[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT MON SEP 16, 2013



BURN ADVISORY:



Recommended times from agricultural burning are from 1 p.m. until 5:30 p.m.



WEATHER DISCUSSION:



A strengthening southwesterly flow aloft deepened the marine air mass across western Oregon overnight.  The Salem sounding this morning showed generally southwesterly flow extending from just above the surface up through a 6000-foot deep marine layer. Mostly cloudy skies extend from the coast to the crest of the Cascades, but there is little to no shower activity on radar.



Some breaks in the marine clouds today should allow surface temperatures to climb into the low 70s.  With quite cool air aloft, even limited surface heating will allow mixing heights to rise above 5000 feet later today.  Favorable gradient-stacking and southwesterly transport winds may allow for the burning of any remaining fields this afternoon, with relative humidity levels dropping below 60 percent.  There is only a slight chance of sprinkles or a light shower.



TODAY'S FORECAST:



Mostly cloudy.  Possible sprinkles or very light showers.



Salem's high temperature today will be near 72 degrees (normal is 77).

Relative humidity:  Drops to 60% by 2 p.m. and to near 55% by 5 p.m.

Surface winds: S 5-10 mph this morning; SW 5-10 mph this afternoon.

Transport winds: SW 10 mph; veering to W 10 mph later this afternoon.

Mixing height: Rising to 3000 feet by 1 p.m. and to near 6000 feet by 5 p.m.

Salem's sunset tonight: 7:20 p.m.



(Salem Airport data for Sunday, Sept. 15th: High 69°F; Rainfall: Trace)

(Maximum Ventilation Index expected today: 60)



EXTENDED DISCUSSION:



Continued southwesterly flow aloft will direct a weak weather system onshore Tuesday, with showers likely across the northern and central Willamette Valley.  Rainfall amounts should be less than one-quarter of an inch.  Drier and warmer conditions are expected Wednesday and Thursday, as an upper-level ridge moves over the region.  Temperatures should recover to slightly above normal by Thursday.  Wet weather may return 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 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 (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 and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.



Pete Parsons

ODF Meteorologist
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