[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Fri Aug 22 11:49:02 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT FRI AUG 22, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A cool but relatively dry upper-level trough moved onshore last night, initiating a moderate "marine push" and driving low clouds across much of the Willamette Valley early this morning.  Skies had cleared significantly by midday, but the upper-level trough will remain over the region today.

With cool air aloft, mixing depths should expand to the top of the marine layer, about 6000 feet, once surface temperatures warm into the mid-70s.  However, the air mass may become unstable enough for shower and thundershower development, especially over the Cascades, which can cause unwanted down-mixing of smoke.  In addition, increasing NNE winds, just above the mixing layer, may inhibit late-day smoke evacuation.

TODAY'S FORECAST:

Partly cloudy and cooler this afternoon.  Risk of afternoon showers or thunderstorms, mainly over the Cascades.

Salem's high temperature today will be near 79°F (average is 82°F).

Relative humidity:  Dropping to near 35% by 5 p.m.

Surface winds: NE 4-7 mph; backing to NNW 5-12 mph this afternoon.

Transport winds: NNE 5-8 mph; backing to NNW 10-15 mph this afternoon.

Mixing height: Rising to near 6000 feet by 5 p.m.

Salem's sunset tonight: 8:06 p.m.

(Salem Airport data for Thursday, August 21st: High 82°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 100)

EXTENDED DISCUSSION:

The upper-level trough over the region is forecast to move eastward, to over the Rockies, this weekend.  A dry northwesterly flow aloft will clear skies and warm temperatures back into the low-to-mid-80s with mostly north winds.

A building upper-level ridge may warm valley temperatures to near 90°F by next Tuesday, with north to NE transport winds.  The ridge should give way to increasing southwesterly flow aloft next Wednesday through Friday, which will begin a cooling trend and possibly provide burning opportunities.

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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