[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Aug 14 08:56:28 PDT 2013

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT WED AUG 14, 2013

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

Middle and high clouds are on the increase across western Oregon this morning, in response to an approaching Pacific weather system.  The offshore storm has more of an early autumn-like appearance on satellite imagery but is forecast to weaken as it very slowly moves onshore and stalls along the coastline later this afternoon and evening.

That should produce a prolonged period of favorable open burning conditions, beginning early this afternoon, as southerly low-level winds veer to the southwest and cooling aloft combines with continued warm surface temperatures to lift mixing heights above 3000 feet.

Onshore pressure gradients are already favorably stacked this morning to promote sustained lifting of smoke, so the limiting factors today will be mixing heights and wind direction.  PIBALS are scheduled to begin early today, at 11 a.m., since favorable burning conditions may begin in the early afternoon.  Rain is likely along the coast later today but should stay west of the Valley until tonight.

TODAY'S FORECAST:

Increasing clouds.  Chance of light rain late.

Salem's high temperature today will be near 85 degrees (normal is 83).

Relative humidity:  Drops to 50% by noon and to near 35% by 5 p.m.

Surface winds:  S 2-7 mph this morning; SW 5-10 mph this afternoon.

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

Mixing height: Rising to 3000 feet by 1 p.m. and to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:19 p.m.

(Salem Airport data for Tuesday, Aug. 13th: High 86°F; Rainfall .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

The weather system will continue to weaken as it moves inland tonight and Thursday, with a chance of light rain in the Willamette Valley.  More clouds and onshore flow will likely bring several degrees of cooling on Thursday, but conditions may remain favorable for additional burning, if fields are able to stay dry.

Another weather system is forecast to move onshore Friday.  If the timing is right and fields are dry enough, it could create another burning opportunity.

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 15mph.

     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 and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.

Pete Parsons

ODF Meteorologist
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