[willamette-fcst] Silverton Hills Midday Forecast_Gary.doc

Wed Aug 31 11:55:27 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT WED AUG 31, 2016

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A few light showers remain near Salem but most valley locations remain dry. The showers have caused higher moisture at Salem than would be typical on fields farther east. A drying trend is expected along with partial clearing by mid-afternoon.

Relative humidity will be slow to drop today and mixing heights slow to rise. Both still have a good chance of reaching burnable levels by late afternoon and transport winds will turn west by late afternoon. Pressure gradients have become slightly negative which also need to balance. All of these factors continue to be marginal and require monitoring for potential burning conditions.

Showers ending and becoming partly sunny by late afternoon.

Salem's high temperature today will be near 70°F (average is 81°F).

Relative humidity:  About 75% by 1:00 p.m., falling to near 60% around 4:00 p.m.

Surface winds: S 5-8 mph becoming SW 8-10 mph about 2:00 p.m. and W 10-14 mph by 5:00 p.m.

Transport winds: SW 8-10 mph becoming W 12-15 mph by 4:00 p.m.

Mixing height: Rising to 1500 feet about 2:00 p.m. then 4000 feet by 5:00 p.m.

Salem's sunset tonight: 7:50 p.m.

(Salem Airport data for Tuesday, August 30th: High 75°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

Thursday still appears to provide the best remaining potential for burning this week as transport winds continue southwesterly as the upper trough gathers strength again offshore. The upper trough will finally move inland on Thursday night and cause a chance of rain again late Thursday afternoon or early evening, then continuing overnight. Rain on Thursday night and Friday morning will likely make Friday an unburnable day.

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

Gary Votaw

ODF Meteorologist

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