[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Thu Aug 24 11:58:08 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 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 incoming weak upper-level trough began breaking up the marine low clouds over the northern Willamette valley late this morning.  Continued clearing is expected this afternoon, but cool air aloft should cap high temperatures near 80°F.  Mixing will be excellent today, but negative gradient stacking, as a result of last night's "marine push," will be difficult to balance.  NE transport winds should turn northerly this afternoon.  There is a slight chance they will back to the NW but could become too brisk for good plume development.  The bottom line is that a brief burning "window" is possible but unlikely late this afternoon.  A PIBAL is scheduled for 3 p.m.

TODAY'S FORECAST:

Partly cloudy, cooler, and breezy.

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

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

Surface winds: NE 5-10 mph; becoming N 7-15 mph.

Transport winds: NE 10-15 mph; becoming N 15 mph.

Mixing height: Rising 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.  Transport winds will likely stay too northerly to allow for burning.

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 and may transport 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 mid-80s during the second half of next week and may create 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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