[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT THU AUG 30, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

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



WEATHER DISCUSSION:



Water vapor imagery shows an upper-level trough slowly dropping southward along the southern British Columbia coastline, with a dry west-southwesterly flow aloft over Oregon. Upper-level moisture is increasing across Washington and northwestern Oregon, which is an indication of rising air aloft moving into the region.  Morning marine clouds have mostly cleared from the Willamette Valley, and sunny skies are expected statewide this afternoon.



The air aloft has warmed since yesterday but is still cool enough to allow for good afternoon mixing.  The approaching upper-level trough should help to provide favorable pressure-gradient-stacking, for open burning, this afternoon.  ODF SODAR was still showing NNE transport winds late this morning over the coastal range.  Transport winds are forecast to turn northerly, and possibly northwesterly, later this afternoon, which would be more favorable for smoke evacuation. Pibals are scheduled to begin at 1:00 p.m. to monitor the changing transport winds.



TODAY'S FORECAST:



Sunny.



Salem's high temperature today will be near 78 degrees (normal is 81).

Relative humidity drops to 60% by noon and to near 35% by 5 p.m.

Surface winds:  N 5-10 mph; becoming NW 5-10 mph later this afternoon.

Transport winds: N 10 mph; becoming NW 10-15 mph later this afternoon.

Mixing height: Rising to 4000 feet by 2 p.m. to near 5000 feet by 5 p.m.

Salem's sunset tonight: 7:50 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 73      77      68

     Relative Humidity:           45%     36%     49%

     Surface Wind Dir/Speed:     N  5   NW  7   NW 10

     Transport Wind Dir/Speed:   N 10   NW 12   NW 15

     Mixing Height:              4000    5000    1800

     Ventilation Index:           40      60      27



EXTENDED DISCUSSION:



The upper-level trough to our north is forecast to slowly move across southern British Columbia on Friday, with a continued dry and stable southwesterly flow aloft over Oregon.  Once again, areas of morning clouds will yield to mostly sunny skies with seasonal temperatures.  However, afternoon transport winds are forecast to stay generally northerly, which is not favorable for open field-burning.



A continued dry and weak westerly flow aloft is forecast for this weekend and early next week, with areas of morning clouds giving way to mostly sunny skies.  Open burning opportunities are possible both Monday and Tuesday.  Temperatures should remain near seasonal normals.



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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