[willamette-fcst] Silverton Hills Midday Forecast_Gary.doc

Wed Aug 10 11:42:22 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT WED AUG 10, 2016

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed now until 2 p.m. with a 50 acre limit.

WEATHER DISCUSSION:

An upper level trough extends from N Montana into E Oregon, which leaves the valley under northerly flow that extends well above the mixing layer. Mixing heights are good but pressure gradients are becoming more negative, and wet fields remain an issue. Persistent northerly wind and unfavorable gradients remain the primary limiting factors for burning today.

Becoming sunny by mid-afternoon and a little warmer.

Salem's high temperature today will be near 78°F (average is 83°F).

Relative humidity:  Falling below 60% about noon and near 45% by 4:00 p.m.

Surface winds: N 6-10 mph.

Transport winds: N 14-18 mph.

Mixing height: Rising to 4000 feet by 5:00 p.m.

Salem's sunset tonight: 8:25 p.m.

(Salem Airport data for Tuesday, August 9th: High 73°F; Rainfall: .07")

(Maximum Ventilation Index expected today: 72)

EXTENDED DISCUSSION:

A low level ridge from the Pacific will build NE into British Columbia over the next couple of days. The ridge will cause low level flow to turn slightly offshore, warming the valley well into the 80s on Thursday and then 90s by Friday. It will also turn the transport winds NNE on Thursday and again on Friday. A weak marine push is expected to return temperatures to near average this weekend which should persist into next week.

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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