[willamette-fcst] Silverton Hills Midday Forecast_Gary.doc

Wed Sep 21 11:54:16 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT WED SEPT 21, 2016

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed.

WEATHER DISCUSSION:

Clouds have been stubborn to burn off and they will require at least a couple more hours on the east side of the valley. The clouds are slowing down the drop of humidity and rise of the mixing height. They are still expected to reach burnable levels by mid-afternoon but it is also slowing down the progress towards balancing of pressure gradients. Transport winds will gradually turn N with a chance of turning NNW in mid-afternoon for a possible burn opportunity. The primary limiting factor remains a north wind and whether pressure gradients can balance. There is still a slight chance of showers though becoming less likely since warming is slower to occur. A PIBAL is scheduled for 2:00 p.m.

TODAY'S FORECAST

Becoming sunny by mid-afternoon.

Salem's high temperature today will be near 69°F (average is 76°F).

Relative humidity:  Dropping to 60% about 2:00 p.m. and near 40% at 5:00 p.m.

Surface winds: N 5-10 mph.

Transport winds: NNE 8-10 mph becoming N 9-12 mph after 2:00 p.m., possibly turning NNW late.

Mixing height: Rising to 2000 feet about 2:00 p.m. and 4500 feet by 5:00 p.m.

Salem's sunset tonight: 7:10 p.m.

(Salem Airport data for Tuesday, September 21st: High 72°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 70)

EXTENDED DISCUSSION:

Thursday still looks like a better burning opportunity since transport winds will begin the day northwesterly and then transition to west. Gradients will again questionable and there will be a few showers in the area, but overall the afternoon conditions appear to encourage burning. Friday looks more likely to be wet as a decaying cold front arrives from the Pacific with showers around noon. Winds do not look especially favorable for burning this weekend, certainly not on Sunday, but so far, early next week still looks like a better week to burn with very small chances of showers from day to 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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