[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Thu Aug 14 08:58:37 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT THU AUG 14, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A slow-moving upper-level trough is keeping skies generally cloudy across most of Washington and Oregon this morning.  There are some breaks in the clouds near the center of circulation, over NW Oregon, with showers across much of Washington and sections of central and SW Oregon.  Daytime heating will enhance the shower activity, especially over the northern Cascades, this afternoon.

Conditions may be favorable for burning again today, with cool air aloft maintaining high afternoon mixing heights.  However, pressure gradients have weakened since yesterday and will need to turn more strongly onshore to allow for smoke evacuation.  Transport winds should turn southwesterly to westerly this afternoon and will also need close monitoring.  There is also a threat of showers, which could render fields too wet for burning or cause unwanted down-mixing of smoke.

TODAY'S FORECAST:

Mostly cloudy.  Chance of showers and thundershowers.

Salem's high temperature today will be near 80°F (average is 83°F).

Relative humidity:  Dropping below 60% by noon and to near 50% by 5 p.m.

Surface winds: S 2-6 mph this morning; SW 5-10 mph this afternoon.

Transport winds: S 4-8 mph this morning; SW 5-10 mph this afternoon.

Mixing height: Rising to 3000 feet by noon and to near 6000 feet by 5 p.m.

Salem's sunset tonight: 8:20 p.m.

(Salem Airport data for Wednesday, August 13th: High 80°F; Rainfall: Tr.)

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

The upper-level trough is forecast to move over eastern Washington on Friday.  The moist circulation around it should maintain mostly cloudy skies, but the threat of showers will decrease.  Onshore flow and good mixing may provide another burning opportunity.

A drier westerly flow aloft is forecast for Saturday through Monday, with onshore flow at the surface.  Mostly sunny skies will help temperatures recover to slightly above average.

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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