[willamette-fcst] Field Burning Forecast - September 12, 2017

Tue Sep 12 08:42:25 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
8:45 AM PDT TUE SEP 12, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

The weather pattern transitions today, with progressive W flow aloft cooling the atmosphere and enhancing daytime mixing for better air quality. Onshore gradients have developed overnight, and are already positively-stacked. Northerly winds will turn W-NW by mid-day and with cooler air aloft, ventilation should be favorable for field burning opportunities shortly after mid-day, ahead of a weak marine push tonight.

TODAY'S FORECAST:

Partly Cloudy

Salem's high temperature today will be near 85°F (average is 78°F).
Relative humidity:  Below 60% by 10 a.m. and nearing 40% by 5 p.m.
Surface winds: N 5-10 mph this morning, becoming NW 5-10 mph by noon and W-NW 8-12 mph this afternoon.
Transport winds: N 5-10 mph this morning, becoming N-NW 5-10 mph by noon and W-NW 10-15 mph this afternoon.
Mixing height: Rising to 3000 feet by 12 p.m. and 4000 feet by 4 p.m.
Salem's sunset tonight: 7:28 p.m.

(Salem Airport data for Monday, September 11th: High 94°F; Rainfall: .00")
(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

W flow aloft increases as an upper-level disturbance shifts out of Canada on Wednesday, with marine clouds just north of the Silverton Hills after sunrise. Daytime heating will produce another day of favorable mixing, with W-NW transport winds and positive gradients in the afternoon hours likely to create another field burning opportunity.

A significant pattern shift occurs late Wednesday night and into Thursday, as the upper-level disturbance creates a weak frontal system near the Oregon coast. This feature will not have time to gather much moisture, so cloud cover will be limited with almost no rainfall. Onshore gradients, wind directions and mixing conditions will be favorable for field burning, but wind speeds may down-mix any smoke and could be a limiting factor. A weak upper-level ridge over the Pacific Ocean will keep the Willamette Valley under W-NW flow aloft with cooler air continuing to move in and promote mixing with daytime heating on Friday. Field burning opportunities could develop ahead of a strong marine push Friday afternoon.

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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