[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Wed Sep 6 11:57:14 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT WED SEP 6, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A strong upper-level ridge is slowly progressing east of the area.  Dense wildfire smoke is making for poor air quality and continues to restrict visibility across NW Oregon. S-SE flow aloft is also transporting elevated wildfire smoke, moisture, and increasingly unstable air northward over the state.  A surface thermal trough is hanging on over NW Oregon but should shift east of the Cascades this afternoon.

Developing onshore flow will slowly begin to clear low-level wildfire smoke from the Willamette Valley later today, but elevated smoke will persist at least through Thursday.  Daytime heating should combine with increasing mid-level moisture to promote shower and thunderstorm development later today and tonight.  Thunderstorms will initially produce more wind than rain but will moisten the air mass over time.

TODAY'S FORECAST:

Mostly cloudy and smoky.  Not as hot.  Increasing chance of showers and thundershowers through tonight.  Possible rainfall totals up to .10"

Salem's high temperature today will be near 86°F (average is 80°F).

Relative humidity:  Dropping to near 45% by 5 p.m.

Surface winds: SW 5-10 mph...stronger gusts possible near showers.

Transport winds: SW 10 mph...stronger and more variable near showers.

Mixing height: Rising to 3000 feet by 2 p.m. and to 3500 feet by 5 p.m.

Salem's sunset tonight: 7:39 p.m.

(Salem Airport data for Tuesday, September 5th: High 89°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 35)

EXTENDED DISCUSSION:

An unstable southerly flow aloft will continue on Thursday, as increasing onshore surface winds cool temperatures back close to average and purge low-level smoke from the valley.  Elevated wildfire smoke will remain, along with the threat of showers and thundershowers.  The potential for wetting rains will also increase.

The flow aloft may turn more southwesterly on Friday, which would push the threat of thunderstorms east of the Cascades.  Continued onshore flow will hold temperatures near average and could create a burning opportunity.

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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