[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Tue Sep 22 08:54:45 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT TUE SEP 22, 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:

After very light rainfall early Monday morning, a drier westerly flow aloft cleared skies Monday afternoon across western Oregon.  The flow aloft has backed to southwesterly this morning, with a band of middle and high clouds covering most of NW Oregon.

There is a 4500-foot deep marine layer over the valley this morning, with unfavorable gradient-stacking, but the absence of low clouds will help to balance the onshore flow this afternoon.  NNE transport winds this morning are predicted to slowly turn to the NNW this afternoon, which could provide an open burning opportunity.

TODAY'S FORECAST:

Partly cloudy with slightly below average temperatures.

Salem's high temperature today will be near 71°F (average is 75°F).

Relative humidity:  Dropping below 60% by noon and to near 35% by 5 p.m.

Surface winds: N 5-10 mph.

Transport winds: NNE 10 mph this morning; becoming NNW 10 mph this afternoon.

Mixing height: Rising to 3000 feet by noon and to 4500 feet by 5 p.m.

Salem's sunset tonight: 7:10 p.m.

(Salem Airport data for Monday, September 21st: High 73°F; Rainfall: .05")

(Maximum Ventilation Index expected today: 45)

EXTENDED DISCUSSION:

Wednesday and Thursday, increasing SW flow aloft should provide very favorable burning conditions both afternoons.  Gradient-stacking will improve with mostly SW transport winds.  The next weather system is predicted to weaken, as it brings a chance of showers onshore Thursday night.

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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