[willamette-fcst] Field Burning Forecast - September 5, 2017

Tue Sep 5 08:38:43 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
8:45 AM PDT TUE SEP 5, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level ridge is trapped over eastern Oregon, creating S flow aloft and offshore flow out of the Columbia Gorge and Cascade passes. Regional wildfire smoke had become trapped in the Willamette Valley, producing smoggy conditions that will keep the air quality degraded throughout today. Temperatures remain well above seasonal averages, but sunlight filtered by smoky skies will keep the Silverton Hills near 90°F. Mixing will not develop very well today with the offshore gradients, and field burning opportunities simply will not develop.

TODAY'S FORECAST:

Smog becoming Hot with Smoke Clouds.

Salem's high temperature today will be near 90°F (average is 80°F).
Relative humidity:  Below 60% by 9 a.m. and nearing 35% by 5 p.m.
Surface winds: Variable < 5 mph all day.
Transport winds: S 5-10 mph this morning, becoming S-SW 5-10 mph this afternoon.
Mixing height: Rising to 2000 feet by 5 p.m.
Salem's sunset tonight: 7:41 p.m.

(Salem Airport data for Monday, September 4th: High 92°F; Rainfall: .00")
(Maximum Ventilation Index expected today: 20)

EXTENDED DISCUSSION:

The weather pattern remains unfavorable for field burning through Wednesday, as the upper-level ridge slowly breaks down and moves into Idaho. S flow aloft persists through the afternoon, with restricted mixing, but the offshore flow will weaken and flat gradients will develop overnight.

Thursday morning will see the transition back to favorable weather arrive, as 2 weak upper-level disturbances move in from the Pacific Ocean, cooling the atmosphere and mixing out the lingering wildfire smoke. Onshore flow will slowly return to the Willamette Valley, with W-SW flow aloft, and partly cloudy skies may form in the afternoon hours over the Cascades. The timing of this improvement will determine whether field burning opportunities arrive before a weak marine push Thursday evening. Friday morning will see marine air warm & mix out quickly, as onshore flow builds with W flow aloft. A strong marine push will move into the Silverton Hills Friday afternoon, and could provide even better conditions for field burning ahead of the weekend.

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

Tom Jenkins, AEM
ODF Meteorologist
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