[willamette-fcst] Silverton Hills PM Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT WED AUG 21, 2013



BURN ADVISORY:



Agricultural Burning:



North of Salem:  Recommended agricultural burn times are 11 a.m. to 6 p.m.

>From Salem south:  Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



Weak upper ridging is in place today over western OR, ahead of a small closed low off the central CA coast. At the surface, a thermal trough has developed over southwestern Oregon extending northward into the Willamette Valley.  Surface winds currently mostly weak from the north-northeast 5-10 mph. Middle and upper cloudiness is scattered through most of the state currently.



The Salem outlook for the remainder of the day shows NNE winds from just above the surface through 3000-3500 feet. Transport winds should remain mostly north to northeasterly today, which is not favorable for burning, and will increase a bit late today.  Very warm surface temperatures will continue this afternoon and mixing heights should be similar to those observed yesterday.



TODAY'S FORECAST:



Partly cloudy and very warm this afternoon. Hazy conditions will persist.



Salem's high temperature today will be near 90 degrees (average is 82).

Relative humidity:  Drops to near 25% by 5 p.m.

Surface winds:  N 5-15 mph this afternoon.

Transport winds: NNE 10-20 mph this afternoon.

Mixing height: Rising to near 3500 feet by 5 p.m.

Salem's sunset tonight: 8:08 p.m.



(Salem Airport data for Tuesday, Aug. 20th: High 86°F; Rainfall .00")

(Maximum Ventilation Index expected today: 50)



EXTENDED DISCUSSION:



The upper level low pressure system is forecast to slowly approach OR from the south on Thursday, with increasing clouds and much weaker transport winds possibly turning to the south/southwest. Cooler surface temperatures are expected. Showers/thunderstorms will mostly be confined to east of the region late in the day, including the eastern portions of the Cascade Range, but some late shower activity is possible over Cascade foothills on the western side.



A more favorable onshore pattern with cooler conditions and southwesterly to westerly flow aloft is expected for Friday. Cloud cover may be a limiting factor as extensive morning cloudiness is possible. How fast that dissipates later will be key to getting mixing heights to a significant level.





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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