[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Tue Sep 15 08:50:21 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT TUE SEP 15, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level trough remains over the Pacific Northwest with the main jet stream directed at California.  The air mass is unseasonably cool.  The freezing level over Salem was measured at just 6000 feet this morning.  A weather disturbance just offshore will be directed mainly south of the region today, but it will keep skies generally cloudy with a threat of light rain.

High relative humidity levels and the threat of rain will be limiting factors for open burning today.  Surface and transport winds should be mostly south to SW today, which sets the humidity threshold for open burning at 65%.  Very cool air aloft should provide favorable mixing heights this afternoon, even with well below average surface temperatures.

TODAY'S FORECAST:

Cloudy.  Chance of sprinkles or very light rain.

Salem's high temperature today will be near 63°F (average is 77°F).

Relative humidity:  Dropping to 65% around noon and to 55% by 5 p.m.

Surface winds: S 5-10 mph this morning; SW 7-12 mph this afternoon.

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

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

Salem's sunset tonight: 7:23 p.m.

(Salem Airport data for Monday, September 14th: High 64°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

A stronger weather system is predicted to move onshore Wednesday, but the main precipitation region will be directed into southern Oregon and California.  Light rain will likely spread across the NW Oregon by Wednesday afternoon and increase Wednesday night and Thursday.  Rainfall totals should exceed one-tenth of an inch and may locally approach one-half inch in the Silverton Hills.

A transitory upper-level ridge will bring dry and warmer weather Friday and Saturday.  The ridge is expected to weaken by Sunday with increasing westerly flow aloft.  Another weather system may bring light rain to NW Oregon Sunday night and Monday.

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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