[willamette-fcst] Field Burning Forecast - September 11, 2017

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
8:45 AM PDT MON SEP 11, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A weak ridge aloft passes into Idaho today, increasing W flow gradually over time. High pressure east of the Cascades will weaken and move away, so current offshore gradients will improve; timing of this reversal will be the key factor, as the easterly winds are keeping temperatures above seasonal averages while helping fields dry out after recent rain events. The heating of the atmosphere from these winds will also limit the mixing that occurs with daytime heating, but a switch to onshore flow could also help improve ventilation.

TODAY'S FORECAST:

Partly Cloudy with Smoke Aloft.

Salem's high temperature today will be near 87°F (average is 78°F).
Relative humidity:  Below 60% by 10 a.m. and nearing 30% by 5 p.m.
Surface winds: N-NE 5-10 mph this morning, becoming N-NW 5-10 mph this afternoon and NW 8-12 mph this evening.
Transport winds: NE 5-10 mph this morning, becoming N-NW 5-10 mph this afternoon and NW 10-15 mph this evening.
Mixing height: Rising to 2000 feet by 12 p.m. and 3000 feet by 4 p.m.
Salem's sunset tonight: 7:35 p.m.

(Salem Airport data for Sunday, September 10th: High 82°F; Rainfall: .00")
(Maximum Ventilation Index expected today: 45)

EXTENDED DISCUSSION:

Onshore flow increases Tuesday, as an upper-level disturbance shifts south across the Canadian border into Washington. Cooler temperatures with improved mixing should provide favorable ventilation for field burning opportunities, as gradients become positively stacked. A sea breeze in the late afternoon brings enough moisture to form evening clouds over the Silverton Hills.

Wednesday, the progressive westerly flow continues and brings elevated moisture into the Willamette Valley. Cloud cover will limit heating and keep the humidity elevated, which may shorten any window for potential field burning operations. Clouds continue to stack against the Silverton Hills Wednesday night as an approaching storm system increases the onshore flow and generates drizzle and damp fog. The storm will move through with its Cold Front on Thursday, and isolated thundershowers may produce wetting rains across the Willamette Valley.



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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