are forecast to remain lightly onshore this afternoon.  Pibals are scheduled to begin either at 1:00 p.m. or when the fog lifts.



An approaching cold front will bring increasing onshore flow tonight, which will aid in smoke evacuation.  However, it will also usher in a very moist air mass, likely ending the 2012 field-burning season.



TODAY'S FORECAST:



Fog giving way to mostly sunny but hazy skies this afternoon.



Salem's high temperature today will be near 67 degrees (normal is 66).

Relative humidity:  Dropping to near 50% by 5 p.m.

Surface winds:  Light and variable.

Transport winds: Light and variable; becoming SW to NW 3-5 mph late.

Mixing height: Rising to near 2500 feet by late this afternoon.

Salem's sunset tonight: 6:33 p.m.



THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 63      66      57

     Relative Humidity:           54%     49%     67%

     Surface Wind Dir/Speed:    Var 3   Var 3    SW 3

     Transport Wind Dir/Speed:   NW 3    SW 3    SW 4

     Mixing Height:              2200    2500    1000

     Ventilation Index:            7       8       4



EXTENDED DISCUSSION:



The upper-level ridge that has been over the Pacific Northwest since July will give way to a strengthening westerly jet stream on Friday.  The first in a series of progressively stronger weather systems will come onshore Friday morning.  The latest computer guidance is showing rain moving into the northern Willamette Valley in the afternoon.  The air mass ahead of the front will likely be too moist to allow for burning.



Look for mostly cloudy skies Saturday and early Sunday with a chance of light rain at times.  A couple of quite potent storms are forecast to bring soaking rains to western Oregon Sunday afternoon through Monday.



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.



Pete Parsons

ODF Meteorologist
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