[willamette-fcst] Silverton Hills AM Forecast_Gary.doc

Wed Sep 14 08:41:13 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:40 AM PDT WED SEPT 14, 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:

An upper level low is now centered over Idaho and moving slowly east. Its eastward movement will allow for a southwest and then westerly transport flow to develop this afternoon over the valley and then a weak marine push by evening. The pressure gradient is still negative between Salem and Redmond however the overall gradients are expected to be positive by about noon. Mixing heights will be slow to rise but sufficient in the afternoon for a burning opportunity. Primary considerations for burning are to confirm a westerly component to the wind, positive gradients and good mixing heights, all expected to be in place by around 2:00 p.m.

Sunny.

Salem's high temperature today will be near 79°F (average is 78°F).

Relative humidity:  Dropping 40% about noon and 30% near 5:00 p.m.

Surface winds: S 5-8 mph becoming SSW by 2:00 p.m. and W 6-9 mph around 5:00 p.m.

Transport winds: SW 6-10 mph becoming W by 5:00 p.m.

Mixing height: Only 1000 feet at 11:00 a.m. rising to 3000 feet about 2:00 p.m. and 4000 feet by 5:00 p.m.

Salem's sunset tonight: 7:23 p.m.

(Salem Airport data for Monday, September 13th: High 84°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 40)

EXTENDED DISCUSSION:

Northerly flow will likely prevent burning on Thursday though there is a chance that transport winds could turn northwesterly late Friday afternoon for burn opportunity. Saturday still looks like the best opportunity of the week as cold front will approaching from the northwest. Transport winds will increasingly be westerly during the afternoon, but models still disagree on the speed on which it arrives. If the faster European model is more correct then rain would likely arrive too soon.

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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