[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:30 AM PDT FRI JUL 27, 2012



BURN ADVISORY:



Recommended burn times for agricultural burning are from 3 p.m. to 6:30 p.m.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A weak upper-level disturbance, cutting across western Washington this morning, further deepened the cool and moist marine layer that surged into the Willamette Valley overnight.  Cloudy skies will likely persist until at least the mid-afternoon, with cool surface temperatures suppressing mixing heights.  Even with some late-day clearing, high temperatures will be about 10 degrees below average.  Pressure gradient stacking became even more pronounced late this morning and will likely persist through the afternoon.  Pibals will only be done after the low clouds break up, and if there is significant improvement in the pressure gradient stacking (unlikely).



TODAY'S FORECAST:



Cloudy and much cooler.  Partial clearing late this afternoon and evening.



Salem's high temperature today will be near 74 degrees (normal is 84).

Relative humidity drops to 60% by 3 p.m. and to near 50% by 5 p.m.

Surface winds:  S 3-6 mph; becoming NW 4-8 mph late this afternoon.

Transport winds: SW 5; becoming NW 5-12 mph late this afternoon.

Mixing height: Rising to 3000 feet by 3 p.m. and to 3500 feet by 5 p.m.

Salem's sunset tonight: 8:44 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 68      73      66

     Relative Humidity:           61%     52%     68%

     Surface Wind Direction:     180     340     310

     Surface Wind Speed:           5       5       8

     Transport Wind Direction:   240     340     310

     Transport Wind Speed:         5       7      10

     Mixing Height:             2800    3500    1800

     Ventilation Index:           14      25      18



EXTENDED DISCUSSION:



A dry and stable southwesterly flow aloft is forecast to continue through next week.  Varying degrees of onshore flow, morning clouds, and afternoon sunshine could present additional burning opportunities.  Onshore flow should hold temperatures to near or slightly below normal.



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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