[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 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 revealed bands of high clouds cutting across northwestern Washington, late this morning, in response to a very weak upper-level disturbance just offshore.  Visible satellite imagery showed low clouds still covering much of western Washington and extending southward over the northwestern corner of Oregon.  Some low clouds had even made it into the central and southern Willamette Valley since the mid-morning report.



The late-morning surface map showed mostly northerly pressure-gradients across the Willamette Valley with most of the onshore flow between Salem and Redmond.  That is not a favorable pattern for evacuating smoke from the valley, even with high afternoon mixing heights expected.  Daytime heating will help to balance the weak onshore flow, across western Oregon, but surface and transport winds are forecast to remain mostly northerly.



TODAY'S FORECAST:



Sunny.



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

Relative humidity drops to near 35% by 5 p.m.

Surface winds:  N 5-10 mph this afternoon; NW 6-12 mph this evening.

Transport winds: NNW 6-12 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:

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

     Temperature:                 78      82      74

     Relative Humidity:           41%     35%     46%

     Surface Wind Direction:     350     350     320

     Surface Wind Speed:           6       8       9

     Transport Wind Direction:   350     350     350

     Transport Wind Speed:         6      10      12

     Mixing Height:             4200    5000    2500

     Ventilation Index:           25      50      30



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.  Skies will remain mostly sunny with little change in temperatures.  Increasing onshore flow may create more favorable burning conditions in the afternoon.



Further strengthening of the southwesterly flow aloft is forecast over the weekend, as a broad upper-lever trough, in the Gulf of Alaska, slowly moves closer to our coastline.  Look for an increase in marine clouds and a few degrees of cooling each day.  Timing of the arrival of the upper-level trough is not certain, but it appears as if a significant surge of onshore flow is likely by Tuesday, which may produce some drizzle or light showers.  Ahead of the trough, there is a chance for a burning opportunity on 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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