[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Aug 13 08:53:55 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT WED AUG 13, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level trough is parked over Oregon with clusters of showers and thunderstorms rotating around it.  The two main areas of showers this morning are located over western Washington and SW Oregon, with mostly dry conditions sandwiched between them across NW Oregon.  As the parent trough very slowly advances eastward today, the area of showers currently across SW Oregon is forecast to circulate northward.  That will increase the threat of showers and thundershowers this afternoon across NW Oregon.

The air aloft is cool, so daytime heating should yield high afternoon mixing heights.  Onshore pressure gradients will increase, with transport winds turning generally westerly this afternoon.  Burning chances will depend upon the initial dampness of fields, additional shower activity, and relative humidity levels.

TODAY'S FORECAST:

Mostly cloudy.  Chance of showers and thundershowers.

Salem's high temperature today will be near 78°F (average is 83°F).

Relative humidity:  Dropping below 60% by 2 p.m. and to near 50% by 5 p.m.

Surface winds: SSW 2-6 mph this morning; SW 4-8 mph this afternoon.

Transport winds: SSW 5-10 mph this morning; W 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:21 p.m.

(Salem Airport data for Tuesday, August 12th: High 76°F; Rainfall: .13")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

The upper-level trough is forecast to move over eastern Oregon on Thursday and Washington on Friday.  The moist circulation around it should maintain mostly cloudy skies, but the threat of showers will decrease.  Onshore flow and good mixing may provide burning opportunities both days, if humidity levels are low enough and fields are not too wet.

A drier westerly flow aloft is forecast for Saturday through Monday, with onshore flow at the surface.  Increasing amounts of sunshine will help temperatures recover to near or slightly above 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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