[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Mon Aug 8 11:48:50 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT MON AUG 8, 2016

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An area of very light showers moved across Marion County late this morning, with mostly just trace amounts of rain reported.  An upper-level trough, centered over NW Washington, will keep skies mostly cloudy this afternoon.  Daytime heating will further destabilize the air mass, leading to more substantial showers and possible thundershowers.  Rainfall totals could locally exceed one-tenth of an inch.

Should any fields remain dry, mixing heights and transport winds may turn favorable for burning, but gradient stacking and general instability could cause smoke down-mixing and will need to be monitored.  The next PIBAL is scheduled for noon.

TODAY'S FORECAST:

Mostly cloudy with scattered showers and a slight chance of thundershowers.

Salem's high temperature today will be near 74°F (average is 83°F).

Relative humidity:  Dropping to near 50% by 5 p.m.

Surface winds: SW 3-6 mph; becoming WNW 5-12 mph later this afternoon.

Transport winds: WSW 3-6 mph; becoming WNW 10-15 mph this afternoon.

Mixing height: Rising to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:28 p.m.

(Salem Airport data for Sunday, August 7th: High 75°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

A slow-moving upper-level trough will progress eastward across Washington, on Tuesday, making it to the Idaho border late in the day.  A drier and more stable NW flow aloft will decrease the threat of showers and bring more afternoon clearing.  WNW winds and good mixing could provide a burning opportunity for dry fields.  Gradient-stacking may be a limiting factor.

High pressure is expected to bring dry and warmer weather Wednesday through Friday.  NW transport winds may provide a burning opportunity on Wednesday, before turning mostly northerly Thursday and Friday.  Temperatures will warm to near average on Wednesday and to above average Thursday and Friday.  A dry marine push is expected to bring a cool-down this weekend.

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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