[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT THU JUL 28, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A dry and stable west-northwesterly flow aloft is finally bringing more typical "summer weather" to the Pacific Northwest.  Late-morning visible satellite imagery showed clear skies virtually statewide.



The surface map shows high pressure pushing into Washington and northwestern Oregon along with a weak thermal trough building into southwestern Oregon.  That is producing north to northeasterly winds across the Willamette Valley, which should aid in the drying of damp fields.  Warmer air aloft will lead to slightly lower mixing heights today, as temperatures return to near normal.



TODAY'S FORECAST:



Sunny and warmer.



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

Relative humidity drops to near 35% by 2 p.m.

Surface winds:  NNE 10-15 G20 mph.

Transport winds: NNE 15-20 mph.

Mixing height: Rises to 4000 feet by 2 p.m. and to near 4500 feet by 5 p.m.

Salem's sunset tonight: 8:43 p.m.





THREE-HOURLY DATA:

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

     Temperature:                 80      83      77

     Relative Humidity:           37%     33%     42%

     Surface Wind Direction:     010     015     005

     Surface Wind Speed:          12      13      12

     Transport Wind Direction:   010     015     010

     Transport Wind Speed:        16      18      20

     Mixing Height:             4000    4300    2800

     Ventilation Index:           64      77      56



EXTENDED DISCUSSION:



A weak system cutting across British Columbia will turn the flow aloft slightly more westerly on Friday, with little change in low-level winds, temperatures or mixing heights from today.



Weak southwesterly flow aloft is forecast for this weekend, with weather systems staying well to our north, across British Columbia.  Northerly low-level winds are forecast to turn more northwesterly next week.  That will keep temperatures moderate and could create favorable burning opportunities.



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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