[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Tue Sep 29 08:52:38 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT TUE SEP 29, 2015

BURN ADVISORY:

Recommended times for agricultural burning are from 3 p.m. until 5 p.m.

Prep burning is not allowed.

WEATHER DISCUSSION:

A warm and dry southwesterly flow aloft is increasing ahead of a slow-moving upper-level trough.  Skies are clear across the interior of western Oregon this morning.  A surface thermal trough is beginning to shift from the coast into the interior of western Oregon.  In response to this shift, low clouds are advancing northward along the southern and central coast and should make it to the north coast later this afternoon.

The thermal trough is expected to shift over the Willamette Valley today, which will result into light and variable winds and warm temperatures.  If the thermal trough shifts far enough east, gradients may turn onshore late this afternoon.  That would turn transport winds southwesterly and could allow for the burning of remaining fields.  However, warm air aloft will keep mixing heights from getting much above 3000 feet.

TODAY'S FORECAST:

Sunny and warm.

Salem's high temperature today will be near 80°F (average is 72°F).

Relative humidity:  Dropping to 45% by 11 a.m. and to near 30% by 5 p.m.

Surface winds: Var 2-5 mph; becoming SW 3-6 mph after about 3 p.m.

Transport winds: Var 3-6 mph; becoming SW 5-10 mph after about 3 p.m.

Mixing height: Rising to 3000 feet after about 3 p.m.

Salem's sunset tonight: 6:56 p.m.

(Salem Airport data for Monday, September 28th: High 80°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 45)

EXTENDED DISCUSSION:

An upper-level trough will slowly advance towards Oregon tonight and Wednesday, which will force the surface thermal trough east of the Cascades. Increasing onshore flow should push some marine clouds into the Willamette Valley Wednesday morning, but no precipitation is expected.  Cooling aloft will improve mixing Wednesday afternoon with SW transport winds likely creating favorable conditions for the burning of any remaining fields.

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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