[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Thu Aug 24 08:48:56 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT THU AUG 24, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

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

WEATHER DISCUSSION:

An upper-level trough brought increasing SW flow aloft and onshore flow at the surface on Wednesday, forcing marine air into the valley overnight.  The marine layer is about 5000 feet thick, so morning low clouds could produce some sprinkles.

Another weak upper-level trough should help to break up the marine clouds this afternoon and keep the air aloft cool enough for excellent daytime mixing.  Even with afternoon sunshine, valley temperatures will struggle to reach 80°F.  The limiting factors for burning today will be negatively-stacked pressure gradients and mostly northerly winds.

TODAY'S FORECAST:

Cloudy with a chance of sprinkles this morning.  Clearing this afternoon.

Salem's high temperature today will be near 78°F (average is 82°F).

Relative humidity:  Dropping to 60% by noon and to 40% by 5 p.m.

Surface winds: N 5-10 mph.

Transport winds: N 10-15 mph.

Mixing height: Rising to 3000 by noon and to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:03 p.m.

(Salem Airport data for Wednesday, August 23rd: High 85°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

A building upper-level ridge will warm temperatures into the low-80s on Friday.  Warming aloft will begin to suppress daytime mixing with mostly northerly transport winds.

A strong ridge is expected to warm temperatures to near 90°F on Saturday and the mid-90s Sunday and Monday.  In addition, the winds aloft will turn more southerly, likely transporting wildfire smoke back into the region.

A very weak upper-level trough should turn the flow aloft southwesterly again next Tuesday, which will help to clear any wildfire smoke from the valley.  Increasing onshore flow is expected to cool temperatures into the 80s and may create mid-week burning opportunities.

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.

     5. In accordance with OAR 837-110-0090, all field burning shall be
         banned when any two of the following criteria are present:
         A. Temperature of 95 degrees Fahrenheit or above
         B. Relative humidity of 30 percent or below
         C. Wind speed of 15 miles per hour or higher

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

Pete Parsons

ODF Meteorologist

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