[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Tue Aug 22 08:56:27 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT WED AUG 22, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

SE-S flow aloft pushed some wildfire smoke back over NW Oregon on Monday, after the eclipse, and nephelometer readings remain slightly elevated this morning.  An upper-level ridge has shifted east of the state today, turning the winds aloft southwesterly.  At the surface, a thermal trough remains over the valley this morning but should advance east of the Cascades this afternoon.  The air aloft is quite warm, so mixing height will be slow to rise this afternoon.  However, the combination of increasing onshore flow and SW winds aloft should begin to clear the wildfire smoke from the valley later this afternoon and could create favorable burning conditions.

TODAY'S FORECAST:

Sunny but hazy due to mostly-elevated wildfire smoke.

Salem's high temperature today will be near 88°F (average is 82°F).

Relative humidity:  Dropping to 60% by 11 a.m. and to 30-35% by 5 p.m.

Surface winds: SW 5 mph this morning; becoming NW 5-10 mph this afternoon.

Transport winds: SW 5 mph this morning; becoming NW 5-12 mph this afternoon; shearing to SW at the mixing height.

Mixing height: Rising to 3000 about 2 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 8:06 p.m.

(Salem Airport data for Monday, August 21st: High 90°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 50)

EXTENDED DISCUSSION:

Increasing SW flow aloft and onshore flow at the surface should drop temperatures back to near average on Wednesday and may provide an afternoon burning opportunity.

An upper-level trough will bring additional cooling on Thursday along with a chance of light showers.  Any rainfall should be quite light.  Temperatures will warm Friday through the weekend, with valley highs climbing back into the 90s by Sunday.

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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