[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Thu Sep 21 08:54:18 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT THU SEP 21, 2017

BURN ADVISORY:

Recommended times for agricultural burning are from 11 a.m. until 5 p.m.

Prep burning is not allowed.

WEATHER DISCUSSION:

Vigorous showers dropped another two-thirds of an inch of rain at the Salem Airport on Wednesday.  Rainfall since Sunday has been just over two-and-a-quarter inches.  Salem's high temperature of 59°F, on Wednesday, was a record-cold maximum for the date (the previous record was 60°F in 1982).

A cold upper-level trough will maintain unseasonably cool weather today but with far less shower activity.  Additional rainfall totals should be less than a tenth of an inch.

TODAY'S FORECAST:

Patchy morning fog, then mostly cloudy and cool with a few showers.

Salem's high temperature today will be near 62°F (average is 76°F).

Relative humidity:  Dropping to near 60% by 5 p.m.

Surface winds: S 5-10 mph this morning; SW 5-10 mph later this afternoon.

Transport winds: SW 5-10 mph this morning; W 5-10 mph later this afternoon.

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

Salem's sunset tonight: 7:11 p.m.

(Salem Airport data for Wednesday, September 20th: High 59°F; Rainfall: .66")

(Maximum Ventilation Index expected today: 50)

EXTENDED DISCUSSION:

A drier and more stable northerly flow aloft will bring more sunshine on Friday, but temperatures will remain about 10 degrees below average.  A slight chance showers will persist, especially in the Cascade foothills.

A building upper-level ridge will bring partly to mostly sunny skies and light NW winds over the weekend, after areas of morning fog.  That will allow fields to begin the drying process.  Temperatures will progressively warm back to near average, by Monday, with light winds turning northerly.

As the ridge strengthens over the region, winds should turn offshore on Tuesday, and perhaps Wednesday, which will promote further drying of fields and help temperatures climb to near 80°F.

The upper-level ridge is expected to shift east of the state by Thursday.  Increasing SW flow aloft will introduce onshore flow back into western Oregon, which should allow for the burning of remaining fields.  A weak cold front is slated to bring back a threat of showers on Friday.

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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