[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:38 AM PDT THU AUG 11, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from noon until 2 p.m. with a 50 acre limit.

Propane flaming is allowed from noon until 5:00 p.m.



WEATHER DISCUSSION:



Infrared satellite imagery shows some high clouds, associated with a weak upper-level disturbance, moving onto the northern Washington coast this morning.  Visible satellite imagery showed low clouds covering virtually all of western Washington; extending southward into the northwestern Oregon.  A few low clouds had made their way into the northern Willamette Valley, north of Marion County, with clear skies over central and southern valley.



A weak westerly flow aloft will send a dry upper-level disturbance onshore this afternoon.  That will maintain weak onshore flow and bring a similar day to what we saw on Wednesday.  The Salem sounding this morning showed some minor warming, below 5000 feet, which will slightly retard the rise in daytime mixing heights, compared to yesterday.



The air aloft is still cool enough for surface heating to produce high mixing heights again this afternoon.  The surface map shows only very weak onshore flow, which may increase enough this afternoon to allow for limited burning.  Transport winds are forecast to maintain a significant northerly component and will need to be monitored closely.



TODAY'S FORECAST:



Sunny.



Salem's high temperature today will be near 84 degrees (normal is 83).

Relative humidity drops below 60% by 11 a.m. and to near 33% by 5 p.m.

Surface winds:  N 5 mph this morning; NNW 5-10 mph this afternoon.

Transport winds: N 5 mph this morning; NNW 6-10 mph this afternoon.

Mixing height: Rises to 3000 feet about noon and to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:25 p.m.





THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 68       78      82      74

     Relative Humidity:           57%      39%     33%     45%

     Surface Wind Direction:     360      340     340     310

     Surface Wind Speed:           5        6       8       9

     Transport Wind Direction:   360      340     340     340

     Transport Wind Speed:         5        6       8      10

     Mixing Height:             2800     4200    5000    2500

     Ventilation Index:           14       25      40      25



EXTENDED DISCUSSION:



The dry upper-level flow is forecast to increase and turn southwesterly on Friday, as a fairly strong trough drops into the Gulf of Alaska.  That should slightly increase the onshore flow, which could create another limited burning opportunity.



The longer-range computer models differ on the timing and strength of a trough forecast to come onshore early next week.  Some models bring it inland on Sunday, but others delay it until late Monday or Tuesday, which would be more favorable for early-week burning.  Some showers are possible with this system, but that is also in question at this time.



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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