[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Fri Aug 29 08:57:00 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI AUG 29, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

The upper-level ridge responsible for the sunny and warm weather this week is breaking down.  A weak weather system pushed high clouds across the region Thursday afternoon; increasing the onshore flow and cooling maximum temperatures from the 90s back into the 80s.  This morning's sounding over Salem showed continued cooling below 5000 feet; an indication that the marine layer is deepening.  Low clouds are more extensive across NW Oregon this morning but do not completely cover the Willamette Valley.

Another, slightly stronger, weather system is already spreading clouds across western Washington and the NW tip of Oregon, with some light rain falling on the northern Washington coast.  This system will bring increasing middle and high clouds to the region this afternoon, with high temperatures likely falling back into the upper 70s.

Very favorable conditions for burning are likely today.  Additional cooling aloft and frontal forcing will promote good lifting of smoke plumes with minimal down-mixing.  Transport winds should slowly veer from southwesterly late this morning to westerly in the early afternoon and northwesterly in the late afternoon.  The limiting factor today will be getting enough surface heating to establish good mixing and to lower the relative humidity enough for burning.  That could happen as early as 11 a.m.

TODAY'S FORECAST:

Areas of morning clouds; increasing middle and high clouds in the afternoon.

Salem's high temperature today will be near 78°F (average is 81°F).

Relative humidity:  Dropping below 65% by 11 a.m. and to below 50% by 2 p.m.

Surface winds: SW 2-6 mph a.m.; W 5-10 mph early p.m.; NW 10 mph late p.m.

Transport winds: SW 5 mph a.m.; W 10 mph early p.m.; NW 15 mph late p.m.

Mixing height: Rising to near 3000 feet by noon and to 5000 feet by 2 p.m.

Salem's sunset tonight: 7:54 p.m.

(Salem Airport data for Thursday, August 28th: High 87°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

A mostly dry upper-level trough is forecast to move across Oregon tonight, which will aid in evacuating any smoke through the Cascades.  A cool NW flow aloft will set up Saturday through Monday.  This pattern looks mostly dry, but some morning drizzle or light showers can't be ruled out, especially on Saturday.  The main impact will be below average temperatures, with highs staying in 70s.  Another mostly dry weather system may bring favorable burning conditions next Tuesday.

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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