[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Tue Sep 20 08:44:22 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT TUE SEP 20, 2016

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level trough, dropping southward just offshore, is producing a WSW flow aloft over Oregon.  The jet stream is also sagging southward over Oregon, pushing an associated band of middle and high clouds across the state.  These clouds are beginning to slide SE of the Silverton Hills, which will allow for more sunshine today.

Cool air aloft and surface heating should promote excellent mixing but also cumulus cloud development this afternoon.  A shower can't be ruled out, especially over higher terrain.  Low-level winds have turned light northerly but should veer to the NW later today.  A burning opportunity is possible this afternoon, if fields can dry out enough.

TODAY'S FORECAST:

Partly cloudy.  Slight chance of an afternoon shower.

Salem's high temperature today will be near 71°F (average is 76°F).

Relative humidity:  Dropping to 50% around 2 p.m. and to near 45% by 5 p.m.

Surface winds: N 5 mph this morning; NNW 5-10 mph this afternoon.

Transport winds: NNE 5-10 mph this morning; NNW 10 mph this afternoon.

Mixing height: Rising to 3000 feet around 1 p.m. and to 5000 feet by 5 p.m.

Salem's sunset tonight: 7:12 p.m.

(Salem Airport data for Monday, September 19th: High 68°F; Rainfall: 0.00")

(Maximum Ventilation Index expected today: 50)

EXTENDED DISCUSSION:

The offshore upper-level trough is projected to move over the region on Wednesday.  Cooling aloft will continue to provide excellent mixing, but the chance of showers will increase.  If fields are dry enough, NW winds may provide a burning opportunity.

The upper-level trough will push SE of the region on Thursday.  A drier and more stable NW flow aloft should end the threat of showers.  However, continued onshore flow may produce favorable burning conditions, if fields are dry enough.  A weakening frontal system may provide another burning opportunity on Friday, before renewing the chance of rain late in the day.

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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