[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT WED AUG 20, 2014



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is allowed from now until 1 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



Marine low clouds began to clear late this morning and will give way to progressively more sunshine this afternoon.  However, cooler air aloft will likely cap valley high temperatures in the upper 70s.



Mixing heights and transport winds should be favorable for burning again this afternoon, but negative pressure gradient-stacking will need to be balanced by daytime heating.  That is unlikely before 2 p.m.  Today's first PIBAL is scheduled for 1 p.m.  Like yesterday, low-level winds may strengthen enough later this afternoon to inhibit good vertical plume development.



TODAY'S FORECAST:



Becoming partly cloudy this afternoon.



Salem's high temperature today will be near 79°F (average is 82°F).

Relative humidity:  Dropping to near 35% by 5 p.m.

Surface winds: NW 5-10 mph; becoming NW 10-15 mph late this afternoon.

Transport winds: NW 10 mph; becoming NW 15 mph late this afternoon.

Mixing height: Rising to near 5000 from 2 p.m. through 5 p.m.

Salem's sunset tonight: 8:10 p.m.



(Salem Airport data for Tuesday, August 19th: High 89°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 100)



EXTENDED DISCUSSION:



A cool NW flow aloft will continue through the week, as an upper-level trough slowly strengthens over the Pacific Northwest. Temperatures should remain a few degrees below average with varying amounts of morning marine clouds.  The combination of high afternoon mixing heights and NW transport winds may provide burning opportunities; limited primarily by gradient-stacking.  There is also a threat of light showers by Friday.



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 15 mph.



     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 (ft) times

         the transport wind speed (mph) 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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.



To add/remove your email address from this list, please go to:



http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst



Pete Parsons

ODF Meteorologist
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