[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT MON AUG 31, 2015



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is allowed from noon until 2 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



Up to an inch of rain fell in the Silverton Hills over the weekend.  The rain had mostly ending by noon Sunday with brisk west winds and dropping humidity levels hastening the drying process.



A strong upper-level trough, in the Gulf of Alaska, is producing a westerly flow aloft over Oregon today.  A frontal system is bringing light rain and gusty south winds to western Washington and cloudy skies to the NW corner of Oregon.  Just ahead of the front, skies are mostly sunny across the central and southern Willamette Valley.  Clouds will advance south and east, across the central Willamette Valley, this afternoon, but rain should stay north of the Silverton Hills.



SW Surface and transport winds will combine with high mixing heights to create favorable ventilation conditions this afternoon.  Wind speeds and the dampness of fields will be potential limiting factors for burning, as well as the threat of light rain.  A PIBAL is scheduled for 1 p.m.



TODAY'S FORECAST:



Increasing clouds.  Slight chance of light rain by this evening.



Salem's high temperature today will be near 76°F (average is 81°F).

Relative humidity:  Dropping below 50% by 2 p.m.

Surface winds: SW 5-15 mph.

Transport winds: SW 15 mph.

Mixing height: Rising to 5000 feet by 2 p.m.

Salem's sunset tonight: 7:51 p.m.



(Salem Airport data for Sunday, August 30th: High 73°F; Rainfall: .44")

(Maximum Ventilation Index expected today: 75)



EXTENDED DISCUSSION:



An upper-level trough in the Gulf of Alaska will spin another cold front onshore Tuesday.  Ahead of the front, SW transport winds and high mixing heights will make for favorable ventilation conditions. However, increasing wind speeds may limit the potential for burning.  Light rain is expected to make it as far south as Salem by Tuesday evening.



Wednesday through Friday, the upper-level trough is predicted to move over Oregon.  That will keep temperatures below average with mostly cloudy skies and scattered showers.  Rainfall totals will range mostly from one-tenth to one-quarter of an inch.  Drier and warmer weather is expected over the Labor Day weekend, with temperatures recovering to near average.



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 15 mph.



     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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.



To add/remove your email address from this list, please go to:



http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst



Pete Parsons

ODF Meteorologist
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