[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Fri Aug 15 08:54:03 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT FRI AUG 15, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

The slow-moving upper-level low-pressure system that has produced mostly cloudy and "relatively" cool weather the past three days is centered over NE Oregon this morning.   The moist flow around it is continuing to spread clouds across virtually all of Washington and most of northern Oregon.  Radar and surface reports indicate some sprinkles across NW Oregon but no significant rainfall.

The low-pressure system will continue to migrate to the north and east today, with the air mass slowly warming and drying.  However, cool air aloft will yield high mixing heights again this afternoon.  Light southwesterly transport winds will slowly veer to westerly, and eventually northwesterly, over the course of the day, as onshore pressure gradients slowly increase.  That should make for favorable burning conditions.

TODAY'S FORECAST:

Cloudy early with possible sprinkles.  Partial afternoon clearing.

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

Relative humidity:  Dropping to 60% by noon and to near 40% by 5 p.m.

Surface winds: S 2-6 mph this morning; becoming WNW 2-6 mph this afternoon.

Transport winds: SW 5 mph this morning; becoming WNW 5-10 mph this afternoon.

Mixing height: Rising to 3000 feet by noon and above 5000 feet by 5 p.m.

Salem's sunset tonight: 8:18 p.m.

(Salem Airport data for Thursday, August 14th: High 80°F; Rainfall: Tr.)

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

A dry and stable westerly flow aloft should clear skies by Saturday afternoon with temperatures warming to slightly above average.  A very weak upper-level trough will likely turn the flow aloft southwesterly on Sunday, which should provide additional slight warming under mostly sunny skies.  The flow aloft is forecast to turn southerly by Monday, as the trough moves onshore.  That may generate scattered afternoon thundershowers over the Cascades.

Long-range computer models show a WNW flow aloft bringing increasing onshore flow and cooler temperatures beginning next Tuesday.  That may create burning opportunities, if fields remain dry.

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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