[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 THU SEP 12, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



A strong ridge of 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.



Mixing heights will continue to be quite low today, due to warm air aloft.  Winds through the mixed layer and aloft will become weak northeasterly, which is not a favorable pattern for burning either.  Low-level wind may shift to NNW late this afternoon. Slightly cooler high temperatures are expected.





TODAY'S FORECAST:



Becoming sunny with warm temperatures continuing and light winds.



Salem's high temperature today will be near 88 degrees (average is 78).

Relative humidity:  Drops below 65% by 11 a.m. and to near 35% by 5 p.m..

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

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

Mixing height: Rising to only about 2000 feet by 5 p.m.

Salem's sunset tonight: 7:28 p.m.



(Salem Airport data for Wed., Sept. 11: High 96°F; Rainfall .00")

(Maximum Ventilation Index expected today: 15)



EXTENDED DISCUSSION:



The upper ridge will begin to weaken on Friday, but dry and unseasonably warm conditions will continue in the low levels. Generally, burning conditions will not be optimal with limited mixing heights, but late-day northwesterly transport winds may allow for some burning.



An approaching upper-level trough is forecast to increase the southerly flow aloft over the weekend. Dry conditions but with some clouds are expected for Saturday, and then showers and thunderstorms are likely by Sunday accompanied with a return to cooler temperatures. An unsettled and much cooler onshore flow pattern should bring autumn-like weather for much of 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.



Doug Wesley

ODF Meteorologist

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