[willamette-fcst] Silverton Hills Midday Forecast_Gary.doc

Fri Sep 16 11:51:46 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT FRI SEPT 16, 2016

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 11:00 a.m. until 2 p.m. with a 50-acre limit.

WEATHER DISCUSSION:

Weak pressure gradients remain in the valley. Very light breezes will turn WNW this afternoon although they would be very light. They will become more consistently W this evening and overnight. Pressure gradients remain negative but are trending towards a balance and should be positive in the afternoon. Mixing heights will again be slow to rise but strong sunshine will get them to burnable levels around mid-afternoon. Limiting factors for burning today are low mixing heights early and weak winds all day.

Sunny and warm.

Salem's high temperature today will be near 85°F (average is 77°F).

Relative humidity:  Dropping to 35% by 2:00 p.m. and below 30% by 4:00 p.m.

Surface winds: NNE 2-5 mph becoming NNW 5-8 mph around 4:00 p.m.

Transport winds: NE 2-4 mph becoming NW around 2:00 p.m. and WNW 5-9 mph around 4:00 p.m.

Mixing height: Currently about 1000 feet then rising to 2500 feet about 2:00 p.m. and 3500 feet by 5:00 p.m.

Salem's sunset tonight: 7:20 p.m.

(Salem Airport data for Thursday, September 15th: High 83°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 32)

EXTENDED DISCUSSION:

Southwest transport winds and good mixing levels still imply an excellent burning opportunity on Saturday, and models show the pressure gradients improving during the day. Transport winds will be strengthening though, possibly reaching 20-25 mph by mid-afternoon. Models remain consistent in bringing rain to the area on Saturday though most indicate that it would be slightly north of the fields early in the afternoon. These models indicate that clouds would limit warming and relative humidity may not fall below 60% even if no rain occurs.

Sunday will likely be a drying day after the valley receives widespread one- to two-tenths of rain late Saturday into early Sunday morning. Monday through Friday looks drier and transport winds in the afternoons will likely be northwest most days. The newest model does suggest a chance of showers each day with the upper trough intensifying over the region. These would be mainly developing in the afternoon and be spotty when they occur.

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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