[willamette-fcst] Silverton Hills AM Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT FRI SEP 13, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



A strong ridge of upper high pressure persists over western Oregon today.  Low cloud coverage is extensive this morning, but clearing is expected later this morning. Warmer than average temperatures and dry conditions continue. A few clouds from a weak upper low well to the southeast may sneak into the picture this afternoon, but otherwise sunny conditions will continue.



Mixing heights will continue to be relatively low by summer standards today, due to continued warm air aloft.  Winds through the mixed layer will become weak northerly, which is not a favorable pattern for burning either.  Slightly cooler high temperatures are expected.





TODAY'S FORECAST:



Becoming mostly sunny with warm temperatures continuing and light winds.



Salem's high temperature today will be in the low 80s (average is 78).

Relative humidity:  Drops below 70% by 11 a.m. and to near 40% by 5 p.m..

Surface winds: Very light this morning; becoming N to NNW 5-10 mph this afternoon.

Transport winds: Becoming NNW 5-10 mph by this afternoon.

Mixing height: Rising to 2000-2500 feet by 5 p.m.

Salem's sunset tonight: 7:26 p.m.



(Salem Airport data for Thurs., Sept. 12: High 85°F; Rainfall .00")

(Maximum Ventilation Index expected today: 15-20)



EXTENDED DISCUSSION:



The upper ridge continues to weaken and by Saturday afternoon an approaching upper-level trough is forecast to increase the southerly flow aloft. Dry conditions will continue Saturday but with early low clouds and some afternoon increase in upper clouds. Then, showers and thunderstorms are likely by afternoon on Sunday with deep southeasterly winds and a marked cooling trend. An unsettled and much cooler onshore flow pattern should bring unsettled autumn-like weather for much of next week, including the daily chance of early clouds over the valley and Silverton Hills.



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.



Doug Wesley

ODF Meteorologist

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