[willamette-fcst] Silverton Hills Midday Forecast_Gary.doc

Thu Sep 15 11:38:22 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT THU SEPT 15, 2016

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

Very weak pressure gradients have left the valley with little wind today although a light north breeze will develop. It may be mid-afternoon before it becomes persistent. Mixing heights will also be slower to rise than occurred yesterday. Primary factors to limit burning will be very little wind and becoming north when it does occur.

Sunny.

Salem's high temperature today will be near 81°F (average is 77°F).

Relative humidity:  Dropping 50% about 11:00 a.m. 30% near 3:00 p.m.

Surface winds: N 2-5 mph increasing to 5-8 mph after 3:00 p.m.

Transport winds: N 2-5 mph increasing to 9-12 mph after 3:00 p.m.

Mixing height: Near 1000 feet currently rising to 2000 feet about 2:00 p.m. and 3500 feet by 5:00 p.m.

Salem's sunset tonight: 7:22 p.m.

(Salem Airport data for Wednesday, September 14th: High 81°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 36)

EXTENDED DISCUSSION:

Models still indicate light northerly breezes on Friday then transport winds would turn northwest late in the afternoon. It would likely be too light and too late in the day for burning to occur. They also show a good burning opportunity on Saturday in all aspects except that the possibility of rain has increased for earlier in the day. Models disagree on this issue and the rain may not be farther south than the Columbia. Southwest transport winds will increase on Saturday as will mixing heights. Pressure gradients are leaning negative but may improve in the afternoon.

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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