[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Tue Aug 19 11:48:30 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT TUE AUG 19, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

Westerly flow aloft directed a dry cold front, with mainly middle and high clouds, across NW Oregon early this morning.  Skies have cleared across the interior of NW Oregon, with marine low clouds persisting along the northern and central coast.  Cooler air aloft will knock 5-8 degrees off of today's valley high temperatures, compared to Monday, even with sunny skies.

Onshore pressure gradients are increasing and favorably "stacked" to keep smoke elevated.  An 11 a.m. PIBAL showed light westerly winds in the mixing layer; sheering to the NNW at and above the mixing height of approximately 3000 feet.  Another PIBAL is scheduled for noon.

Favorable onshore pressure gradients should persist today with good afternoon mixing and predominantly NW transport winds. Low-level winds may strengthen enough later this afternoon to inhibit good vertical plume development.

TODAY'S FORECAST:

Sunny but cooler.

Salem's high temperature today will be near 85°F (average is 82°F).

Relative humidity:  Dropping to near 40% by 5 p.m.

Surface winds: W 2-6 mph; becoming NW 5-15 mph this afternoon.

Transport winds: W 5 mph; becoming NW 8-18 mph this afternoon.

Mixing height: Above 3000 feet; rising to near 5000 feet by 2 p.m.

Salem's sunset tonight: 8:11 p.m.

(Salem Airport data for Monday, August 18th: High 93°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 120)

EXTENDED DISCUSSION:

A cool NW flow aloft will continue through the week, as an upper-level trough slowly strengthens over the Pacific Northwest. Temperatures should generally cool to about 5-10 degrees below average with varying amounts of morning marine clouds and partly cloudy afternoons.  The combination of high afternoon mixing heights and NW transport winds should provide additional burning opportunities.  Pressure gradient-stacking may be an issue at times, as the marine layer deepens across the valley.  There is also a risk of light showers later in the week.

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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