[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 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.  Surface winds currently mostly weak from the northeast. Middle and upper cloudiness is extensive over portions of the southern half of the state this morning.



The Salem sounding this morning continued to show brisk NNE winds from just above the surface through 4000 feet, and ODF SODAR measurements are similar for the coast range.  Brisk transport winds should remain mostly north to northeasterly today, which is not favorable for burning.  Significant warming aloft has occurred overnight, and this will lead to even warmer surface temperatures this afternoon while mixing heights should be similar to those observed yesterday.



TODAY'S FORECAST:



Becoming partly cloudy, warm, and breezy 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 10-15 mph this afternoon.

Transport winds: NNE 15-20 mph this afternoon.

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

Salem's sunset tonight: 8:09 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. Cooler surface temperatures are expected. Showers/thunderstorms will likely be confined to east of the region late in the day, including the eastern portions of the Cascade Range. Wetting rains are not expected over the Silverton Hills.



A more favorable onshore pattern with cooler conditions and southwesterly flow aloft is expected for Friday. Cloud cover may be a limiting factor as extensive morning cloudiness is expected and how fast that dissipates later will be key to getting mixing heights to a significant level.  We will monitor new model guidance for Friday's forecast.



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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