[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 TUE SEP 17, 2013



BURN ADVISORY:



Recommended times for agricultural burning are from 10 a.m. until 5:30 p.m.



WEATHER DISCUSSION:



Southwesterly flow aloft is directing a weak upper-level disturbance across the state today.  Satellite imagery is showing mostly cloudy skies statewide, but most of the shower activity is south of a line from Newport to Corvallis to Madras.  A few showers are likely to develop over the northern Willamette Valley this afternoon with high temperatures 5-10 degrees below average.



The air aloft is still unseasonably cool today, so afternoon mixing heights are likely to rise to near 5000 feet.  Rainfall totals in the Silverton Hills should be less than one-tenth of an inch with some areas possibly staying completely dry.  If fields remain dry, onshore transport winds will be favorable for burning again today, should the need arise.



TODAY'S FORECAST:



Mostly cloudy.  A few light showers possible.



Salem's high temperature today will be near 69 degrees (normal is 77).

Relative humidity:  Drops below 60% by noon and to near 50% by 5 p.m.

Surface winds: NW 5-10 mph this morning; WNW 7-13 mph this afternoon.

Transport winds: NW 10 mph this morning; NW 15 mph this afternoon.

Mixing height: Rising to 3000 feet by 11 a.m. and to near 5000 feet by 5 p.m.

Salem's sunset tonight: 7:18 p.m.



(Salem Airport data for Monday, Sept. 16th: High 72°F; Rainfall: Trace)

(Maximum Ventilation Index expected today: 75)



EXTENDED DISCUSSION:



A transitory upper-level ridge will bring drier and warmer conditions Wednesday and Thursday.  Temperatures should recover to slightly above normal by Thursday.  Increasing southwesterly flow aloft is forecast for Friday, with the first in a new series of weather systems likely pushing rain onshore Friday afternoon and night.



Expect showery and cool weather on Saturday and brief drying on Sunday.  Rain should return by Sunday night.  A generally cool and somewhat damp weather pattern is forecast for next week.



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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